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Semiconductor Physics, Electronic Devices and Circuits (BCA Part-I)
Ashish Sharma M.Sc. (Physics)
Anupama Upadhyay Revised By: Mr Vijay M.Sc. (Physics)
Lecturer Deptt. of Information Technology Biyani Girls College, Jaipur
2
Published by :
Think Tanks Biyani Group of Colleges
Concept & Copyright :
Biyani Shikshan Samiti Sector-3, Vidhyadhar Nagar, Jaipur-302 023 (Rajasthan) Ph : 0141-2338371, 2338591-95 Fax : 0141-2338007 E-mail :
[email protected] Website :www.gurukpo.com; www.biyanicolleges.org
ISBN: 978-93-81254-35-9
Edition : 2011 Price :
While every effort is taken to avoid errors or omissions in this Publication, any mistake or omission that may have crept in is not intentional. It may be taken note of that neither the publisher nor the author will be responsible for any damage or loss of any kind arising to anyone in any manner on account of such errors and omissions.
Leaser Type Setted by : Biyani College Printing Department
Semiconductor Physics, Electronic Devices and Circuits
3
Preface
I
am glad to present this book, especially designed to serve the needs of the
students. The book has been written keeping in mind the general weakness in understanding the fundamental concepts of the topics. The book is self-explanatory and adopts the “Teach Yourself” style. It is based on question-answer pattern. The language of book is quite easy and understandable based on scientific approach. Any further improvement in the contents of the book by making corrections, omission and inclusion is keen to be achieved based on suggestions from the readers for which the author shall be obliged. I acknowledge special thanks to Mr. Rajeev Biyani, Chairman & Dr. Sanjay Biyani, Director (Acad.) Biyani Group of Colleges, who are the backbones and main concept provider and also have been constant source of motivation throughout this Endeavour. They played an active role in coordinating the various stages of this Endeavour and spearheaded the publishing work. I look forward to receiving valuable suggestions from professors of various educational institutions, other faculty members and students for improvement of the quality of the book. The reader may feel free to send in their comments and suggestions to the under mentioned address. Author
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Syllabus B.C.A. Part-I
SEMICONDUCTOR PHYSICS, ELECTRONIC DEVICES AND CIRCUITS [This course is of introductory nature as such rigorous mathematical analysis should be avoided and emphasis should be on concepts and contents of mathematical expressions] 1.
2.
3.
4.
5.
6. 7.
Structure of Matter (Molecule, Atom), Atomic Structure (Energy Levels and Electronic Configuration), Intermolecular Forces, Phases of Matter, Types of Solids, Crystal Structure of Solids, Atomic Bonding (Ionic Covalent and Metallic Bondings), Energy Band Theory of Crystals, Energy Band Structure of Insulators, Semiconductors and Metals. Mobility and Conductivity, Electrons and Holes in Intrinsic, Semiconductor Elementary Properties of Germanium and Silicon, Donor and Acceptor impurities, Extrinsic Semiconductors, Generation and Recombination of Charges, Diffusion. Energy Band Structure of Open Circuit P-N Junction, Depletion Region, P-N Junctions as a Rectifier, Current Components of a P-N Diode, Ideal Voltage, Ampere Characteristics, Temperature Dependence of the V-I Characteristics, Diode Resistance Varactor Diodes, Junction Diode Switching Times, Breakdown Diodes, Tunnel Diode, Semiconductor Photodiode, Photovoltaic Effect, Light Emitting Diodes. Half-wave and Full-wave Rectifiers, Ripple Factor, Efficiency, Voltage Regulation, Inductor Filters, Capacitor Filters, L and Section Filters, Regulated Power Supplies, Information about SMPS Supply. Bipolar Junction Transistors, Bipolar Transistor Action, Basic Principle of Operation Open Circuited Transistor, Transistor Biased in Active Region, Current Components in a Transistor, Characteristic Curves in Common Emitter, Common Base and Common Collector Configurations, Expression for α, β and γ, Hybrid Parameters of a Transistor. Transistor as an Amplifier, Frequency Response of an Amplifier, RC Coupled Amplifier, Feedback Concepts and Oscillator. Elementary Information about Field Effect Transistors, Thyristors, Optoelectronic Devices and Display Devices.
□□□
Semiconductor Physics, Electronic Devices and Circuits
Content S. No. 1.
2.
3.
4.
Name of Topic Structure of Solid 1.1
Types of Solids
1.2
Crystal Structure of Solids
1.3
Atomic Bonding
1.4
Energy Band Structure
Transport Phenomena in Semiconductor 2.1
Mobility and Conductivity
2.2
Semiconductor Ge and Si (Intrinsic Semiconductors)
2.3
Extrinsic Semiconductors
2.4
Diffusion
Semiconductor Diodes 3.1
Open Circuit P-N Junction
3.2
Temperature Dependence of the V-I Curve
3.3
Types of Diodes
Rectification & Power Supply 4.1
Half Wave and Full Wave Rectifier
4.2
Filters
4.3
SMPS Supply
5
6
S. No. 5.
6.
7.
8.
Name of Topic Bipolar – Junction – Transistor 5.1
Basic Principle
5.2
Characteristic Curves in Different Configurations
5.3
Hybrid Parameters of a Transistor
Amplifiers and Oscillators 6.1
Transistor as an Amplifier
6.2
RC Coupled Amplifier
6.3
Feed Back Concepts and Oscillator
FETs, Thyristors and Opto-electronic Devices 7.1
Elementary Information about FET
7.2
Thyristors
7.3
Opto Electronic Devices
Unsolved Papers 2011 to 2006
□□□
Semiconductor Physics, Electronic Devices and Circuits
7
Chapter-1
Structure of Solid Q.1
What do you understand by Interatomic and Intermolecular Forces? How are they different from each other?
Ans.: The Net force of attraction between atoms is called Interatomic Force whereas the net force of attraction between molecules is called Intermolecular Force. Interatomic Forces are mainly of four types : (i) Ionic Bond
(ii) Covalent Bond
(iii) Metallic Bond
(iv) Vander Waal’s Bond
Types of Intermolecular Forces : (i)
Dipole –Dipole Interactions
(ii)
Instantaneous Dipole – Induced Dipole Interactions
(iii)
Hydrogen Bond
Differences between Interatomic an Intermolecular Forces :
Q.2
(i)
Intermolecular Forces are generally weaker than the Interatomic Forces.
(ii)
The equilibrium separation of two molecules is large compared to the equilibrium separation of two atoms.
(iii)
Intermolecular Forces are generally dependent on the relative orientation of two molecules in addition to the dependence on the distance between then but for Interatomic Forces it s not so.
What do you understand by Space Lattice? List the name of Fundamental Crystal System.
8
Ans.: Lattice : The Lattice is defined as an array of points in space such that the environment about each point is same with the environment about any other point. Linear Lattice : It is are array of infinite points is one dimension. Two Dimensional Lattice : It is a repetition of linear lattice in another direction by a regular interval of distance. Space Lattice or Three Dimensional Lattice : If the points of two dimensional lattice are repeated in another non-coplanar direction by a regular interval of distance, then they form three dimensional lattice called Space Lattice. Fundamental Types of Lattice :
Q.3
(i)
Cubic
(ii)
Tetragonal
(iii)
Orthorhombic
(iv)
Rhombohedral
(v)
Hexagonal
(vi)
Monoclinic
(vii)
Triclinic
What do you mean by Crystalline and Amorphous Solids? Point out some important differences between them.
Ans.: The solids which have regular and periodic arrangement of atoms or molecules in a definite and long range order are called Crystalline Solids. Amorphous or Glassy Solids : Solids which do not have any permanent shape or do not have any regular and periodic arrangement of atoms or molecules are said to be Amorphous Solids.
Difference between Crystalline and Amorphous Solids
Semiconductor Physics, Electronic Devices and Circuits
S.No.
Amorphous Solids These are isotropic in nature.
9
Crystalline Solids These are anisotropic.
All the bonds in the All bonds have same bond Amorphous Solids are not strength. equally strong. These are not bounded by flat The Crystalline Solids are surfaces. bounded by the flat surfaces. They lack a long-range order There is definite and long range of bounding in their structure. order of arrangement of atoms or molecules in a Crystalline Solid.
Q.4
What are Conduction Band, Valance Band and Forbidden Gap. Explain these bands by proper diagrams.
Ans.: Valance Band : The energy band in which the valance electrons are situated is called Valance Band. Conduction Band : The allowed energy band above the Valance Band (i.e. having higher energy state) which is almost empty in the unexcited state is called Conduction Band. Forbidden Energy Gap : In between the Conduction and Valance Bands there is an energy gap in which no allowed energy level is situated. This energy gap is called Forbidden Energy Gap.
10
As shown in above figure PQ represents Conduction Band, RS is Valance Band and QR is Forbidden Energy Gap. Q.5
Compare the resistivities of Conductors, Semiconductors and Insulators. What the order of Forbidden Energy Gap in there substances?
Ans.: Conductors : In conductors, forbidden energy gap is not present i.e. ∆Eg = 0. At normal temperatures their electrical conductivity is very high of the order of 10 6 to 108 mho/m and resistivity is very low of the order of 10-8 to 10-6 Ohm–m. Insulators : The forbidden energy gap ∆Eg in insulators is very large. Their conductivity is very small of the order of 10-12 to 10-18 mho/m. The resistivity of such materials is very high of the order of ∆Eg > 5 ev. Semiconductors : In these materials there is a finite forbidden energy gap between their conduction and valance bands but it is much less their the forbidden energy gap of insulators, It is of the order of 1 ev.
Semiconductor Physics, Electronic Devices and Circuits
11
The conductivity of semiconductors is of the order of 10-7 to 10 mho/m and resistivity of the order of 10-1 to 107 ohm-m. Q. 6
Write down the Postulates of Bohr’s Atomic Model.
Ans.: Postulates of Bohr’s Atomic Model : According to Bohr’s Model, an atom is composed of electrons moving in various fixed circular or elliptical orbits around a heavy nucleus made up of protons and neutrons.
Diagram : An Atomic Structure Electrons : Electrons are negatively charged elementary particles moving around the positively charged nucleus in different but fixed orbits. Mass of an electronic is 9.1 x 10-31 kg and charge on electron is -1.6 x 10-19 coulomb. Nucleus : It is a central hard care of an atom and contains protons and neutrons. Neutrons are electrically neutral while protons carry positive charge equal to the magnitude of electrons charge. Mass of Both protons and electron is about 1840 times of electron’s mass. Atomic Number (Z) : The number of protons on atom has is called its Atomic Number (Z). Atomic Mass Number (A) : The total number of protons (Z) and neutrons (N) contained in the nucleus of an atom is called its Atomic Mass Number (A). Multiple Choice Questions: 1. According to Dalton’s atomic theory, an atom can:
12
(a) (b) (c) (d) 2.
3.
5
6
7
8
9
( )
A matter has neither definite volume nor fixed shape is: (a) solid (b) liquid (c) gas (d) None of the above
( )
The maximum number of electrons possible in the shell of principal quantum number n1 is (a) (c)
4.
be destroyes be created either be destroyed ar created neither be destroyed nor created
n2 2n
The strongest bond is: (a) Ionic bond (c) Molecular bond
(b) (d)
2n 2 n(n+1)
( )
(b) (d)
Covalent bond Metallic bond
( )
The reason for inter-molecular forces is: (a) Only dipole-dipole interactions (b) Only induced dipole interactions (c) Dispersive forces (d) All of the above The strongest bond is: (a) lonic (c) vander Waal
( )
Covalent Metallic
( )
In the metals, the most common interaction is: (a) ion-ion (b) (c) election-electron (d)
ion-electron repulsive
( )
Election-volt is the unit of: (a) Momentum (c) Energy
Velocity Potential
( )
The electron orbiting the nucleus possesses: (a) Potential energy only (b) Kinetic energy only
(b) (d)
(b) (d)
Semiconductor Physics, Electronic Devices and Circuits
(c) (d) 10
11
12
13
14
16
Partially potential and partially kinetic energy None of the above
( )
The cause of conductivity in metal is: (a) Proton (c) Bound electron
(b) (d)
Free electron Ions
( )
The valence bond at OK is: (a) Completely filled (c) Partially
(b) (d)
completely empty Nothing can be said
( )
Semi-conductors such as Germanium and silicon exhibit. (a) Covalent bonding (b) Metallic bonding (c) Ionic bonding (d) van der walls bonding
( )
Holes are present in: (a) Valence band (b) Conduction band (c) Conduction and valence band (d) None of the above
( )
The maximum intertomic distances exists in :
(a) (c) 15.
13
Solids Gases
(b) (d)
Liquids Both in liquids and solids
( )
Which of the following elements is a covalently bonded crystal? (a) Ar (b) NaCL (c) Ge (d) Na
( )
The maximum intertomic distances exists in : (a) Solids (b) (c) Gases (d)
( )
Liquids Both in liquids and solids
17
Each energy band of a small piece of solid containing 100 atoms will have closely spaced energy levels equal to: (a) 50 (b) 100 (c) 200 (d) 500 ( )
18
Which of the following elements is a covalently bonded crystal?
14
(a) (c)
19
20.
21.
22
23
24
25.
26
27.
Ar Ge
(b) (d)
NaCL Na
( )
The maximum number of electrons in nth shell of an atom is: (a) n2 (b) 2n (c) n (n + 1) (d) 2n2
( )
The range of intermolecular force is: (a) infinite (c) 10 A
(b) (d)
1 –3 A 0
( )
The bonds in silicon crystals are: (a) Covalent (c) metallic
(b) (d)
ionic None of the above
( )
Example of a semiconductor is: (a) Al (c) Fe
(b) (d)
Cu Si
( )
The unit cell with crystallographic dimension, a = b c and (a) cubic (b) hexagonal (c) monoclinic (d) tetragonal The number of fundamental crystal structures are: (a) 7 (b) (c) 21 (d) On heating resistance of conductor: (a) increase (b) (c) does not change (d)
0
is: ( )
14 28
( )
decreases becomes zero
(
Bonds in semiconductor are: (a) Covalent (c) Metallic
(b) (d)
Ionic neutral
( )
Band gap in Ge and Si lies in range: (a) 0.5 –1.5 eV (c) 2.5 – 3.5 eV
(b) (d)
1.5 –2.5 eV 3.5 – 4.5 eV
( )
Semiconductor Physics, Electronic Devices and Circuits
28.
29
In intrinsic semiconductors: (a) nh < ne (c) nh = ne
(b) (d)
nh > ne nh = ne = 0
The unit of resistance is: (a) Mho
(b)
Ohm
(d)
Ampere
(c)
S.No. Ans S.No. Ans S.No. Ans
15
Farad
Q:01 D Q:10 B Q:19 D
02 B 11 C 20 B
03 B 12 A 21 A
04 A 13 A 22 D
05 D 14 C 23 B
06 B 15 B 24 A
( )
07 A 16 C 25 C
08 C 17 B 26 A
09 C 18 B 27 B
16
Chapter-2
Transport Phenomena in Semiconductor Q.1
Write down the relation between Drift Velocity and Current Density.
Ans.: Current Density : The electric current passing normally through a units area of the metal wire is called Current Density (J). I J A As shown in figure, let a potential difference V is applied between the ends of the wire of length L and cross section area A. If number of free electrons per unit volume in wire are n then total number of electrons passing through unit area of the wire in time t seconds will be (nAV dt) and flow of charge will be Q qnAVd t Where q → Charge of an electron
qnAVd t t
I
Q t
I
qnAVd
Current Density J
J Q.2
I A qnVd
What is Intrinsic Semiconductor? Give examples of Intrinsic Semiconductors? What are the charge carriers in them?
Semiconductor Physics, Electronic Devices and Circuits
17
Ans.: Intrinsic Semiconductor : When a semi conducting material is pure and in its natural from it is called Intrinsic Semiconductor. The examples of Intrinsic Semiconductors are Germanium (Ge) and Silicon (Si). Crystal Structure : Both Silicon and Germanium has four valance electrons. For stability every atom shares one electron from the neighboring four atoms to complete the valance orbit. Thus the valance electrons of each atom form covalent band with one electron of each of its four neighbors. The crystal structure of both Silicon and Germanium is diamond structure in which the bands form a tetrahedron. Charge Carriers : At absolute zero temperature, all semiconductor behaves as insulator and no charge carrier exist. As the temperature increases, some electron from valance band, reach the conduction band after crossing the forbidden energy gap. Due to this a vacant site is created is valance band is called a Hole. Thus free electrons and holes are created in pairs simultaneously. Therefore in intrinsic semiconductors both electrons and holes are the charge carriers. Q.3
What are Extrinsic Semiconductor? Explain the construction of N and P-Types Semiconductors with necessary diagrams.
Ans. Extrinsic Semiconductor : The conducing of Intrinsic Semiconductor is low. When semi conducting material Germanium and Silicon are doped with a controlled amount of an impurity element, the conductivity increases. The semiconductors so obtained are called Extrinsic Semiconductors. Extrinsic Semiconductors are of two types : (i)
N-Type Semiconductor
(ii)
P-Type Semiconductor
N-Type Semiconductors : If an element of fifth group as arsenic or antimony is added to Germanium or Silicon whose valancy is four, the semiconductor obtained is called N-Type Semiconductor.
18
As shown in fig.(1), when a pentavalent impurity atom replaces a tetravalent atom Germanium or Silicon, four valance electrons of the impurity atom form covalent band respectively with one electron of each of the neighboring atoms, the fifth electron remains almost free. Thus atom of the impurity element of fifth group contributes one free electron as a charge and due to this property fifth group impurity is called Donor Impurity.
of
while every carrier
The energy level of the valance electron of the pentavalent element lies in forbidden energy gap of the intrinsic semiconductors just below the conduction band. This energy level is called Donor Level. In N-Type Semiconductors, electrons are majority charge carries.
P-Type Semiconductor : This type of semiconductor is formed when third group element like boron is added as impurity in Ge or Si material. Due to adding of trivalent impurity, three valance electrons of impurity atom form covalent bands respectively with one electron of each of the three neighboring atoms but the band of the fourth atom remains incomplete and a deficiency of electron exists there. This vacancy is called Hole. Hole has ability to capture available electron there on the impurity of third group is called Acceptor Impurity.
Semiconductor Physics, Electronic Devices and Circuits
19
In P-Type Semiconductor, acceptor level lies slightly above the valance band of semiconductor. Holes are the majority charge carries in P-Type Semiconductor.
Q.4
Derive the expression of conductivity of an Intrinsic Semiconductor in terms of mobility.
Ans.: In Intrinsic Semiconductors, both electrons and holes are the energy charge carries. When a battery is connected between the two ends of a semi-conducting material, both electrons and holes move in definite direction. The drift velocity of both electrons and is proportional to intensity of applied electric field. i.e.
And
Vn E
Vn
n
Vp E
Vp
p
Here →
n
&
p
holes,
E E
are mobilities of electrons and holes respectively.
If current density of electrons is Jn and of holes is Jp then total current density is an intrinsic semiconductor is J = Jn + JP But we know that Current Density = Number Density of Charge Carriers X Charge X Drift Velocity Jn = nqVn = μnnqE And
Jp = nqVp = μppqE J = Jn + Jp
OR
J = qE (nμn + pμp ) amp/m2
The electrical conductivity of the semiconductor
20
J E Q.5
q (nμn + pμp ) mho/m.
Derive of the expression of conductivity on N and P Types of Semiconductors.
Ans.: We have derived the expression for current density in a semiconductor in last question and got J = q (nμn + pμp ) E And therefore conductivity J q (nμn + pμp ) mho/m E For N-Type Semiconductors n = ND Where ND is the number density of donor impurities Further, n >> p Therefore,
= q NDμn
For P-Type Semiconductors p ≈ NA Where NA is number density of acceptor impurities Further, P>>n Therefore, σ ≈ q Na µp Thus is Extrinsic Semiconductors the conductivity depends on the number density of donor or acceptor impurity atoms. Q.6
Explain of formation of depletion larger in P-N Junction Semiconductor.
Ans.: In a P-N Junction, P-type semiconductor material is joined to Ntype material in atomic sense. The Pmaterial has a large hole density while the N- material has a large free electron density. In this way at the
Semiconductor Physics, Electronic Devices and Circuits
21
junction a gradient of density of charge carriers developed. Due to this, the holes diffuse from P-side to N-side while electrons diffuse from N-side to P-side. This process of diffusion affects only a narrow region near the junction between X1 and X2. Due to the diffusion of electrons from N to P-side and holes from P to N-side, in a thin layer at the junction the holes capture the electrons and the charge carriers no longer remain free. This region of the junction thus depleted of free charge a carriers and the larger so formed is called Depletion Layer.
Multiple Choice Questions: 1
2
3
4
5
6
7.
For producing a p-type semiconductor which of these would used: (a) Acceptor atoms (b) Donar atoms (c) Pentavalent impurity (d) Arsenic
( )
The merging of a free electron with a hole is called: (a) decomposition (b) recombination (c) neutralization (d) covalent bonding
( )
Free electron exits in: (a) First band (c) Valence band
(b) (d)
Forbidden band Conduction band
( )
Depletion layer has: (a) Both free electrons and holes (c) Only free electrons
(b) (d)
None of the free charge carriers Only free holes ( )
A P-N junction in reverse bias acts as a: (a) Very high resistance (b) Very low resistance (c) Zero resistance (d) None of the above Current in a semi-conductor diode is due to the drift of: (a) Free electrons (b) Free electrons and holes (c) Positive and negative ions (d) Protons In n-type semiconductor the fermi energy level is displaced: (a) towards the valence band
( )
( )
22
(b) (c) (d) 8
9
10
11
12
13
14
towards the conduction band not displace d and depend on quantity of impurity
( )
The electronic configuration of Si is: (a)
1s2, 2s2, 2p6, 3s2, 3p4
(b)
1s2, 2s2, 2p6, 3s1, 3p3
(c)
1s2, 2s2, 2p6, 3S2, 3p2
(d)
1s2, 2s2, 2p6, 3s0, 3p4
The depletion region of a-p-n junction contains: (a) Electrons only (b) A rectifier (c) Electrons and holes (d) Neither electrons nor holes A p-n junction can be used as: (a) An amplifier (c) An oscillator
(b) (d)
( )
( )
A rectifier A modulator
( )
Depletion layer increases: (a) When p-n junction is formed (b) On applying forward bias (c) On applying the reverse bias (d) When temperature is decreases ( ) The number of free electrons and holes in an intrinsic semiconductor increase when the temperature: (a) Decreases (b) Increases (c) Stays the same (d) None of the above ( ) The electronic configuration of Si is: (a)
1s2, 2s2, 2p6, 3s2, 3p4
(b)
1s2, 2s2, 2p6, 3s2, 3p2
(c)
1s2, 2s2, 2p6, 3S1, 3p3
(d)
1s2, 2s2, 2p6, 3s0, 3p4
To make an n-type material, following is added to Ge: (a) Al (b) B (c) P (d) In
( )
( )
Semiconductor Physics, Electronic Devices and Circuits
23
15
The energy needed to detach the fifth valence electron from the Antimony (Sb) impurity atom surrounded by Ge atoms is approximately: (a) 0.001 eV (b) 0.01 eV (c) 0.1 eV (d) 1.0 eV ( )
16
In an N-type semiconductor, the position of Fermi level: (a) Is lower than the centre of energy gap (b) Is at the centre of the energy gap (c) Is higher than the centre of energy gap (d) Can be anywhere in energy gap depending upon the
17.
18
19
20
21
S.NO. Ans S.NO. Ans S.NO. Ans
concentration of impurity atoms The resistivity of a semiconductor at Ok is: (a) Zero (b) (c) 105 (d)
Infinite None of the above
( )
The unit of current density is: (a) C/m2 (c) Ampere/m2
C/m3 Ampere/m3
( )
(b) (d)
( )
The merging of a free electron with a hole is called: (a) Neutralization (b) Decomposition (c) Recombination (d) None of the above The example of intrinsic semiconductor is: (a) Ge (c) Sb Cut in voltage of Si diode is of the order of: (a) 0.2 V (c) 0.2 mV
Q:01 A 10 B Q:19 C
02 B 11 C 20 A
03 D 12 B 21 B
04 B 13 B
05 A 14 C
( )
(b) (d)
Ca Ag
( )
(b) (d)
0.6 mV 0.6 V
( )
06 B 15 B
07 B 16 C
08 C 17 B
09 C 18 C
24
Chapter-3
Semiconductor Diodes Q.1
What is a P-N Junction Diode?
Ans.: P-N Junction : When a P-type semi conducting material is joined to a N-type material is the atomic sense, the device is called P-N junction. During the formation of a P-N Junction a potential barrier is developed at the junction. This potential barrier abstracts the flow of free charge carriers (electrons & holes). P–N junction has a special feature that it allows the flow of current easily in one direction but obstruct the flow of current in opposite direction. Thus P-N Junction acts as a rectifier diode therefore this device is called P-N junction Diode. Q.2
Explain the working of P-N Junction Diode. Discuss its Forward and Reverse Bias.
Ans.: Working of P-N Junction Diode : P-N Junction diode has a very special feature that it allows the flow of current only in our direction and obstruct the flow of current ins other direction so this device is used in rectifies circuits. Forward Biasing of a P-N Junction Diode : P-N junction, If P-terminal is joined to positive terminal of battery and N-terminal negative terminal then the diode is said to Forward Biased.
to be
By a Forward Bias, the height of the potential barrier at the junction is reduced because the field applied by the external source at the depletion larger is opposite to the field present there due to the potential barrier.
Semiconductor Physics, Electronic Devices and Circuits
25
In Forward Bias, current flows in the circuit from P-region to N-region of the diode; By increasing the biasing voltage current increase exponentially the dynamic resistance offered by the P-N junction is very low of the order of 100 ohm. Reverse Bias : If the P-terminal of P-N junction is connected to negative terminal of the battery and N-terminal of the diode to positive terminal of the battery, the applied bias is called Reverse Bias. In Reverse Bias, the height of potential barrier as well as the thickness of depletion layer is increased. Thus the availability of free charge carriers is reduced and their flow is obstructed by the barrier the current in this state is almost zero only a small current in microamperes flow due to the thermally generated holes in a N-material and free electrons in P-region. In Reverse Bias, state the dynamic resistance of PN Junction diode is very large (106 ohms) Thus PN Junction allows current to flow easily in forward bias state and obstructs the flow of current in reveal bias state.
26
Q.3
Give circuit diagrams to obtain characteristic curves of a P-N Junction Diode.
Ans.:
The circuit of Forward Biasing P-N Junction Diode
The circuit of Biasing of a P-N Diode
Volt-Ampere Characteristics of a Junction Diode
of
a
Reverse Junction
P-N
Semiconductor Physics, Electronic Devices and Circuits
Q.4
27
Explain the phenomena of Avalanche Breakdown and of Zener Breakdown.
Ans. Avalanche Breakdown : This breakdown takes place due to strong electric field in depletion layer. Due to this, there is a direct rupture of covalent bands and electron-hole pairs so generated acquire a large amount of kinetic energy from this field and collide with immobile ions, thereby generating further electronhole pairs. This process is cumulative in nature and results in avalanche of charge carriers in very short time. This mechanism is called Avalanche Multiplication. Zener Breakdown : When P and N regions are heavily doped, the change charge carrier density at the junction is abrupt and depletion layer of very thin width is formed. The charge carriers of opposite nature are concentrated on both sides of this layer. Due to this, strong electric field is generated is the depletion layer due to it’s very small width even at nearly zero bias. When diode is reverse biased, field across depletion layer becomes very high. In this state the valance band of P-region comes in front of conduction band of Nregion and the electrons start flowing from valance band of P-region to conduction band of N-region by tunnel effect due to strong electric field. This type of breakdown is called Zener Breakdown. Q.5
Explain the working of Zener Diode. How is it used for the Voltage Stabilization?
Ans.: In reverse bias state, when applied voltage exceeds a certain limit, the current through the P-N diode increases abruptly. This reverse voltage at which current increases sharply, is called Zener Voltage. The ordinary diode may got damaged. But zener diode is made to work in this breakdown region.
Symbol of Zener Diode To obtain constant DC Voltage, regulated power supply is used. Zener diode is used in many different type of Voltage Regulators for Voltage Stabilization.
28
Example : In Transistor, Shunt Regulator, Transistor Series Regulator, Zener Diode Shunt Regulator. Zener Diode Shunt Regulator : As shown in circuit diagram Vi = VZ + RS I or
VZ = Vi – RS I _ _ _ _ (1)
and current diode will be IZ = I – IL
through
zener _ _ _ _ _ (2)
We know that variation in input voltage Vi and load current IL makes output voltage unregulated. When a zener diode is connected in parallel with load RL, then zener diode keeps the output voltage constant is all conditions for example, If voltage variation DC takes place at any instant in the input voltage Vi. Let it changes the current by an amount dI. This current variation simultaneously changes the current through zener with the same amount, but the load current IL remains constant and thus voltage across load remains equal to zener voltage Vz. Similarly if the load current changes, the zener current increases or decreases to keep the current through RS. Constant as seen from equation (1). Q.6
Explain the principle of Varacter Diode.
Ans.: Varacter Diodes: The varacter is a reverse biased diode having a useful property that the diodes in reverse bias state that the diodes in reverse bias state acts as a capacitor whose capacitance varies with the applied reverse voltage. The width of depletion layer increases with reverse voltage. In this state P and N region the diode are like the plates of the capacitor the depletion layer is like the dielectric. We know that capacitance is inversely
Circuit Symbol
the of and
Semiconductor Physics, Electronic Devices and Circuits
29
proportional to the distance between the plates. Consequently larger the reverse voltage, the larger is the width of depletion layer and hence the smaller the capacitance. It means that the barrier capacitance is controlled by the reverse voltage. The varacter diode is widely used in FM Radio, TV receiver etc. Characteristic Curve of Varacter Diode Q.7
Explain the principle and working of Photodiode.
Ans.: Photodiode : It is a light a sensitive device which converts light signals into electrical signals. It is a special type of diode which operates in reverse bias. Construction of Photodiode : Photodiode is made of a semiconductor P-N diode kept in a sealed opaque plastic of glass casing. It has a small transparent window through which light falls junction.
at
Working of Photodiode : When light falls or junction of PN diode, electron-hole pairs are generated in the semiconductor material. Due to movement of these pairs, current flow in reverse bias. The magnitude of current depends on the intensity of light. Higher the intensity of light IL, higher is the reverse current that flows through the circuit. Use of Photodiode : Photodiode is generally used in photo-detection devices, logic circuits etc. Q.8
Explain working of Light Emitting Diode. Mention it’s applications.
Ans.: Light Emitting Diode (LED) : P-N junction diode emits light when it is forward biased. Such type of diode is called Light Emitting Diode.
30
These diodes are made from Gallium Phosphide (Ga) Materia or Gallium Arsenide Phosphide (GaAsP) material. The amount of light emitted by the diode is directly proportional to the forward current i.e. higher the forward current, higher is the intensity of light emitted. When LED is forward biased, the electrons from N region and holes from P region move towards the junction and recombine. In this process the electrons lying in the conduction band of N-region fall into the holes in the valance band of P region. As a result energy of electrons decreases and the energy equivalent to the difference of energy between the conduction band and valance band is radiated in the form of light in LED. Q.9
Describe the construction and circuit symbol of a Photovoltaic Cell.
Ans.: Photovoltaic Cell : If the Potential difference is generated between the ends of the semiconductor material due to light radiator fall on it, then this effect is called photovoltaic effect and semiconductor device using this effect is called Photovoltaic Cell. The generated potential difference depends upon the intensity of light and the load. A Photovoltaic Cell consists of semiconductor material such as silicon, germanium etc. which is joined with the metal plate as shown in fig. below :
Semiconductor Physics, Electronic Devices and Circuits
31
Multiple Choice Questions:
1 A P-N junction in reverse bias acts as a: (a) Very high resistance (c) Zero resistance 2
(b) (d)
The characteristic curve of a P-N junction diode:
( ) 3
Symbol of P-N junction diode is:
4.
( ) The varactor diode is used as:
Very low resistance None of the above
( )
32
(a) (c) 5
6
7
9
10
11.
12.
13
(b) (d)
Regulator Switching circuits
The color of light emitted by a LED depends upon: (a) its reverse bias (b) its forward bias (c) the amount of forward current (d) the type of semiconductor material used
( )
( )
Zener diode is used as: (a) Amplifier (b) Oscillator (c) Regulator (d) Rectifier ( ) The ratio of the resistance of a-p-n junction diode under forward biased and reverse biased condition is: (a) 102 :1 (b) 10–2 :1 (c)
8
Rectifier Oscillator
1 : 10–4
(d)
1 : 104
The voltage where the avalanche occurs is called the: (a) Barrier potential (b) Peek voltage (c) Knee voltage (d) Breakdown voltage In LED, blue light is emitted by: (a) GaAs (b) GaP (c) GaAsP (d) GaN Zener diode used in circuit is always biased: (a) forward (b) reverse (c) zero (d) none of the above Zener diode is: (a) Rectifier diode (c) constant voltage device
(b) (d)
constant current device forward biased diode
Zener diode is used is: (a) amplifier (b) oscillator (c) regulator (d) rectifier For highly doped diode: (a) Zener break down is likely to take place (c) Avalanche break down is likely to take place (c) Either will take place (c) Neither will take place
( )
( )
( )
( )
( )
( )
( )
Semiconductor Physics, Electronic Devices and Circuits
14
Zener diode is generally operated: (a) In forward bias (c) In zero bias
(b) (d)
33
In reverse bias Both in same bias
( )
S.NO. Ans
Q:01
02
03
04
05
06
07
08
09
A
B
C
B
A
C
D
D
D
S.NO.
10
11
12
13
14
Ans
B
A
B
B
B
34
Chapter-4
Rectification & Power Supply Q.1
Define Rectification and Rectifier. What are the types of Rectifier?
Ans.: Rectification : Rectification is the process by which alternating voltage is converted into direct voltage. Rectifier : The electrical device which rectifies an alternating voltage is called a Rectifier. In a rectifier the characteristic property of a diode of unidirectional conduction is used by which a low resistance is offered to the flow current in on direction and a high resistance is offered for flow in opposite direction. Rectifiers are mainly of two types : (i)
Half Wave Rectifier : In which only one diode is used and only half of the cycle of input alternating voltage is used.
(ii)
Full Wave Rectifier : In which the full wave of the input alternating voltage is used.
Q.2
Describe working of Half Wave Rectifier with the help of circuit diagram.
Ans.: Half Wave Rectifier :
Semiconductor Physics, Electronic Devices and Circuits
35
As shown in figure, in Half Wave Rectifier, the source of alternating voltage is connected to primary coil of a transformer and the secondary coil to a diode D and load resistance in series. Suppose voltage on transformer is Vi = Ep sin t the voltage induced in secondary is Vs = n Ep Sin t = Em sin t, where n → ratio of turns in secondary and primary coil. For positive half cycle of Vi, the end A of secondary coil is at positive voltage and B at negative voltage. In this state, diode is forward biased and current i flow in the circuit. During other half cycle, A is at negative voltage and end B is at positive voltage. In this state P-N diode is reverse biased and no current slows. Thus thorough the load RL connected in the output circuit current flows only for half cycle of the cycle of input voltage Vi. The output voltage is always positive (unidirectional) but varies with time i.e., the output voltage is pulsating. Q.3
Determine following quantities in Full Wave Rectifier : (i) Average Value of Pulsating Current (ii) Efficiency (iii) Ripple Factor
Ans.: Full Wave Rectifier : (i)
Average Value of Pulsating Current : Idc =
1 T
T
idt 0
36
OR
Idc
i1dt +
i2 dt T /2
0
T
T /2
1 = T
I m sin t dt + T /2
Cos t
-
Cos t
0
=
Im t
cos t
=
Im 2
Cos
Idc =
( I m )sin t dt T /2
0
I = m T
(ii)
T
T /2
1 = T
T /2 0
T
T /2
-
cos t
T T /2
2 T . Cos0 T 2
2Im
Cos
2 2 T .T Cos . T T 2
2 T
Efficiency : Rectification Efficiency of Full Wave Rectifier Output DC Power of Full Wave Rectifier Pdc Input AC Power of Full Wave Rectifier Pac
Output DC Power Input AC Power I dc 2 RL
4Im2
I rms 2 ( R RL )
2
Pdc Pac
RL
Im2 ( R RL ) 2
4 I m 2 RL 2
Efficiency
2
I m ( R RL ) 2
Percentage Efficiency
2
8RL ( R RL )
0.812 R 1 RL
81.2 % R 1 RL
Maximum Efficiency of Full Wave Rectifier will be 81.2% when R < < RL.
Semiconductor Physics, Electronic Devices and Circuits
(iii)
37
Ripple Factor : The alternating variation present in the output of Full Wave Rectifier is called Ripple. It is measured by a constant called Ripple Factor. Ripple Factor r
OR
r
The effective valueof ac component in output The dc component in output
I rms 2 1 I dc 2
I ac I dc
1/2
Substituting the value of Irms and Idc for a Full Wave Rectifier r
2 Im2 . 1 2 4I m2
1/2
2
r
8
1/2
1
r = 0.482 r =48.2% Q.4
Draw the circuit diagram of a Bridge Rectifier and explain its working.
Ans.: Bridge Rectifier :
As shown in figure Bridge Rectifier consists of four diodes D1, D2, D3 & D4 which are connected to form a network just like wheat stone bridge.
38
Working of Bridge Rectifier : When AC voltage Vi = Ep sin t is applied across the ends of the primary coil of transformer then Vs = nEp sin t is induced between the ends E and F of its secondary coil. During the half cycle of Input voltage when E is positive with respect to Point F, diodes D1 & D3 are forward biased and D2 & D4 are reverse biased. Consequently a current slows through the diodes D1, & D3. During Next half cycle, diode D2 & D4 conduct and current flows only through then. Thus it is clear that for both half cycles the currents flows through RC in one direction only. Q.5
What are Filters? Describe the working of Shunt Capacitor Filter.
Ans.: Filter : Filter is an electric circuit used for smoothing output current i.e. to separate the AC component from the pulsating output current of the rectifier. It is made by combining inductors and capacitors. Shunt Capacitor Filter :
In this type of filter, a capacitor C is connected is parallel with the load resistance RL. Filtration of AC component from diode output voltage depends upon the energy storage capacity of the capacitor C. When diode (Suppose D1) output voltage increases (Say from t1 to t2 as shown in figure) the capacitor stores energy by charging to the peak B of the corresponding Input cycle upto time t 2. Beyond t2 when input voltage decreases from its peak value, the diode D1 disconnects from the source and load. So it stops conducting and capacitor starts discharging energy thorough the load RL. In the next half cycle of Input, this process takes place through diode D 2. The process continues for other cycles and the rectifier with capacitor filter gives the wave form of the output voltage like ABCD curve shown in figure.
Semiconductor Physics, Electronic Devices and Circuits
Q.6
39
Explain the working of Series Inductor Filter.
Ans.: Series Inductor Filter : The ripple in the rectifier output can be reduced by connecting high independence inductor L in series with lead Resistance RL.
Working of Series Inductor Filter : When diode output current tends to rise above the average value, of the inductor L stores energy in due to rise in the current in the form of magnetic energy and offers impedance to AC component and no resistance to DC component. Thus AC component is blocked and only DC component reaches at the load. When the load current decreases below average, it develops additional only in the inductor due to outflow of magnetic energy and resists the decrement of the output load current. In this way the fluctuation in the output current decreases by the inductor L. Q.7
Sketch block diagram of SMPS.
Ans.: SMPS :
Block Diagram of Switching Mode Power Supply
40
Multiple Choice Questions Q:1 Which rectifier requires four diodes? (a) Half-wave rectifier (c) Full-wave rectifier 2
(b) (d)
Bridge rectifier All of the above
( )
Output wave of full-wave rectifier given:
( ) 3
4
5
The efficiency of a half-wave rectifier is: (a) 50% (c) 40%
(b) (d)
81% 100%
( )
The current obtained from a rectifier without filter is: (a) Varying d.c. (b) Constant d.c. (c) Pulsating d.c. (d) d.c. mixed with a.c.
( )
The filter circuit which has better voltage regulation is: (a) Choke input filter (b) -filter (c) Capacitor filter (d) LC filter
( )
Semiconductor Physics, Electronic Devices and Circuits
41
6.
The output of a half-wave rectifier is suitable only for: (a) Charging batteries (b) Running a.c. motors (c) Running car radios (d) Running TV The output voltage for a half wave rectifier is: (a) Pure AC (b) Pure DC (c) Pulsating (d) None of the above
( )
( )
7
A full wave rectifier is connected with an AC source of frequency 50 Hz. The output current contains a ripple of what frequency? (a) 25 Hz (b) 50 Hz (c) 100 Hz (d) 60 Hz ( )
8
Which of the following V-I graphs is correct for a p-n junction? (a) (b) I
I
v
v (c)
(d) I
I
v
v
S.NO.
Q:01
02
03
04
05
06
07
08
Ans
B
B
C
C
B
A
C
C
09
42
Chapter-5
Bipolar – Junction - Transistor Q.1
What is Transistors? Draw notations for PNP and NPN Transistors.
Ans.: Transistor : When a third doped element is added to semiconductor diode in such a way that
two P-N Junctions are formed, the resulting device is known
so Transistor. Transistor is small in size and very light in weight. It needs a very low voltage source and has longer life. There are two types of Transistor : (i)
PNP Transistor
Semiconductor Physics, Electronic Devices and Circuits
(ii)
Q.2
43
NPN Transistor
Explain different types of Transistor Biasing.
Ans.: The connection of correct polarity of voltage across it’s two junctions for proper working of transistor its known as Transistor Biasing. There are three ways of biasing transistor : (i)
Forward Active : In this biasing, emitter base junction is made forward biased and collector base junction is reverse biased. This biasing is called Forward Ative and generally used in Amplification.
(ii)
Saturation : IN this biasing, both emitter base junction and collector base junction are forward biased. In this mode the transistor has very large current to flow and used as a closed switch.
44
(iii)
Q.3
Cut Off : In this biasing both junction are reversed biased and transistor has practically no current to flow and used as open switch.
Discuss sign convention and signs of various voltage and current in a Junction Transistor.
Ans.: Sign Conventions for Voltages and Currents in Transistor : In Junction Transistor one of its terminal for input and output circuits. Sign Convention for Current : Normally, current flowing into a Junction Transistor is taken positive where as current flowing out of it is takes as negative. Sign Convention for Voltages : The voltage is taken as positive when terminals symbol of transistor used as first subscript in symbol indicating voltage is positive with respect to terminals symbol of transistor used as second subscript in voltages symbol.
Semiconductor Physics, Electronic Devices and Circuits
Q.4
45
In NPN
In PNP
VEB
-
+
VCB
+
-
VCE
+
-
What do you understand from Junction Transistor Configurations and Characteristic Curves.
Ans.: In Junction Transistor, all the electrical quantities at the output are in general controlled by the electrical quantities at the Input. The four electrical quantities related with input and output are : a.
Input Current
b.
Input Voltage
c.
Output Current
d.
Output Voltage
The dependence of these quantities on one another is represented by different curves called Characteristic Curves. There are three basic configuration of Transistor :
Q.5
(i)
Common Base Configuration
(ii)
Common Emitter Configuration
(iii)
Common Collector Configuration
Give the circuit diagram of Common Base NPN Transistor and give the method of plotting input and output characteristics.
Ans.: Common Base Configuration : In this configuration base terminal is common in input and output circuits. To represent the behaviour of NPN Transistor in this configuration two sets of Characteristic Curves are drawn :
46
(i)
(ii)
Input Characteristics : In this configuration IE is input current and VEB is input voltage. IC is output current VCB is output voltage. Keeping constant, the graph between IE VEB are called Input Characteristics. Output Characteristics : In common base configuration, Ic output current and VCB is output voltage. Collector Current IC is a function of IE and VCB i.e. Ic =
(VCB, IE)
and VCB and
is
Semiconductor Physics, Electronic Devices and Circuits
47
Keeping IE constant the set of waves drawn between IC and VCB are called Output Characteristics. The output characteristics of transistor in CB configuration may be divided into three region of operation :
Q.6
(i)
Active
(ii)
Saturation
(iii)
Cutoff
What do you understand from α and β of Transistor. Establish relation between them.
Ans.: Current Amplification Factor (α) : In common base configuration, this is defined as the ratio of collector current IC and emitter current IE keeping VCB constant.
IC IE Current Amplification Factor (β) : In common emitter configuration, the ratio of IC and IB for constant VCE is called DC Current Gain and given as IC IB
VCE
Cons tan t
Relation between α and β : For Junction Transistor, we know that IE = IB + IC
_ _ _ _ _ (i)
Dividing this equation by the IC, we get =
IB +1 IC
1
=
1
1
=
1
1
+1
_ _ _ _ _ (ii)
48
α =
_ _ _ _ _ (iii)
1
Equation (ii) can be written as 1
=
=
1
1 1
β=
Q.7
1
-1
_ _ _ _ _ (iv)
1
Describe Hybrid Parameters of a Transistor.
Ans.: Hybrid Parameters of Transistor : To stand a four terminal network two variables are taken as independent variables while two are taken as dependent variables. In transistor Input current I1 and output voltage V2 control the behaviour of internal circuit so there are taken as independent variables and remaining the two are dependant variables. V1 = f (I1, V2)
_ _ _ _ _ (i)
I2 = f ( I1 , V2)
_ _ _ _ _ (ii)
On taking total differentials dV1 =
V1 dI1 + I1
And
dI1 =
I2 I2 dI1 + dv2 I1 V2
OR
V1 = h11 I1 + h12 V2
_ _ _ _ _ (v)
I2 = h21 I1 + h22 v2
_ _ _ _ _ (vi)
V1 dV2 V2
_ _ _ _ _ (iii) _ _ _ _ _ (iv)
Semiconductor Physics, Electronic Devices and Circuits
49
Here coefficient h11, h12, h21 & h22 are of different dimension so they are referred as hybrid parameters. hi = h11 =
V1 I1
Short circuited input impedance when V2 =0 its unit is ohm.
hr = h12 =
V1 V2
Open circuited reverse voltage ratio when I1 =0 it dimension less parameter.
hf = h21 =
I2 I1
Short circuited forward current gain when V2 =0 it is dimension less parameter.
H0 = h22 =
I2 V2
Open circuited output admittance when I1 =0 its unit is mho.
50
Multiple Choice Questions Q:1 In transistor, the relation among emitter current I g , base current I B and Collector Current I B is:
2
3.
4.
5.
6.
7
8.
(a)
Ic
IE
IB
(b)
IE
IB
IC
(c)
IE
IB
IC
(d)
IE
IC
IB
In a transistor leakage current mainly depends on: (a) Temperature (b) (c) Size of emitter (d)
Doping of base Rating of transistor
( )
Transistor is known as bipolar transistor because it has: (a) Electrons (b) Holes (c) Electrons and holes (d) None of the above
( )
In NPN or PNP transistor: Ic I E (a) Ic I E (c)
( )
(b)
Ic
IE
(d)
None of the above
Which of the following transistor amplifiers has the highest power gain? (a) Common base (b) Common emitter (c) Emitter follower (d) Common collector
( )
The voltage gain of a common base amplifier is: (a) Zero (b) (c) Unity d) In N-P-N transistor both emitter and collector transistor will operate in: (a) Active region (b) (c) Cut-off region (d)
Saturation region Inverted region
( )
Faraday Schockley
( )
Who invented the first junction transistor? (a) Bell (c) Marconi
(b) (d)
Less than unity Greater than unity ( ) junctions are forward biased. The
Semiconductor Physics, Electronic Devices and Circuits
9
10.
11.
12
13
The current amplification factor in common collector configuration as: (a) (b) (c) (d)
( )
The base of an n-p-n transistor is thin and: (a) Heavily doped (b) Lightly doped (c) Marconi (d) Doped by pentavalent material
( )
I bell is equal to: (a) 1 db (c) 100 db
(b) (d)
10 db 1000 db
In a transistor the value of a is 100, the value of is: (a) 0.01 (b) 0.1 (c) 0.99 (d) 1 The common-emitter gain ( ) of a transistor is defined as: (a)
(b)
(c)
(d)
14
Figure represents a: (a) PNP transistor (b) NPN transistor (c) Unipolar transistor (d) None of the above
15.
In case of a bipolar transistor, is: (a) +ve and greater than one (b) +ve and less than one (c) -ve and less than one (d) None of the above
16.
51
The relation between (a)
=
and
None of the above
( )
( )
( )
( )
is given by: (b)
=
–
52
(c) 17.
18
19.
20.
21.
22
23
24
25.
(d)
None of the above
( )
In a transistor leakage current mainly depends on: (a) Temperature (b) (c) Size (d)
Doping None of the above
( )
The smallest part of a transistor is: (a) Emitter (c) Collector
(b) (d)
Base None of the above
( )
The unit of impedance is given by: (a) Ampere (c) Volt
(b) (d)
Ohm None of the above
( )
The current gain of CC amplifier is: (a) Zero (c) Unity
(b) (d)
Greater than unity Less than unity
( )
The decibel is a measure of: (a) Power ration (c) Current ratio
(b) (d)
Voltage ratio None of the above
( )
In a Cc amplifier voltage gain: (a) Remains constant (b) Equal to one (c) Is independent of input (d) Depends on output For a n-p-n transistor the collector voltage is: (a) –ve (b) (c) O (d)
+ve a.c. voltage
( )
Highly doped region in a transistor is: (a) Emitter (c) Base
(b) (d)
Collector All are equally doped
( )
Thinnest region in a transistor is: (a) Emitter (c) Both of the above
(b) (d)
Base All of the above
( )
( )
Semiconductor Physics, Electronic Devices and Circuits
26
27
S.NO.
Ans S.NO
Ans S.NO
Ans
53
Transistor is a: (a) Voltage operated device (c) Both of the above
(b) (d)
Current operated device None of the above
( )
Transistor can be used as: (a) Switch (c) Oscillator
(b) (d)
Amp lifer All of the above
( )
Q:01 B Q 10 B Q 19 B
02 B 11 C 20 B
03 C 12 C 21 A
04 B 13 A 22 B
05 A 14 B 23 B
06 D 15 A 24 B
07 B 16 B 25 A
08 A 17 B 26 B
09 C 18 B 27 B
54
Chapter-6
Amplifiers and Oscillators Q.1
Explain how a Transistor works as an Amplifier?
Ans.: A Transistor raises the strength of a weak signal and thus acts as an Amplifier.
The weak signal is applied between emitter base junction and output is taken across the load R L connected in the collector circuit. In order to achieve faithful amplification, emitter base junction should always remain forward biased. To do so, a DC voltage EE is applied in the input circuit in addition to the signal. As the Input circuit has low resistance c being forward biased), a small change in signal voltage causes an appreciable change in emitter current. This almost causes same change in collector current due to transistor action. The collector current flowing through load resistance RL produces large voltage across it. Thus, a weak signal applied in the input circuit appears in the amplified form in the collector circuit. It is in this way that a transistor acts as an amplifier.
Semiconductor Physics, Electronic Devices and Circuits
Q.2
55
Draw circuit diagrams for Transistor Amplifier Circuits in different configuration.
Ans.: (1)
Common Emitter Amplifier :
(2)
Common Base Amplifier :
(3)
Common Collector Amplifier :
56
Q.3
Describe R-C Coupled Transistor Amplifier.
Ans.: This amplifier is usually employed for voltage amplification, in this, a coupling capacitor Cc is used its connect the output of first stage to the base (i.e. input) of second stage and so on. As the coupling from Due stage to next stage is achieved by a coupling capacitor followed by a connection to a shunt resistor, therefore, such amplifiers are called resistance capacitance coupled amplifiers.
CC
OUTPUT
CE
As shown in fig. R1, R2, & RE form the biasing and stabilization network. The emitter by pass capacitor CE offers low reactance path to signal. The coupling capacitor Cc transmits ac signal but blocks DC. Operation : When A.C. signal is applied to the base of first transistor, it appears in amplified form across its collector load Rc. The amplified signal across Rc is given to base of next stage through coupling capacitor C c. The second stage does further amplification o the signal. In this way cascaded stages amplify the signal and overall gain is considerably increased. First stage gain G1 = β
RAC Rin
Where RAC = RC11Rin =
RcxRin Rin → input resistance of second stage RC Rin
Second stage gain G2 = β Total gain G = G1 x G2
RC Rin
Semiconductor Physics, Electronic Devices and Circuits
57
The Rc coupled amplifier has excellent audio fidelity one a wide range of frequency. Therefore it is widely used in voltage amplifiers. The main drawback of R-C coupled amplifier is that it has poor impedance matching. Q.3
What do you understand by Feedback? Describe its types.
Ans.: Feedback : Transfer of energy from output of the electric network to its input is called Feedback, when a fraction of total voltage or current of the output of the amplifier is fed back to its input, its working efficiency changes. There are two types of Feedback :
Q.4 Ans.:
(i)
Positive Feedback : When the Input signal and feedback signal have the same phase, the magnitude of the input signal increases by the addition of feedback signal. This type of feed is also called Regenerative Feedback.
(ii)
Negative Feedback : When the input signal and the feedback signal have the opposite phase, the magnitude of the input signal decreases, such type of feed back is called Negative or Degenerative Feedback.
Prove that for an Amplifier with Feedback. AF =
A 1 A
As shown in fig., if the resultant voltage at the input of the amplifiers Vif = Vi + B1 Vf { With Positive Feedback } If the input voltage Vi is amplified directly by a factor A to the value of the output voltage then A =
V0 Vi
………………………. (i)
If Vif is input voltage with feedback and V0’ is the output voltage then A =
V0 ' Vif
58
Vif =
V0 ' ………………………..(ii) A
If the fraction β of output voltage V0’ is fed in to the input of amplifier then Vf = βV0’ Vif = Vi + βV0’ ……………………..(iii) Using equation (ii) and (iii), we have V 01 = Vi + βV0’ A
V0’ =
AVi 1 A
The Amplifier gains with feedback is given by
Q.5
Af =
V0 ' Vi
Af =
A 1 A
Hence proved.
Explain that stability in gain and bandwidth increases due to Negative Feedback.
Ans.: Gain Stability: We know that Af =
A 1 A
{ for Negative Feedback }
For Negative Feedback in an Amplifier to be effecting the designer deliberately makes AB>>1 therefore above relation becomes Af =
1
This means that gain now depends upon the feedback fraction B i.e. characteristics of feed back circuit. As feed back circuit is usually a resistive network, the gain of the amplifier becomes independent of the ageing of transistor, variation in supply voltage, temperature and frequency.
Semiconductor Physics, Electronic Devices and Circuits
Frequency Response : As shown above Af =
59
1
. This means that voltage gain of
the amplifier is independent of signal frequency. The result is that voltage gains of the amplifier will be substantially constant over a wide range of signal frequency. The Negative Feedback, therefore improves frequency response i.e., increases bandwidth. Q.6
Describe Oscillator. Draw the circuit diagram of a general Oscillator.
Ans.: Oscillator : It is an active electronic circuit which produces electrical oscillations of any desired frequency or we can say it converts energy from DC source into a periodically varying voltage or current. Oscillators are self excited active electronic circuits as no external signal is applied at the input of the oscillators.
An oscillator employs positive feedback for its operation. Whereas negative feedback tends to reduce the magnitude of the signal applied to the amplifier, positive feedback has the opposite effect. Consider the block diagram of Fig.(1), here A is the voltage gain of the amplifier and m is the feedback fraction. The output voltage is E2 = AE1 where E1 is the input voltage. Clearly, feedback voltage = m E2 = AmE1. If the link L were removed and the feedback circuit output connected to the input circuit with +ve to +ve and –ve to –ve, the feedback would be positive. This means that feedback voltage (= A mE1) is in phase with the input voltage. Since E1 is the required input voltage to produce the output voltage E2, it means that for continuous
60
output (i.e., oscillations), the feedback voltage must be equal to the input voltage i.e. Feedback Voltage = E1 Or
A m E1 = E1
Or
Am=1
Under these conditions (i.e., Am=1), even if there were no input generator (i.e., E1), a continuous output will be obtained across RL immediately after connecting the necessary power supplies (i.e, VCC). The general layout of an oscillator is shown in Fig. 31.2. It is clear from Fig.(2) that continuous output (E2) will be obtained with no extremely applied input signal. It provides its own input signal via the feedback circuit. Thus, output voltage E2 = AE1 and feedback voltage = A mE1 = E1 (because Am = 1). An amplifier with infinite gain is one that can produce an output signal without any extremely applied input signal. It provides its own input signal via the feedback circuit. Such a circuit is known as an oscillator.
Multiple Choice Questions 1
2
3
4
Which of the following transistor amplifiers has the highest power gain? (a) Common base (b) Common emitter (c) Emitter follower (d) Common collector The voltage gain of a common base amplifier is: (a) Zero (b) (c) Unity d) An oscillator converts: (a) A.C. power into D.C. power (b) D.C. power into A.C. power (c) Mechanical power into A.C. power (d) A.C. power into rediation power In a Cc amplifier voltage gain: (a) Remains constant (b) Equal to one
Less than unity Greater than unity
( )
( )
( )
Semiconductor Physics, Electronic Devices and Circuits
(c) (d) 5.
6
7
8.
9
.10
61
Is independent of input Depends on output
( )
Ina practical oscillator, A is: (a) slightly less than one (b) I (c) –1 (d) slightly greater than one In a Cc amplifier voltage gain: (a) Remains constant (b) Equal to one (c) Is independent of input (d) Depends on output The gain of an amplifier with feedback is given by: (a) (b) (c)
(d)
The value of feedback fraction is: (a) (c)
(b) (d)
( )
( ) –
( )
–
( )
The negative feedback increase: (a) gain (b) bandwidth (c) noise (d) All of the above The gain of an amplifier with feedback is given by: (a)
Af =
(c)
Af =
–
(b)
Af =
(c)
Af =
( )
– ( )
–
S.NO.
Q:01
02
03
04
05
06
07
08
09
Ans
A
D
A
C
A
B
B
A
C
S.NO.
10
Ans
B
62
Chapter-7
FETs, Thyristors and Opto-electronic Devices Q.1
Describe Field Effect Transistor and its types.
Ans.: Field Effect Transistor (FET) : It is a semiconductor device in which flow of current through semiconductor is controlled by an electric field (voltage) i.e. it is a field (voltage) controlled device. FET is a unipolar device i.e. current flow by only one type of majority carrier electrons or holes. Types of FET : FET
JFET
MOSFET
(Junction Field Effect Transistor)
N-channel JFET
(Metal Oxide Semiconductor Field Effect Transistor)
P-Channel JPET
E-MOSFET (Enhancement MOSFET)
Q.2
Describe the construction of JFET.
D-MOSFET (Deplction MOSPET)
Semiconductor Physics, Electronic Devices and Circuits
63
Ans.: JFET : It consists of N or P type semiconductor bar whose ends are heavily doped with same type impurity. Types of JFET : JFET is of two types :
N-Channel JFET : If the bar is made from type semiconductor Then the transistor is be N-Channel JFET.
N said to
P-Channel JFET : When bar is made from JFET.
P-type
In JFET one end of bar is said to be Source (S). Majority carriers of bar enter the bar from this terminal. Another end of bar is said to be drain majority carriers leave the bar through terminal. Gate (G) is contracted on lateral surface of bar by doping heavily opposite type impurity. Q.3
State advantages of a JFET over a Bipolar Junction Transistor.
Ans.: Advantage of JFET over Bipolar Junction Transistor : JFET is a voltage-controlled device.
this with
64
JFET operation depends only one of majority charge carriers (electrons or holes) therefore it is called unipolar transistor.
type
Its thermal stability is quite good. JFET are much easier to fabricate are particularly suitable for ICs.
and
FETs are costlier than BJTs. Q.4
Describe how the characteristics of a JFET are determined experimentally.
Ans.: Characteristics Curve of JFET : In JFET drain current depend
the
drain
source voltage (VDS)
and
the
upon
(ID)
gate-source
voltage Vgs. The dependence of
one
quantity over the other can be represented by plotting the curves between them. These curves are called Characteristic Curves of JFET. These Characteristic Curves are of two types:(i)
Drain characteristics : Keeping gate-source voltage VGS constant, graph plotted between drain current ID and drain source voltage VDS is called Drain Characteristics. These characteristics can be divided into three regions :
Semiconductor Physics, Electronic Devices and Circuits
(a)
Ohmic Region
(b)
Saturation Region
(c)
Break-down Region
65
(ii) Transfer Characteristics : Keeping output voltage VDS constant in the pinch off region, if a graph is plotted between the saturated drain current IDS and the gate voltage VGS, then the family of curves so obtained, is called Transfer Characteristics. Q.5
What is the principle of MOSFET? Explain its
types. Ans.: Metal Oxide Semiconductor Field Effect Transistor (MOSFET) : MOSFET works on the principle that the thickness and hence the resistance of the conducting channel of a Semiconductor material can be controlled by a transverse electric field across the insulator deposited on the semi conducting material. Types of MOSFET : MOSFET is of two types : (i)
E-MOSFET (Enhancement MOSFET) : In this the controlling transverse electric field across the insulator deposited on the semi conducting
66
material causes an increase (enhance) of the majority carrier density in the conducting region of the transistor. (ii)
Depletion MOSFET : In D-MOSFET the controlling electric field reduces the majority carrier density in the conducting region. E-MOSFET and D-MOSFET both are also of two types :
Q.6
(a)
N-Channel
(b)
P-Channel
What are the advantages of MOSFET over an FET.
Ans.: Advantages of MOSFET over an FET:
Q.7
(1)
The input resistance of the MOSFET is very high of the order of (10) 10 to (10)15 ohm where as the input resistance of the JFET is less i.e. of the order of 108 ohm.
(2)
The drain resistance of the MOSFET is much lower than that of a JFET.
(3)
Comparing to JFET, MOSFET are easier to fabricate.
(4)
MOSFET is very susceptible to overload voltage and needs special handling.
(5)
JFET are operated only in the depletion mode where as depletion MOSFET may be operated in both depletion and enhancement mode.
Describe the principle and construction of LCD.
Ans.: Liquid Crystal Display (LCD) :
Semiconductor Physics, Electronic Devices and Circuits
67
In a liquid crystal cell, liquid crystal material is sandwiched between two transparent glass plates with transparent SnO2 film electrodes deposited in inside faces. When both glass sheets are transparent, the cell is known as Transmittive Type Cell. When only one glass sheet is transparent and other has a reflecting coating, then cell is known as Reflective Type Cell. When cell is not activated, both transmittive and reflective type cell do not appear bright. When cell is activated, the incident light is scattered in all directions by both type of cells and the cell appears quite bright. This type of LCD is known as Dynamic Scattering Type LCD. Another type of LCD is Field Effect LCD. Q.8
Draw and explain a Seven Segment and Dot Matrix Display.
Ans.: Seven Segment Display : This can display all numerals and nine alphabets. It consists of 7 rectangular LED/LCD which can be turned on/off to form desired digit.
(1)
(2)
As shown in Fig.(2), external resistance is required to limit the current through LED/LCD.
68
Dot Matrix Display : The another method of displaying numerals and letters is a dot matrix formation of LED’s / LCD’s in a monolithic structure. Commonly used Dot Matrix for display are 5 x 7, 5 x 8, 7 x 9. Which can display 64 different alphanumeric characters by deriving the appropriate horizontal and vertical inputs.
Multiple Choice Questions Q:1
A FET consists of a: (a) Drain (b) Source (c) Gate (d) All of the above
( )
Semiconductor Physics, Electronic Devices and Circuits
2.
3
4 5
6
7.
8.
9.
10.
69
The best electronic device for switching is: (a) Triode (b) (c) FET (d)
BJT MOSFET
( )
An SCR is a semiconductor device consisting of: (a) One PN junction (b) (c) Three PN junctions (d)
Two PN junction Four PN junction
( )
a set of TRIACs Two DIACs
( )
A thyristor is basically: (a) PNPN device (c) Two TRIACs The gate- source diode of a JEFT should: (a) be forward biased (b) be reverse biased (c) be either forward or reverse biased (d) not be biased at all A JFET: (a) Is a voltage controlled device (b) is a current controlled device (c) Has low input resistance (d) Has a very large voltage gain
(b) (d)
( )
( )
The gate-source diode of a JFET should: (a) Be forward biased (b) be reverse biased (c) Be either forward or reverse biased (d) Not be biased at all
( )
When a JFET is operated above pinch-off voltage its drain current: (a) Increase sharply (b) Becomes constant (c) Starts increasing (d) Becomes zero
( )
Positive feedback is used in: (a) Rectifier (c) Amplifier
(b) (d)
Detector Oscillator
( )
A Shockley diode is the same as: (a) A four layer diode (c) A diac
(b) (d)
An SCR A triac
( )
70
11.
12.
13
14.
15.
Which is an optoelectronic device? (a) Phototransistor (c) LCD
(b) (d)
LED All of the above
Which one of the following has the negative resistance region? (a) Tunnel diode (b) Schottky diode (c) Step recovery diode (d) Optocoupler A thyristor is basically: (a) A PNPN device (b) A combination of DIAC and TRIAC (c) A set of TRIACs (d) None of the above
( )
( )
( )
Which of the following needs light to be activated? (a) LED (b) LCD (c) LASCR (d) All of the above
( )
An FET consists of a: (a) Drain (c) Gate
( )
(b) (d)
Source All of the above
16.
After drain source voltage reaches pinch off value in a JFET, drain current becomes: (a) Zero (b) Low (c) Saturated (d) Reversed ( )
17.
Compared to a bipolar transistor JFET has a much higher: (a) Voltage gain (b) Input resistance (c) Supply voltage (d) Drain current Thyristors are used in: (a) Power supply (b) Amplifiers (c) Oscillators (d) Display devices
18
( )
( )
Semiconductor Physics, Electronic Devices and Circuits
71
S.NO.
Q:01
02
03
04
05
06
07
08
09
Ans
D
D
B
A
A
A
B
A
D
S.NO.
10
11
12
13
14
15
16
17
18
Ans
A
D
A
A
C
D
A
B
A
72
BACHELOR OF COMPUTER APPLICATIONS (Part-I) EXAMINATION (Faculty of Science) (Three – Year Scheme of 10+2+3 Pattern)
PAPER - 117 SEMICONDUCTOR PHYSICS,ELECTRONIC DEVICES & CIRCUITS OBJECTIVE PART- I Year - 2011 Time allowed : One Hour Maximum Marks : 20 The question paper contains 40 multiple choice questions with four choices and student will have to pick the correct one (each carrying ½ mark). 1.
2.
3.
According to Dalton’s atomic theory, an atom can: (a) be destroyes (b) be created (c) either be destroyed ar created (d) neither be destroyed nor created
( )
A matter has neither definite volume nor fixed shape is: (a) solid (b) liquid (c) gas (d) None of the above
( )
The maximum number of electrons possible in the shell of principal quantum number n1 is (a) (c)
4.
n2 2n
The strongest bond is:
(b) (d)
2n 2 n(n+1)
( )
Semiconductor Physics, Electronic Devices and Circuits
(a) (c) 5.
6.
7.
8.
9.
10.
Ionic bond Molecular bond
(b) (d)
73
Covalent bond Metallic bond
( )
For producing a p-type semiconductor which of these would used: (a) Acceptor atoms (b) Donar atoms (c) Pentavalent impurity (d) Arsenic
( )
The merging of a free electron with a hole is called: (a) decomposition (b) recombination (c) neutralization (d) covalent bonding
( )
Free electron exits in: (a) First band (c) Valence band
(b) (d)
Forbidden band Conduction band
( )
Depletion layer has: (a) Both free electrons and holes (c) Only free electrons
(b) (d)
None of the free charge carriers Only free holes ( )
A P-N junction in reverse bias acts as a: (a) Very high resistance (c) Zero resistance
(b) (d)
Very low resistance None of the above
The characteristic curve of a P-N junction diode:
( )
74
( ) 11.
Symbol of P-N junction diode is:
( ) 12.
The varactor diode is used as: (a) Rectifier (c) Oscillator
(b) (d)
Regulator Switching circuits
( )
Semiconductor Physics, Electronic Devices and Circuits
13.
14.
15.
16.
75
The color of light emitted by a LED depends upon: (a) its reverse bias (b) its forward bias (c) the amount of forward current (d) the type of semiconductor material used
( )
Zener diode is used as: (a) Amplifier (c) Regulator
(b) (d)
Oscillator Rectifier
( )
Which rectifier requires four diodes? (a) Half-wave rectifier (c) Full-wave rectifier
(b) (d)
Bridge rectifier All of the above
( )
Output wave of full-wave rectifier given:
( ) 17.
18.
The efficiency of a half-wave rectifier is: (a) 50% (c) 40%
(b) (d)
81% 100%
The current obtained from a rectifier without filter is: (a) Varying d.c. (b) Constant d.c.
( )
76
(c) 19.
20.
21.
Pulsating d.c.
(d)
d.c. mixed with a.c.
( )
The filter circuit which has better voltage regulation is: (a) Choke input filter (b) -filter (c) Capacitor filter (d) LC filter
( )
The output of a half-wave rectifier is suitable only for: (a) Charging batteries (b) Running a.c. motors (c) Running car radios (d) Running TV
( )
In transistor, the relation among emitter current I g , base current I B and Collector Current I B is:
22.
23.
24.
(a)
Ic
IE
IB
(b)
IE
IB
IC
(c)
IE
IB
IC
(d)
IE
IC
IB
In a transistor leakage current mainly depends on: (a) Temperature (b) (c) Size of emitter (d)
Transistor is known as bipolar transistor because it has: (a) Electrons (b) Holes (c) Electrons and holes (d) None of the above In NPN or PNP transistor: Ic I E (a) (c)
25.
26.
27.
Doping of base Rating of transistor
Ic
IE
(b)
Ic
(d)
None of the above
( )
( )
IE ( )
Which of the following transistor amplifiers has the highest power gain? (a) Common base (b) Common emitter (c) Emitter follower (d) Common collector
( )
The voltage gain of a common base amplifier is: (a) Zero (b) (c) Unity d)
( )
A PET consists of a: (a) Drain
Less than unity Greater than unity
Semiconductor Physics, Electronic Devices and Circuits
(b) (c) (d) 28.
29.
30.
31.
32.
33.
35.
Source Gate All of the above
( )
The best electronic device for switching is: (a) Triode (b) (c) FET (d)
BJT MOSFET
( )
An SCR is a semiconductor device consisting of: (a) One PN junction (b) (c) Three PN junctions (d)
Two PN junction Four PN junction
( )
A thyristor is basically: (a) PNPN device (c) Two TRIACs
(b) (d)
a set of TRIACs Two DIACs
( )
Semiconductor materials have bonds: (a) Ionic (c) Covalent
(b) (d)
Metallic Mutual
( )
Current flow in a semiconductor depends on the phenomenon: (a) Drift (b) Diffusion (c) Recombinationt (d) All of the above Electronic configuration of sodium is: 1s 2 2 s 2 2 p 6 3s1 (a) (c)
34.
77
1s 2 2s 2 2 p 2 3s 5
An example of p-type semiconductor: (a) Pure germanium (b) Pure silicon (c) Silicon doped with phosphorous (d) Germanium with Indium The energy gap is the largest in the case of: (a) Metals (b) Semiconductor (c) Insulators
(b)
1s 2 2s 2 2 p 6 3s 2
(d)
1s 2 2 s 2 2 p 5 3s1
( )
( )
( )
78
(d)
Superconductors
( )
36.
For a P-N junction diode, the approximate ratio of resistances in reverse and forward bias is: (a) 1:1 (b) 103 :1 (c) (d) ( ) 10 3 :1 106 :1
37.
The device which converts d.c. into a.c. is called: (a) Rectifier (b) Oscillator (c) Amplifier (d) Modulator
( )
In filter circuit which component is connected in serier to load? (a) R (b) L (c) Diode (d) C
( )
Low power display device is: (a) LED (c) Monitor
( )
38.
39.
40.
(b) (d)
LCD Seven-segment display
Electrical energy is converted into light by: (a) Electronic device (b) Optoelectronic device (c) Optical device (d) All of the above
( )
Answer Key 1. ( ) 2. ( )
3. ( )
4. ( )
5. ( )
6. ( )
7. ( )
8. ( )
9. ( )
10. ( )
11. ( )
12. ( )
13. ( )
14. ( )
15. ( )
16. ( )
17. ( )
18. ( )
19. ( )
20. ( )
21. ( )
22. ( )
23. ( )
24. ( )
25. ( )
26. ( )
27. ( )
28. ( )
29. ( )
30. ( )
31. ( )
32. ( )
33. ( )
34. ( )
35. ( )
36. ( )
37. ( )
38. ( )
39. ( )
40. ( )
Semiconductor Physics, Electronic Devices and Circuits
79
_____________
DESCRIPTIVE PART-II Year- 2011 Time allowed : 2 Hours Maximum Marks : 30 Attempt any four questions out of the six. All questions carry 7½ marks each. Q.1
Q.2
Q.3
(a)
Name and explain different types of bonds in solids with examples.
(b)
Explain the difference among conductors, insulators and semiconductors.
(a)
Explain intrinsic and extrinsic semiconductors with examples.
(b)
What do you mean by mobility of charge carrier?
(c)
Explain formation of depletion layer in PN junction semiconductor/diode.
(a)
Explain the working of P-N junction diode. Discuss its forward and reverse biased characteristics.
(b)
Describe the construction and working of light emitting diode (LED) and mention its two applications.
80
Q.4
(a)
What is meant by bipolar junction transistor? Draw a neat circuit diagram to study the characteristics of a PNP transistor in common base configuration. Sketch the input and output characteristics curves and indicate the active cut-off and saturation regions.
Q.5
(b)
Prove the relation:
(a)
Describe the working of a half-wave rectifier. Find out its:
(b)
1
(i)
Average value of pulsating current and voltage.
(ii)
RMS value of pulsating current and voltage
(iii)
Efficiency
(iv)
Ripple factor.
Write down short notes on: (i)
F.E.T.;
(ii)
S.M.P.S;
(iii)
Opto-electronic devices.
______
Semiconductor Physics, Electronic Devices and Circuits
81
SEMICONDUCTOR PHYSICS,ELECTRONIC DEVICES & CIRCUITS PAPER - 117 OBJECTIVE PART- I Year - 2010 Time allowed : One Hour Maximum Marks : 20 The question paper contains 40 multiple choice questions with four choices and student will have to pick the correct one (each carrying ½ mark). 1.
2.
3.
The reason for inter-molecular forces is: (a) Only dipole-dipole interactions (b) Only induced dipole interactions (c) Dispersive forces (d) All of the above The strongest bond is: (a) lonic (c) vander Waal
( )
(b) (d)
In the metals, the most common interaction is: (a) ion-ion (b)
Covalent Metallic
ion-electron
( )
82
(c) 4.
5.
6.
7.
8.
9.
10.
11.
12.
election-electron
Election-volt is the unit of: (a) Momentum (c) Energy
(d)
repulsive
( )
(b) (d)
Velocity Potential
( )
The electron orbiting the nucleus possesses: (a) Potential energy only (b) Kinetic energy only (c) Partially potential and partially kinetic energy (d) None of the above
( )
The cause of conductivity in metal is: (a) Proton (c) Bound electron
(b) (d)
Free electron Ions
( )
The valence bond at OK is: (a) Completely filled (c) Partially
(b) (d)
completely empty Nothing can be said
( )
Semi-conductors such as Germanium and silicon exhibit. (a) Covalent bonding (b) Metallic bonding (c) Ionic bonding (d) van der walls bonding
( )
Holes are present in: (a) Valence band (b) Conduction band (c) Conduction and valence band (d) None of the above
( )
Current in a semi-conductor diode is due to the drift of: (a) Free electrons (b) Free electrons and holes (c) Positive and negative ions (d) Protons
( )
The resistivity of a semiconductor depends upon its: (a) Size (b) Type of atoms (c) Length (d) Size and type of atom
( )
In n-type semiconductor the fermi energy level is displaced: (a) towards the valence band
Semiconductor Physics, Electronic Devices and Circuits
(b) (c) (d) 13.
14.
15.
16.
17.
19.
20.
towards the conduction band not displace d and depend on quantity of impurity
( )
The electronic configuration of Si is: (a)
1s2, 2s2, 2p6, 3s2, 3p4
(b)
1s2, 2s2, 2p6, 3s1, 3p3
(c)
1s2, 2s2, 2p6, 3S2, 3p2
(d)
1s2, 2s2, 2p6, 3s0, 3p4
The depletion region of a-p-n junction contains: (a) Electrons only (b) A rectifier (c) Electrons and holes (d) Neither electrons nor holes A p-n junction can be used as: (a) An amplifier (c) An oscillator
(b) (d)
( )
( )
A rectifier A modulator
Depletion layer increases: (a) When p-n junction is formed (b) On applying forward bias (c) On applying the reverse bias (d) When temperature is decreases
( )
( )
The ratio of the resistance of a-p-n junction diode under forward biased and reverse biased condition is: (a) 102 :1 (b) 10–2 :1 (c)
18.
83
1 : 10–4
(d)
1 : 104
( )
The voltage where the avalanche occurs is called the: (a) Barrier potential (b) Peek voltage (c) Knee voltage (d) Breakdown voltage
( )
The output voltage for a half wave rectifier is: (a) Pure AC (b) (c) Pulsating (d)
( )
Pure DC None of the above
A full wave rectifier is connected with an AC source of frequency 50 Hz. The output current contains a ripple of what frequency?
84
(a) (c) 21.
25 Hz 100 Hz
(b) (d)
50 Hz 60 Hz
( )
Which of the following V-I graphs is correct for a p-n junction? (a) (b) I
I
v
v (c)
(d) I
I
v
22.
23.
24.
The thinnest part of a transistor is: (a) emitter (c) collector
(b) (d)
base none of the above
( )
In p-n-p or n-p-n transistor: (a)
Ie = Ic + Ib
(b)
Ic = Ie + Ib
(c)
Ie = Ib – Ic
(d)
Ie = Ic– Ib
Relation between (a) (c)
25.
v
–
and
( )
for a transistor is: (b) (d)
–
The martial used generally for the fabrication of a transistor is: (a) Capper (b) Silicon (c) Ebonite (d) Silver
( )
( )
Semiconductor Physics, Electronic Devices and Circuits
26.
27.
28.
29.
An oscillator is nothing but an amplifier with: (a) positive feedback (b) (c) no feedback (d)
85
high gain negative feedback
( )
The diamond is the hardest material because: (a) It has covalent bonds (b) It has large cohesive force (c) It has high melting point (d) It is not soluble
( )
The formation of depletion layer in a-p-n junction is due to: (a) Drift of holes (b) Diffusion of carries (c) Transport of impurity ions (d) Drift of electrons
( )
Transistor can be used as: (a) Amplifier (c) Oscillator
( )
(b) (d)
Rectifier All of the above
30.
The minimum energy required to ionine the hydrogen atom from its ground state is: (a) + 13.6 eV (b) –10.2 eV (c) + 10.2 eV (d) – 13.6 eV ( )
31.
Structure of crystals is determined by: (a) Infra-red Rays (c) Ultraviolet rays
32.
33.
34.
(b) (d)
Visible light rays X-rays
The color of light emitted by an LED depends upon: (a) Its reverse bias (b) its forward bias (c) The amount of forward current (d) The type of semiconductor material used The ripple factor of a bridge rectifier is: (a) 0.482 (c) 1.11
(b) (d)
( )
( )
0.812 1.21
The primary function of a filter is to: (a) Minimize a.c. input fluctuation (b) Suppress odd harmonics in the rectifier output (c) Remove ripple from rectifier output
( )
86
(d)
Stabilize d.c. level of output voltage
( )
35.
In N-P-N transistor both emitter and collector junctions are forward biased. The transistor will operate in: (a) Active region (b) Saturation region (c) Cut-off region (d) Inverted region ( )
36.
Diffusion current in a P-N junction is caused by: (a) Chemical energy (b) (c) Junction formation (d)
37.
38.
39.
40.
Heat energy None of the above
( )
The gate- source diode of a JEFT should: (a) be forward biased (b) be reverse biased (c) be either forward or reverse biased (d) not be biased at all
( )
The frequency range of the signal produced by audio frequency oscillator is: (a) (103 to 2 X 104) KHz (b) (103 to 2 X 104) MHz (c) (103 to 2 X 104) Hz (d) None of the above
( )
In case of a bipolar transistor is: (a) positive and greater than one (b) positive and less then one (c) negative and very small (d) negative and less than one
( )
An oscillator converts: (a) A.C. power into D.C. power (b) D.C. power into A.C. power (c) Mechanical power into A.C. power (d) A.C. power into rediation power
( )
Answer Key 1. (d) 2. (a)
3. (b)
4. (c)
5. (c)
6. (b)
7. (a)
8. (a)
9. (a)
10. (b)
Semiconductor Physics, Electronic Devices and Circuits
87
11. (d)
12. (b)
13. (c)
14. (d)
15. (b)
16. (c)
17. (d)
18. (d)
19. (c)
20. (d)
21. (c)
22. (b)
23. (a)
24. (d)
25. (b)
26. (b)
27. (a)
28. (b)
29. (d)
30. (a)
31. (d)
32. (d)
33. (a)
34. (c)
35. (b)
36. (c)
37. (b)
38. (c)
39. (b)
40. (b)
_________
DESCRIPTIVE PART-II Year- 2010 Time allowed : 2 Hours Maximum Marks : 30 Attempt any four questions out of the six. All questions carry 7½ marks each. Q.1
(a)
Explain the principle of energy bands in solids and classify the solids on the basis of electrical conductivity.
Q.2
(b)
What is the difference between covalent and metallic bond?
(a)
Derive the expression of conductivity of an intrinsic semiconductor in terms of mobility.
(b)
Explain the working of P.N. junction diode. Discuss its forward and reverse biased characteristics.
Q.3
(a)
Describe the construction and working of light emitting diode (LED) and mention its four applications.
Q.4
(b)
Define ripple factor and efficiency of a rectifier.
(a)
Draw the equivalent circuits of a transistor using h-parameters is CB and CE configurations.
(b)
Discuss the role of
section filter.
88
Q.5
(a)
Describe the action of a transistor as an amplifier. Derive the relation for current gain in CE configuration transistor amplifier using h-parameters.
Q.6
(b)
Write elementary information about Field Effect Transistor (FET).
(a)
Derive the feedback requirements for oscillation in feedback oscillator.
(b)
Write down the postulates of Bohr's atomic model.
(c)
Sketch the block diagram of switching mode power supply (SMPS) _________
SEMICONDUCTOR PHYSICS,ELECTRONIC DEVICES & CIRCUITS PAPER - 117 OBJECTIVE PART- I Year - 2009 Time allowed : One Hour Maximum Marks : 20 The question paper contains 40 multiple choice questions with four choices and student will have to pick the correct one (each carrying ½ mark). 1.
The maximum intertomic distances exists in : (a) Solids (b) (c) Gases (d)
Liquids Both in liquids and solids
( )
2.
Each energy band of a small piece of solid containing 100 atoms will have closely spaced energy levels equal to: (a) 50 (b) 100 (c) 200 (d) 500 ( )
3.
Which of the following elements is a covalently bonded crystal? (a) Ar (b) NaCL (c) Ge (d) Na
4.
( )
In an intrinsic semiconductor at any finite temperature, the number of free electrons:
Semiconductor Physics, Electronic Devices and Circuits
(a) (b) (c) (d)
89
Is equal the number of holes Is greater than the number of holes Is lower than the number of holes Is zero
( )
5.
The number of free electrons and holes in an intrinsic semiconductor increase when the temperature: (a) Decreases (b) Increases (c) Stays the same (d) None of the above ( )
6.
The electronic configuration of Si is:
7.
(a)
1s2, 2s2, 2p6, 3s2, 3p4
(b)
1s2, 2s2, 2p6, 3s2, 3p2
(c)
1s2, 2s2, 2p6, 3S1, 3p3
(d)
1s2, 2s2, 2p6, 3s0, 3p4
To make an n-type material, following is added to Ge: (a) Al (b) B (c) P (d) In
( )
( )
8.
The energy needed to detach the fifth valence electron from the Antimony (Sb) impurity atom surrounded by Ge atoms is approximately: (a) 0.001 eV (b) 0.01 eV (c) 0.1 eV (d) 1.0 eV ( )
9.
In an N-type semiconductor, the position of 89ermi level: (a) Is lower than the centre of energy gap (b) Is at the centre of the energy gap (c) Is higher than the centre of energy gap (d) Can be anywhere in energy gap depending upon the concentration of impurity atoms
( )
10.
When the graph of the Current Versus Voltage is a straight line, the device is referred as: (a) Active (b) Linear (c) Nonlinear (d) Passive ( )
11.
The voltage where the Avalanche Occurs is called the:
90
(a) (b) (c) (d)
Barrier potential Peak voltage Knee voltage Breakdown voltage
knee
12.
The reverse saturation current in diode (Ge) at room temperature is of the order of: (a) InA (b) 1 A (c) ImA (d) 1A ( )
13.
If the input frequency is 60 Hz, the output frequency of hal-wave rectifier is: (a) 30 Hz (b) 60 Hz (c) 120 Hz (d) 240 Hz
( )
Photodiode is normally: (a) Forward biased (b) Reverse biased (c) Neither forward nor reverse biased (d) None of above
( )
Introduction of negative feedback in an amplifier increases: (a) Gain (b) Noise level (c) Band width (d) Harmonic distortions
( )
Varactor diodes are: (a) Reverse biased (c) Band width
(b) (d)
Voltage sensitive capacitor None of the above
( )
Who invented the first junction transistor? (a) Bell (c) Marconi
(b) (d)
Faraday Schockley
( )
14.
15.
16.
17.
18.
19.
The current amplification factor in common collector configuration as: (a) (b) (c) (d) The base of an n-p-n transistor is thin and: (a) Heavily doped (b) Lightly doped
( )
Semiconductor Physics, Electronic Devices and Circuits
(c) (d) 20.
21.
91
Marconi Doped by pentavalent material
I bell is equal to: (a) 1 db (c) 100 db In a transistor the value of a (a) 0.01 (c) 0.99
( )
(b) (d)
10 db 1000 db
is 100, the value of is: (b) 0.1 (d) 1
( )
( )
22.
The d.c. output voltage drops from 6 V with no load to 5.8 v at full load. The percentage regulation is: (a) 1.72 % (b) 3.33 % (c) 3.45 % (d) 6.67 % ( )
23.
If Em is the maximum voltage across secondary coil of transformer, then PIV of a half wave rectifier circuit is: (a) Em (b) 2 Em (c)
24.
3 Em
Which rectifier requires four diodes? (a) Half wave rectifier (b) Full wave rectifier (c) Bridge rectifier (d) Half wave rectifier with filter
(d)
0.5 Em
( )
( )
25.
A power supply with a.d.c. output of 140 V has 60 Hz ripple of 1.4 V what is the percentage ripple: (a) 0.1 % (b) 1.0 % (c) 0.14 % (d) 2.4 % ( )
26.
A JFET: (a) Is a voltage controlled device (b) is a current controlled device (c) Has low input resistance (d) Has a very large voltage gain
27.
The gate-source diode of a JFET should:
( )
92
(a) (b) (c) (d) 28.
29.
30.
31.
32.
33.
34.
35.
36.
Be forward biased be reverse biased Be either forward or reverse biased Not be biased at all
( )
When a JFET is operated above pinch-off voltage its drain current: (a) Increase sharply (b) Becomes constant (c) Starts increasing (d) Becomes zero
( )
Positive feedback is used in: (a) Rectifier (c) Amplifier
(b) (d)
Detector Oscillator
( )
A Shockley diode is the same as: (a) A four layer diode (c) A diac
(b) (d)
An SCR A triac
( )
Which is an optoelectronic device? (a) Phototransistor (c) LCD
(b) (d)
LED All of the above
( )
Which one of the following has the negative resistance region? (a) Tunnel diode (b) Schottky diode (c) Step recovery diode (d) Optocoupler
( )
Mobile holes are found in : (a) Valance band (c) Band gap
( )
(b) (d)
Fermi energy level Conduction band
Silver is the best conductor, how many valence electrons do you think it has: (a) I (b) 4 (c) 18 (d) 29
( )
Which one of the following cannot move is solids: (a) Holes (b) Free electrons (c) Majority charge carries (d) Ions
( )
Most of the electrons that flow through the base will: (a) Flow into the collector (b) Flow out of the base lead
Semiconductor Physics, Electronic Devices and Circuits
(c) (d) 37.
38.
39.
40
93
Recombine with the base holes Recombine with the collector holes
( )
Fermi level of an intrinsic semiconductor lies: (a) Near conduction bank but in bank gap (b) Near valence bank but in band gap (c) Half way between the condition and valence band (d) In the valence band
( )
The density of charge in an intrinsic semiconductor is proportional to: (a)
Exp (–Eg/ kt)
(b)
Exp (–2Eg/kt)
(c)
Exp (–Eg/kt2)
(d)
Exp (–Eg/2kt)
( )
an oscillator converts : (a) Ac power into DC power (b) DC power nto AC power (c) Mechanical power into AC powe (d) None of the above
( )
When a H-atom raised from the ground state to an excited state? (a) PE decreased and KE increases (b) PE increases and KE decreases (c) Both PE and KE increase (d) Both PE and KE decreases
( )
Answer Key 1. © 2. (b)
3. ©
4. (a)
5. (b)
6. (b)
7. ©
8. (b)
9. (d)
10. (b)
11. (b)
12. (b)
13. (b)
14. (b)
15. (a)
16. ©
17. (d)
18. ©
19. (b)
20. (b)
21. ©
22. ©
23. (a)
24. ©
25. (b)
26. (a)
27. (b)
28. (b)
29. (d)
30. (a)
31. (d)
32. (a)
33. (a)
34. (a)
35. (d)
36. (a)
37. ©
38. (d)
39. (b)
40. (b)
94
___________
Semiconductor Physics, Electronic Devices and Circuits
95
DESCRIPTIVE PART-II Year- 2009 Time allowed : 2 Hours Maximum Marks : 30 Attempt any four questions out of the six. All questions carry 7½ marks each. Q.1
(a)
What is Bohr's quantum condition for angular momentum of electron in an atom? Write the name of symbols used in the condition.
Q.2
(b)
Give five examples of covalent bonding.
(a)
What are the characteristics of a semiconductor? What do you mean by depletion layer in a-p-n junction?
(b)
Classify each of the following as n-type or p-type semiconductors: (i)
Doped by acceptor atoms
(ii)
Crystal with pentavalent impurities
(iii)
Majority carriers are holes
(iv)
Minority carriers are free electrons
(v)
Donor atoms were added to crystal.
Q.3
Discuss the V-I characteristics of a tunnel diode and write its applications.
Q.4
(a)
Define mobility and electrical conductivity and give their SI units.
(b)
Draw an energy level diagram showing the donor and acceptor energy levels with respect to valence and conduction bands.
Q.5
Q.6
(a)
Derive an expression for ripple factor for a full ware rectifier.
(b)
Discuss the role of L section filter.
(a)
Write elementary information about thyristors and display devices
(b)
Explain bipolar junction transistor. __________
96
SEMICONDUCTOR PHYSICS,ELECTRONIC DEVICES & CIRCUITS PAPER - 117 OBJECTIVE PART- I Year - 2008 Time allowed : One Hour Maximum Marks : 20 The question paper contains 40 multiple choice questions with four choices and student will have to pick the correct one (each carrying ½ mark). 1.
2.
3.
The charge of neutron is: (a) 1.6 X 10–19 Coulomb (c) Zero
(b) (d)
1.6 X 10+19 Coulomb None of the above
( )
Total energy of electron in atom is: (a) +ve (c) Zero
(b) (d)
–ve None of the above
( )
In cubic crystal all the three crystal axes are: (a) Perpendicular (b) (c) Diagonal (d)
Parallel None of the above
( )
4.
The electrons that usually do not contribute to the electrical behaviours of the materials are known as: (a) Bound electrons (b) Free electrons (c) Valence electrons (d) None of the above ( )
5.
The resistivity of a semiconductor at Ok is: (a) Zero (b) 5 (c) 10 (d)
Infinite None of the above
( )
The unit of current density is: (a) C/m2 (c) Ampere/m2
C/m3 Ampere/m3
( )
6.
7.
(b) (d)
The merging of a free electron with a hole is called: (a) Neutralization (b) Decomposition
Semiconductor Physics, Electronic Devices and Circuits
(c) 8.
9.
10.
11.
12.
13.
Recombination
(d)
97
None of the above
( )
The example of intrinsic semiconductor is: (a) Ge (b) (c) Sb (d)
Ca Ag
( )
In LED, blue light is emitted by: (a) GaAs (c) GaAsP
GaP GaN
( )
(b) (d)
Diffusion current in a PN junction is caused by: (a) Heat energy (b) Chemical energy (c) Junction formation (d) None of the above
( )
The ripple before using any filter in a full wave rectifier is: (a) 1% (b) 41 % (c) 48.2 % (d) None of the above
( )
The filter circuit which has better voltage regulation is: (a) L section (b) Choke input (c) filter (d) None of the above
( )
When Vn is the no load voltage and VL is voltage on load then % voltage regulation is given by: (a) (b) (c) (d)
14.
–
–
–
None of the above
( )
In a full wave rectifier operating at 50 Hz main frequency, the fundamental frequency of the ripple will be: (a) 25 Hz (b) 50 Hz (c) (d) None of the above ( )
98
15.
The common-emitter gain ( ) of a transistor is defined as: (a)
(b)
(c)
(d)
16.
Figure represents a: (a) PNP transistor (b) NPN transistor (c) Unipolar transistor (d) None of the above
17.
In case of a bipolar transistor, is: (a) +ve and greater than one (b) +ve and less than one (c) -ve and less than one (d) None of the above
18.
The relation between (a)
and
20.
21.
22.
( )
( )
is given by:
=
(c) 19.
None of the above
(b)
–
None of the above
( )
In a transistor leakage current mainly depends on: (a) Temperature (b) (c) Size (d)
Doping None of the above
( )
The smallest part of a transistor is: (a) Emitter (c) Collector
(b) (d)
Base None of the above
( )
The unit of impedance is given by: (a) Ampere (c) Volt
(b) (d)
Ohm None of the above
( )
The current gain of CC amplifier is:
(d)
=
Semiconductor Physics, Electronic Devices and Circuits
(a) (c) 23.
24.
25.
26.
27.
28.
Zero Unity
The decibel is a measure of: (a) Power ration (c) Current ratio
99
(b) (d)
Greater than unity Less than unity
( )
(b) (d)
Voltage ratio None of the above
( )
In a Cc amplifier voltage gain: (a) Remains constant (b) Equal to one (c) Is independent of input (d) Depends on output
( )
Ina practical oscillator, A is: (a) slightly less than one (b) I (c) –1 (d) slightly greater than one
( )
A thyristor is basically: (a) A PNPN device (b) A combination of DIAC and TRIAC (c) A set of TRIACs (d) None of the above
( )
Which of the following needs light to be activated? (a) LED (b) LCD (c) LASCR (d) All of the above
( )
An FET consists of a: (a) Drain (c) Gate
( )
(b) (d)
Source All of the above
29.
After drain source voltage reaches pinch off value in a JFET, drain current becomes: (a) Zero (b) Low (c) Saturated (d) Reversed ( )
30.
Compared to a bipolar transistor JFET has a much higher: (a) Voltage gain (b) Input resistance
100
(c) 31.
32.
33.
34.
35.
36.
37.
38.
39
Supply voltage
(d)
Drain current
( )
Which of the following semiconductor is electrically positive? (a) P-type (b) N-type (c) Intrinsic SC (d) None of the above
( )
A P-type semiconductor contains holes and: (a) +ve ions (b) (c) Pentavalent atoms (d)
( )
–ve ions Donor atoms
The voltage across the zener assistance is equally: (a) Small (b) Large (c) Zero (d) None of the above
( )
Zener diode is used as: (a) Amplifier (c) Regulator
( )
(b) (d)
Oscillator Rectifier
Which of the following is always used in forward biased arrangement? (a) Zener diode (b) LED (c) Photodiode (d) Varactor diode
( )
Tunnel diode is used as: (a) Oscillator (c) Rectifier
( )
(b) (d)
Amplifier Regulator
Which rectifier required four diodes? (a) Half rectifier (b) Full rectifier (c) Bridge rectifier (d) None of the above
( )
A transistor is in active region then: (a)
IB= Ic
(b)
IC= IB
(c)
IC= IE
(d)
IC= IB
The output voltage of CE amplifier is: (a) Amplified
( )
Semiconductor Physics, Electronic Devices and Circuits
(b) (c) (d) 40
101
Inverted 180o out of phase with the input All of the above
( )
An amplifier power is charged from 10 to 20 . The equivalent d B gain will be: (a) 2 dB (b) 3 dB (c) 6 dB (d) 10 dB ( )
Answer Key 1. (c) 2. (b)
3. (a)
4. (a)
5. (b)
6. (c)
7. (c)
8. (a)
9. (d)
10. (c)
11. (c)
12. (c)
13. (c)
14. (c)
15. (a)
16. (b)
17. (b)
18. (b)
19. (a)
20. (b)
21. (b)
22. (b)
23. (a)
24. (b)
25. (d)
26. (a)
27. (c)
28. (d)
29. (c)
30. (b)
31. (d)
32. (b)
33. (a)
34. (c)
35. (d)
36. (a)
37. (c)
38. (b)
39. (d)
40. (b)
__________
102
DESCRIPTIVE PART-II Year- 2008 Time allowed : 2 Hours Maximum Marks : 30 Attempt any four questions out of the six. All questions carry 7½ marks each. Q.1
(a)
What are conduction band, valence band and forbidden gap? Explain these bands by proper diagrams.
(b)
Q.2
Write down the postulates of Bohr's atomic model
What is intrinsic semiconductor? Give examples of intrinsic semiconductor. Describe the crystal structure of intrinsic semiconductor. What are the charge carriers in them?
Q.3
Explain V-I characteristics of a PN junction diode and also explain the effect of temperature on a diode.
Q.4
Sketch bridge rectifier showing direction of current flowing through the circuit and explain its working.
Q.5
Discuss the current components in a junction transistor and hence prove that IE=IB+IC
Q.6
Derive the feedback requirement for oscillations in feedback oscillators
_________
Semiconductor Physics, Electronic Devices and Circuits
103
SEMICONDUCTOR PHYSICS,ELECTRONIC DEVICES & CIRCUITS PAPER - 117 OBJECTIVE PART- I Year - 2007 Time allowed : One Hour Maximum Marks : 20 The question paper contains 40 multiple choice questions with four choices and student will have to pick the correct one (each carrying ½ mark). 1
2.
3.
4.
5.
6.
7.
The maximum number of electrons in nth shell of an atom is: (a) n2 (b) 2n (c) n (n + 1) (d) 2n2
( )
The range of intermolecular force is: (a) infinite (c) 10 A
(b) (d)
1 –3 A 0
( )
The bonds in silicon crystals are: (a) Covalent (c) metallic
(b) (d)
ionic None of the above
( )
Example of a semiconductor is: (a) Al (c) Fe
(b) (d)
Cu Si
( )
The unit cell with crystallographic dimension, a = b c and (a) cubic (b) hexagonal (c) monoclinic (d) tetragonal
0
is: ( )
The number of fundamental crystal structures are: (a) 7 (b) (c) 21 (d)
14 28
( )
On heating resistance of conductor: (a) increase (c) does not change
decreases becomes zero
( )
(b) (d)
104
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
The forbidden gap of germanium crystal is: (a) 0.7 eV (b) (c) 0.07 eV (d)
7 eV Zero
( )
Holes are minority carriers in .....................semiconductor. (a) N-type (b) P-type (c) intrinsic (d) extrinsic
( )
Free electrons exist in: (a) First band (c) valence band
( )
(b) (d)
forbidden band conduction band
The large depletion layer in P-N junction diode is due to: (a) forward bias (b) reverse bias (c) zero bias (d) none of the above
( )
Zener diode used in circuit is always biased: (a) forward (b) (c) zero (d)
reverse none of the above
( )
Zener diode is: (a) Rectifier diode (c) constant voltage device
(b) (d)
constant current device forward biased diode
( )
Zener diode is used is: (a) amplifier (c) regulator
(b) (d)
oscillator rectifier
( )
2 4
( )
Number of diodes used in a bridge rectifier is: (a) 1 (b) (c) 3 (d)
Which of the following has negative resistance regions? (a) varactor (b) rectifier (c) tunnel diode (d) photodiode
( )
A photodiode is: (a) forward biased (c) zero biased
( )
(b) (d)
reverse biased none of the above
Semiconductor Physics, Electronic Devices and Circuits
18.
19.
20.
21.
22
23.
24.
25.
26.
105
Efficiency of half wave rectifier is: (a) 81.6 % (c) 20.2 %
(b) (d)
40.6 % 90 %
( )
Ripple factor of a bridge rectifier is: (a) 0.48 (c) 1.11
(b) (d)
0.81 1.21
( )
Average d.c. of a bridge rectifier is (where V0 is peak value): (a) (b) (c)
(d)
A transistor has number of pins: (a) 1 (c) 3
(b) (d)
2 4
( )
Emitter junction of a transistor is biased: (a) forward (c) zero
(b) (d)
reverse all of the above
( )
The common base current gain
( )
of a transistor is:
(a)
(b)
(c)
(d)
The relation between transistor parameter
and
(a)
(b)
(c)
(d)
The thinnest part of a transistor is: (a) emitter (c) collector
(b) (d)
In a transistor correct relation in Ib Ic and Ie is: (a) IC = IE + IB (b)
( ) is:
( )
base None of the above
IE= IC+IB
( )
106
(c) 27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
IE = IB–IC
In general gain of a transistor is: (a) real (c) complex
(d)
IE = IC–IB
( )
(b) (d)
imaginary non-complex
( )
The gain of an amplifier with feedback is given by: (a) (b)
–
(c)
(d)
The value of feedback fraction is: (a) (c)
(b) (d)
The negative feedback increase: (a) gain (c) noise
(b) (d)
bandwidth All of the above
( )
FET is used as: (a) Amplifier (c) regulator
(b) (d)
rectifier detector
( )
–
( )
( )
An SCR is a semiconductor device consisting of number of diodes: (a) 1 (b) 2 (c) 3 (d) 4
( )
Which of the following needs light to activate? (a) LED (b) (c) LASCR (d)
( )
LCD None of the above
The minimum number of segments required in LED/LCD to be: (a) 3 (b) 7 (c) 9 (d) 11
( )
Which material is used as light sensing? (a) Se (c) CdS
( )
(b) (d)
PbS All of the above
A seven segment display used maximum current when it display:
Semiconductor Physics, Electronic Devices and Circuits
(a) (c) 37.
38.
39.
40.
0 8
107
(b) (d)
5 4
( )
(b) (d)
1 4
( )
Computer has SMPS for it’s: (a) memory (c) display
(b) (d)
power I/O device
( )
The input resitance of BJT is: (a) 102 – 104 (c) 106 – 108
(b) (d)
104 – 106 1–10
( )
Band gap of GaAs is: (a) 1.1 eV (c) 1.3 eV
(b) (d)
0.72 eV 2.4 eV
( )
Number of capacitors in (a) 0 (c) 2
section filter is:
Answer Key 1. (d) 2. ©
3. (a)
4. (d)
5. (d)
6. (a)
7. (a)
8. (a)
9. (a)
10. (d)
11. (b)
12. (b)
13. ©
14. ©
15. (d)
16. ©
17. (b)
18. (b)
19. (a)
20. (b)
21. ©
22. (a)
23. ©
24. (b)
25. (b)
26. (b)
27. ©
28. (b)
29. (a)
30. (b)
31. (a)
32. ©
33. ©
34. (b)
35. (d)
36. ©
37. ©
38. (b)
39. (a)
40. ©
______________
108
DESCRIPTIVE PART-II Year- 2007 Time allowed : 2 Hours Maximum Marks : 30 Attempt any four questions out of the six. All questions carry 7½ marks each.
Q.1
(a)
Describe differences between three states of matter.
(b)
Name fundamental types of crystals
Q.2
Explain classification of solids on the basis of electrical conductivity.
Q.3
(a)
Explain difference between P and N type semiconductors.
(b)
Describe working of half-wave rectifier.
Q.4
Describe working of PNP transistor and its characteristics in CB configuration.
Q.5
Explain transistor amplifier in CE configuration with feedback.
Q.6
Describe in brief: (a)
SCR
(b)
FET
(c)
SMPS
__________
Semiconductor Physics, Electronic Devices and Circuits
109
SEMICONDUCTOR PHYSICS,ELECTRONIC DEVICES & CIRCUITS PAPER - 117 OBJECTIVE PART- I Year - 2006 Time allowed : One Hour Maximum Marks : 20 The question paper contains 40 multiple choice questions with four choices and student will have to pick the correct one (each carrying ½ mark). 1.
2.
3.
4.
5.
6.
7.
Bonds in semiconductor are: (a) Covalent (c) Metallic
(b) (d)
Ionic neutral
( )
Band gap in Ge and Si lies in range: (a) 0.5 –1.5 eV (c) 2.5 – 3.5 eV
(b) (d)
1.5 –2.5 eV 3.5 – 4.5 eV
( )
In intrinsic semiconductors: (a) nh < ne (c) nh = ne
(b) (d)
nh > ne nh = ne = 0
( )
The unit of resistance is: (a) Mho (c) Farad
(b) (d)
Ohm Ampere
( )
Cut in voltage of Si diode is of the order of: (a) 0.2 V (b) (c) 0.2 mV (d)
0.6 mV 0.6 V
( )
One electron volt (eV) is equal to: (a) 1.6 X 10–19V (c) 1 V
1.6 X 10–19 V 1 V
( )
(b) (d)
For highly doped diode: (a) Zener break down is likely to take place (c) Avalanche break down is likely to take place
110
(c) (c) 8.
9.
10.
11.
12.
13
14.
15.
16.
Either will take place Neither will take place
Zener diode is generally operated: (a) In forward bias (c) In zero bias
( )
(b) (d)
In reverse bias Both in same bias
( )
For a n-p-n transistor the collector voltage is: (a) –ve (b) (c) O (d)
+ve a.c. voltage
( )
Highly doped region in a transistor is: (a) Emitter (c) Base
(b) (d)
Collector All are equally doped
( )
Thinnest region in a transistor is: (a) Emitter (c) Both of the above
(b) (d)
Base All of the above
( )
Transistor is a: (a) Voltage operated device (c) Both of the above
(b) (d)
Current operated device None of the above
( )
Transistor can be used as: (a) Switch (c) Oscillator
(b) (d)
Amp lifer All of the above
( )
The gain of an amplifier with feedback is given by: (a)
Af =
(c)
Af =
–
(b)
Af =
(c)
Af =
– ( )
–
Crystal structure of NaCl is: (a) FCC (c) Both of above
(b) (d)
BCC None of above
Photodiode operates in: (a) Forward bias
(b)
Reverse bias
( )
Semiconductor Physics, Electronic Devices and Circuits
(c) 17.
18.
19.
20.
21.
22.
23.
24.
25.
Zero bias
111
(d)
All of the above
( )
Light emitting diode is made of: (a) Ge (c) GaAs
(b) (d)
Si All of the above
( )
Functional break down occurs in: (a) LED (c) Varactor diode
(b) (d)
Photo diode Zener diode
( )
The relation in IB, IC and IE of a transistor is: (a) IB=Ic+IE (b) (c) IE=IB+IC (d)
IC=IB+IE All of the above
( )
Input resistance of an ideal amplifier is: (a) 0
(b)
1
(c)
(d)
None of the above
( )
Charge on a hole in magnitude is equal is: (a) One electron (c) Oscillator
(b) (d)
Amplifier None of the above
( )
Tunnel diode is used in: (a) Rectifier (c) Oscillator
(b) (d)
Amplifier All of the above
( )
The value of in transistor lies in the range: (a) 0.5 – 0.99 (b) (c) 10-100 (d)
1–10 100–1000
( )
The value of in transistor lies in the range: (a) 0.01 – 0.99 (b) (c) 10-100 (d)
1–10 100–1000
( )
No. of pins in an transistor: (a) 1 (c) 3
2 4
( )
(b) (d)
112
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
No. of pins in a FET: (a) 1 (c) 3
(b) (d)
2 4
( )
Advantage of negative feedback amplifier is: (a) Highly stabilized gain (b) (c) Reduce noise (d)
Improved frequency response All of the above ( )
No. of h parameters in transistor: (a) 2 (c) 4
(b) (d)
3 5
( )
No. of p-n junction in BJT: (a) 1 (c) 3
(b) (d)
2 4
( )
No. of p-n junctions in UJT are: (a) 1 (c) 3
(b) (d)
2 4
( )
Gain of a transistor with negative feedback: (a) Decreases (b) (c) No-charge (d)
Increases None of the above
( )
Positive feedback is used in: (a) Amplifier (c) Oscillator
(b) (d)
Rectifier None of the above
( )
Efficiency of a half wave rectifier is: (a) zero (c) 40.6 %
(b) (d)
20.3 % 81.2%
( )
Ripple factor bridge rectifier is: (a) 1.21 (c) 0.24
(b) (d)
0.47 None of the above
( )
Low power display can device is: (a) LED (c) Monitor
(b) (d)
LCD Seven segment display
( )
Semiconductor Physics, Electronic Devices and Circuits
36.
37.
38.
39.
40.
Colour of a display can be changed by: (a) Voltage (c) Frequency
(b) (d)
Electrical energy is converted into light using: (a) Electronic device (b) (c) Opto electronic device (d)
113
Current None of the above
( )
Optical device All of the above
( )
Frequency of ripple in full wave rectifier working at 50 Hz AC is: (a) 25 Hz (b) 50 Hz (c) 100 Hz (d) 20 kHz
( )
Frequency of ripple in SMPS working at 50 Hz AC is: (a) 25 Hz (b) 50 Hz (c) 100 Hz (d) 20 kHz
( )
Thyristors are used in: (a) Power supply (c) Oscillators
( )
(b) (d)
Amplifiers Display devices
Answer Key 1. (a) 2. (a)
3. (c)
4. (b)
5. (b)
6. (b)
7. (a)
8. (b)
9. (b)
10. (a)
11. (b)
12. (b)
13. (d)
14. (b)
15. (a)
16. (b)
17. (c)
18. (d)
19. (c)
20. (d)
21. (a)
22. (c)
23. (c)
24. (a)
25. (c)
26. (c)
27. (d)
28. (c)
29. (b)
30. (a)
31. (a)
32. (c)
33. (c)
34. (b)
35. (b)
36. (d)
37. (c)
38. (c)
39. (a)
40. (a)
_____________
114
DESCRIPTIVE PART-II Year- 2006 Time allowed : 2 Hours Maximum Marks : 30 Attempt any four questions out of the six. All questions carry 7½ marks each. Q.1
Q.2
Q.3
Q.4
Q.5
Q.6
(a)
Name and explain different types of bonds in solid with examples.
(b)
Explain the differences between conductors and semiconductors.
(a)
Explain different types of semiconductors with examples.
(b)
Define the donor and acceptor impurities.
(a)
Name different types of diodes and their uses.
(b)
Explain - V-I characteristics of p-n junction and its temperature
(a)
Define parameters of a transistor and explain their relationship
(b)
Compare the n-p-n and p-n-p transistors.
(a)
Explain construction and working of a power supply with electronic component.
(b)
Explain voltage regulation using zener diode.
Define and explain: (a)
Electronic oscillator
(b)
SMPS
(c)
Display devices.
dependence.
Semiconductor Physics, Electronic Devices and Circuits
115
Key words Amplification: a method for increasing the amplitude (or loudness) of electrical signals Amplifier: An electronic device which generates a high power signal based on the information supplied by a lower powered signal. A perfect amplifier would add or subtract nothing from the original except additional power - these have not been invented yet Amplitude: the loudness of sound waves and electrical signals. Amplitude is measured in decibels (dB) or volts Crystal - solid featuring periodic spatial arrangement of atoms throughout the entire piece of material Semiconductors, III-V - III-V semiconductors are fabricated using elements from 3rd and 5th group of periodic table; e.g. GaAs, Gap, GaN, GaAlAs. Current Density - Current per unit area. Energy gap - a range of forbidden energy levels between two permitted bands. Fermi Level - the Fermi level is the energy at which the probability of occupation by an electron is exactly one half. Impurity - a substance that is incorporated into a semiconductor material and provides free electrons (n-type impurity) or holes (p-type impurity). Intrinsic Semiconductor - a semiconductor that has no impurities to change the conduction properties of the material. Lattice - a regularly space array of points that represents the structure of a crystal. Crystals are composed of groups of atoms repeated at regular interval in three dimensions with the same orientation. LASER - Light Amplification by the Stimulated Emission of Radiation
116
Mobility - in an electrical conductor, the ratio of the drift speed and the applied electric field. n- type Semiconductor - material doped with donors to increase the population of free electrons. p-n junction - semiconductor material doped in with donors and acceptors to form a sharply defined region with over which the doping type changes from p to n. A single p-n junction acts as a diode p- type Semiconductor - semiconductor material doped with acceptors to create 'holes' Semiconductor - a material that is characterised by ability to conduct a small a electrical current. In terms of the band model, the lower energy levels are filled at low temperatures Valence Band - the highest occupied energy level in a solid crystal semiconductor or insulator that is occupied by electrons at T= 0K. The number identifying an element in the periodic table, the atomic number of an element equals the number of protons (which also equals the number of electrons) in the nucleus of a neutral atom. Energy Band: The continuous range of allowed energies for electrons in a solid. Individual atoms can have only certain quantized energies. As atoms bond to form solids, each energy level "spreads" to accommodate the shifted levels of adjacent atoms. Atoms in the solid can occupy any energy in one of the newly-formed "bands". Band Gap: The range of energies between two allowed energy bands in a solid. Electrons in the solid can only take energies in the allowed bands, so electrons cannot have one of the energies in the gap Charge Carrier A particle having electric charge that can move freely through the material. In conductors, electrons are free to move, so they are the charge carriers. Holes are the charge carriers in p-type semiconductors. Positive or negative ions can be charge
Semiconductor Physics, Electronic Devices and Circuits
117
carriers in some liquids. Conduction Band The unfilled top Energy band in a solid. Since this band is not filled, electrons with energies in this band can move easily through the solid, creating an electric current. Conductor A material with low resistivity used for contacts and interconnects in semiconductor processing. Conductors have a partially filled valence band, through which electrons can move freely Conductivity ():A measure of how freely current can flow through a material. Coulomb (C):The SI unit of electric charge. One coulomb is a fairly large amount of charge, equaling the charge of 6.25 x 1018 protons. Covalent Bond An interaction between atoms by which they "share" valence electrons in the outermost energy shell, Diffuse To spread out, particularly when substance was originally concentrated Diffusion Current The current occurring at a p-n junction due to diffusion of charge carriers. The n-side of the junction contains conduction electrons, while the p-side contains holes. These electrons and holes will move across the junction as they diffuse, causing a net (positive) electric current from the p-side to the n-side. Diode A semiconductor device which allows current to flow in only one direction. A is composed of a single p-n junction, and the words can often be used interchangeably. When a forward bias is applied (positive terminal of a battery connected to the p-side of the junction and negative terminal to the n-side).
118
Donor An element that "donates" an electron to a semiconductor atom. Donors will have one more valence electron than the semiconductor they accept from. Drift Velocity The average velocity of charge carriers in a material. This is not the same as the average speed. Electron A fundamental particle with negative charge found in atoms. It has a mass of 9.11 x 1031 kilograms and a charge of - 1.6 x 10-19Coulombs. ElectronVolt (eV) A unit of energy, equal to the energy gained by an electron that passes through a potential difference of 1 Volt. An electronVolt is related to the SI energy unit Joule by the charge of the electron e as follows: 1 eV = 1.6 x 10-19 J. \ Forward Bias A voltage applied to a diode (a p-n junction) in a direction that does produce electric current. When a p-n junction is forward biased, the positive terminal of a battery is connected to the p-side of the diode, and the negative terminal to the n-side. Intrinsic (Pure) Semiconductors A semiconductor comprised of a single type of molecule, with no impurities. Pure semiconductors are typically poor conductors of electricity. Light-Emitting Diode (LED) A diodethat has been constructed a) to optimize the emission of light as conduction electrons and holes recombine at a p-n junction. Neutron A fundamental particle with no electric charge found in the nuclei of atoms. It has a mass of 1.675 x 10-27 kilograms. Proton A fundamental particle with positive charge found in the nuclei of atoms. It has a
Semiconductor Physics, Electronic Devices and Circuits
119
mass of 1.673 x 10-27 kilograms and a charge of 1.6 x 10-19 Coulombs. Resistance (R) The ratio between the voltage applied to a device and the electric current I that flows through it: R = V/I. Reverse Bias A voltage applied to a diode (a p-n junction) in a direction that does NOT produce significant electric current Transistor An electronic switch. Transistors allow a (relatively) large amount of current to flow when a (relatively) small voltage is applied, Voltage (V), or Potential Difference The change in energy per unit of electric charge, measured in the SI unit of Volt (V). For example, a 1.0 Volt battery increases the energy of each Coulomb of charge by one Joule
120
Bibliography S. no. Book Name
Author
1
Dr A k Nagawat ,Dr S S C B H
Semiconductor physics
Publication
rawat 2
Fundamentals
of Rolf Enderlein & Norman J Springer, 2004
Semiconductor Physics and Horing Devices 3
Semiconductor Physics: An Introduction
LINKS:
1) Wikipedia, internet, Guru k p o etc .
Karlheinz Seeger
Springer, 2004
