Electronic Devices and Circuit

Semiconductor Material & Devices AN OVERVIEW OF THE SUBJECT

Usman Ali Khan

Contents 1. 2. 3. 4. 5.

Subject Information The study of Electronics History Semiconductor Materials Atomic Structure

Subject Information Code: EE120 Text Book: Electronic Devices & Circuits by Theodore F. Bogart 6th ed. Electronic Devices & Circuits by David A Bell 4th ed. Electronic Devices & Circuits by Floyd Electronic Devices & Circuits by Manzar Saeed Basics of Electronic Device by NIIT

Marks distribution

Total Marks: 150 Theory: 100 Practical: 50 Session Marks: 20 • Assignments: 05 • Quiz: 05 • Project + Presentations: 05 • Attendance: 05

Introduction Semiconductor Devices

Building blocks of useful electronic devices Semiconductor devices include: Diodes PN junction Light Emitting Diode (LED) Zener Diode Tunnel Diode Varactor Diode Laser Diode Photo Diode

Transistors Bipolar Junction Transistor (BJT) • NPN BJT • PNP BJT Junction Field Effect Transistor (JFET) Amplifier Fundamentals Small Signal Transistor Amplifier Integrated Circuits (ICs) Analog ICs Digital ICs

Basic Atomic Theory Every chemical element is composed of atoms All atoms within a single element have same structure Every element is unique because the structure of its atoms is unique Nucleus Atom is composed of three basic particles: Protons (+ive charge) Neutrons Electrons (-ive charge)

Silicon Atom

Orbits or Shells K, L, M,N

Draw the atomic structure of Ge (32)

P=14 N=14 Valence Shell

+ Ne( Electrons in nth orbit) = 2n2

Sub-shells

Shell

Sub-shell

Capacity

K

s

2

s

2

p

6

s

2

p

6

d

10

s

2

p

6

d

10

f

14

L

M

N

Free Electrons

When electrons get enough energy (e.g. from heating), they leave their parent atoms and become free electrons. Flow of free electrons is called current. Therefore more free electrons and more current.

+ Valence electrons have more tendency to become free electrons because of less attraction force between nucleus and valence shell Free electrons in (i) conductors (ii) Insulators & (iii) Semiconductors

Flow of Free Electrons (Current)

Material containing free electrons Force of attraction

Force of repulsion -

-

-

+

-

Excess of electrons

Lack of electrons

Silicon Crystal (Covalent Bonding)

Si Crystal

* *

+

* * *

*

* *

*

*

*

*

* *

* * * *

+

* * *

* *

+

+

*

*

+

*

* * * * *

* * * *

+

* *

+

+

*

*

+

*

* *

*

+

+

* *

*

+

*

* *

+

*

* *

+

*

* *

+

* *

+

* *

*

*

+

*

For stability there should be 8 electrons in valence shell

Current in Semiconductors HOLE CURRENT

Usman Ali Khan

Contents 1. 2. 3. 4. 5.

Basics Electron Energy Energy Bands Temperature & Resistance Holes & Hole Current

Basics:

Rupturing of covalent bond The unit of energy is electronvolt(eV) Energy acquired by one electron if it is accelerated through potential difference of one volt 1 eV = 1.602 x 10-19 J Valence Electron energy considerably large and need a few amount of energy to release Electrons in inner shell possess little energy and need a large amount of energy to release Electrons can lose energy in the form of heat and light Free electrons can alco lose and fall into valence shell

Important Quantities

Quantity

Symbol

Unit

Unit Symbol

Current

I

Ampere

A

Voltage

V

Volt

V

Charge

Q

Coulomb

C

Energy

W,E

Joule

J

Electric Field Strength

E

Volt/meter

V/m

Volume

V

Cubic meter

m3

Area

A

Squared meter

m2

Resistance

R

Ohm

Ω

Conductance

G

Moh, Siemens

S

Resistivity

ρ

Ohm-meter

Ω-m

Conductivity

σ

Siemens/meter

S/m

Important Relations V = IR (Ohm's Law) I = Q/t W = QV R = ρl/A G = 1/R σ = 1/ ρ Charge on electron = e = 1.602 X 10-19 C Electron energy = 1 eV = 1.602 X 10-19 J

Rupturing of Covalent Bonds

Hole created

Electron Freed (Conduction Band)

+

Covalent bond ruptured

+ Valence band

+ +

+ Energy is supplied in the form of heat to rupture covalent band

Electron Energy

Electrons closer to nucleus are more tightly bound and need more energy to become free

E2 Therefore: P=14 N=14

E3

E1

E1 > E2 > E3

If free electron loses energy and falls back to valence band, this process is called “Annihilation” or “Recombination”

Lost energy emits as light

Energy Bands: Quantum theory explain these bands as

Conduction Band : Free electrons accommodate there Valence Band :

Electrons having lesser energy accommodate there Forbidden band: The region between valence and conduction band No electrons can stay at this energy levet

Energy Bands

eV Conduction Band (Free Electrons)

Energy Gap

Forbidden Band

Valence Band (Electrons in Valence Shell)

Energy gap is the energy required to rupture covalent bond

Energy Bands for Different Materials

Conduction Band

Conduction Band Forbidden Band

Forbidden Band

Valence Band

Valence Band

Conductors

Insulators

Conduction Band

Conduction Band

1.1eV

≤0.01eV

Forbidden Band

0.67eV

Forbidden Band

Valence Band Valence Band

Silicon

Germanium Temperature dependent

Temperature & Resistance

dR Temp. Coeff. = α α = dT α = -ive α = +ive I I R R

T Conductors

T Semiconductors

+

+

+

+

Holes & Hole Current

Hole Movement

+ +

+

+

+

+

+

+

+

Electron Movement

Hole Current Vs Electron Current The movement of holes and electrons is in opposite directions There are no holes in pure conductors, they are only created in semiconductors There are two currents in semiconductors: Hole current (Band ? Charge ?) Free electron current (Band ? Charge ?)

The total current in semiconductor materials is the sum of hole current and electron current Number of holes = ?

Charge Carriers

Holes are called positive charge carriers Free electrons are called negative charge carriers For pure (Intrinsic) semiconductors: Number of positive charge carriers = Number of negative charge carriers Is there any way to make charge carriers unequal? Let hole density be pi (holes/m3) and electron density be ni (electrons/m3) where i denotes intrinsic semiconductor, then: ni = pi -

+

+ +

-

+

Intrinsic Semiconductor

-

+ -

+

Charge Carriers at Room Temperature Silicon Carriers/m3

Germanium Carriers/m3

Copper Carriers/m3

1.5 X 1016

2.4 X 1019

8.4 X 1028

Thank You