Semiconductors cbse notes

SEMICONDUCTOR DEVICES

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P-N JUNCTION: - When a P-type crystal is joined with a N-type crystal in such a manner that crystal structure remains continuous then this structure is called as P-N Junction. Formation of P-N junction: - Diffusion method is used to form a P-N Junction. In this method an element of III group (like Boron) is coated on a slice of N-type semiconductor called wafer or an element of V group (like phosphorus) is coated on P-type semiconductor. When this semiconductor is heated at a high temperature (about 500°C) the impurity is diffused inside the semiconductor. Diffusion is more at surface and decreases as the depth increases. The depth up to which the diffusion takes place, a junction is formed which is called P-N Junction. On the one side of this junction there is P-type semiconductor and on the other side there is Ntype semiconductor. What happens at the time of formation of P-N Junction (formation of depletion region and potential barrier): As soon as a junction is formed the holes from p-region diffuse towards nregion and electron from n- region diffuse towards p-region due to the high concentration of holes and electron into two different regions. In the vicinity of junction the Potential Barrier P-Type

N-Type

Electrons majority carrier

Holes majority carrier

Depletion region Immobile +ve ions

Immobile - ve ions

electrons and holes recombines with each other and vanishes, due to which there is a excess of immobile +ve ions in n-region and –ve ions in p-region. Thus an electric field and hence a potential difference called potential barrier is developed across the junction which stops the further diffusion of holes and electrons. The region free form charge-carriers on both side of junction is called depletion region or space charge region. The thickness of the depletion region is of the order of 10-6 meter while the potential barrier is about 0.7 volt. Therefore P N 0.7 ElectricField  6  7  105 Vm1 10 Biasing of p-n junction: B A (I) Forward Bias: - When p-region of a p-n junction is joined to the (+) ve pole of a battery and n-region to -ve pole then the junction is said to be forward biased. Action of p-n junction: - When the p-n junction is made forward bias the (+) ve pole of the battery repels the Depletion layer holes towards n-region and the (-) ve pole repels the electron towards p-region. Due to which the electrons FORWARD BIASING and holes enter the depletion region and the thickness of depletion region decreases. If the external potential is greater than the potential barrier then near the junction electrons recombine with holes. For each electron-hole combination that

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takes place near the junction, a covalent bond breaks in p-region near the positive pole of battery. Due to which electrons and holes are produced in pair, the electron is captured by the (+) ve terminal, while the hole moves towards the junction. At the same time an electrons enters the n-region from the –ve terminal of the battery, thus a forward current flows in the circuit due to the flow of electrons and hole. During the forward bias the applied D.C. voltage opposes the potential barrier due to which the thickness of the depletion layer decreases. Thus p-n junction offers low resistance in forward bias. (II) Reverse Bias: - When p-region of a p-n junction is joined to the (-) ve pole of a battery and n-region to P N +ve pole then the junction is said to be reversed biased Action of p-n junction: - when p-n junction is reversed biased, the –ve pole of the battery attracts the holes present in P-region, while the +ve pole of the battery attracts the electrons present in the n-region. Thus the electrons and holes get away from the junction and the thickness of depletion region increases. But a very small current flows through the junction due to the recombination of minority carriers. This current is called as reverse current. If the reverse bias voltage is REVERSE BIASING made very high, all the covalent bonds near the junction break and a large number of electron-hole pairs are created due to which reverse current increases abruptly. This phenomenon is called avalanche breakdown and the reverse voltage at which this phenomenon occurs is called as reverse break down voltage or zener voltage which depends upon the density of impurity atoms. Due to the over heating at this voltage, the p-n junction may be damaged. During the reverse bias the applied D.C. voltage aids the potential barrier due to which the thickness of the depletion layer increases and hence it offers the high resistance in reverse bias. p n Symbol of p-n junction diode:Characteristics of p-n junction: -There is two type of characteristics(I) forward bias Characteristics- First of all makes the connection according to the circuit shown in fig.1. By changing the forward voltage with the help of potential divider note down the corresponding forward current and plot the graph between forward voltage and forward current. The graphs so obtained are called as forward characteristic curve of p-n junction. If

Vf

R h

+

Forward Current If

N BATTER Y

P

Knee Voltage

0.2 0.4 0.6 0.8 1.0 FORWARD VOLTAGE Vf

From the graph it is clear that initially there is no current. When the applied voltage is less than the potential barrier, the current flow through the junction is very small. As the forward PHYSICS DEPARTMENT, V.B.P.S., NOIDA

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voltage increases above the potential barrier, current increases approximately linearly. When the forward voltage is equal to voltage of potential barrier then the curve becomes like a knee and called as knee voltage. At this voltage the thickness of depletion layer becomes negligibility and the diffusion of electrons and holes across the junction take place easily i.e. the p-n junction offer low resistance when it is forward bias and the resistance is of the order of 100 ohm. (II) Reverse Characteristic Curve: Make the connection according to the circuit shown in the following figure. Change the reverse voltage and note the corresponding reverse current. The graph plotted between reverse voltages and reverse current is called as reverse bias curve. Practically in reverse bias there is no current if the applied voltage is low but a very small flow due to minority carriers. On increasing the reverse REVERSE VOLTAGE Reverse Current Ir (

Zener Voltage

Voltage to a very high value, the current increases abruptly, which is clear from graph. It is due to the fact that at very high voltage, the entire covalent bond near the junction is broken. Due to which a large number of holes & electrons are liberate and the corresponding voltage is called as Zener voltage. In reverse bias the thickness of depletion layer increases due to which the further diffusion of charge carriers stops and no current flows through the junction. Thus in reverse bias the junction offers very high resistance. DYNAMIC RESISTANCE: - The ratio of the small change in voltage to the small change in the current is called as dynamic or a.c. resistance of the junction diode. It is represented by Vd. Vd 

V I

The region of the characteristic curve where dynamic resistance is almost independent of the applied voltage is called the linear region of junction diode.

Junction diode as Rectifier: - An electronic device, which converts a.c. power in to D.C. power, is called rectifier. Half – wave Rectifier: - A rectifier, which rectifies only one half of each a.c. input supply cycle is called half wave rectifier. Principle: - It is based on the principle that the diode offers low resistance when it is forward bias and high resistance when it is reversed bias i.e. current can flow through the diode when it is forward biased.

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A.C. Input Voltage

D.C. 0utput Voltage

Arrangement:- The p-region of the junction diode is joined to the one terminal of the secondary coil of a step down transformer and the load resistance is joined between n-region and the IInd terminal of the secondary coil. Working:- Let during the first half cycle of the input a.c. upper end i.e. point S1 of secondary is at +ve potential and the lower end i.e. point S2 is at –ve potential. Thus the diode is forward bias. During first half cycle and current flows through diode in loadresistance from C to D. During the next half cycle the upper end becomes –ve and lower end becomes +ve and thus the diode gets reverse biased and no current fows through it. In the next half cycle diode gets forward biased and current flows through it from C to D and this process repeated again and again. The current obtain in output is discontinuous and pulsating d.c. due to which there is a huge loss of energy. Full-wave Rectifier:- A rectifier which rectifies both halves of the a.c. input is called a full wave rectifier. Principle:- It is based on the principle that the diode offers low resistance when it is forward bias and offers high resistance when it is reverse biased.

A.C. Input Voltage

D.C. 0utput Voltage

Arrangement:- The a.c. supply is fed across the primary coil P of a step down transformer. Two two ends of the secondary coil S of the transformer are connected to the p- regions of the junction diodes D1 and D2 . A load resistance RL is connected beteen the n-regions of the two diodes and the ncentral tapping of the secondary coil. The out put d.c. is obtained across the load reistance. Working:-Suppose that during first half of the input, the upper end S1 of the secondary is at + ve pot. and lower end S2 is at (–) ve pot. So the diode D1 gets forward bias and D2 gets reverse bias hence current flows through D1 in load resistance from C to D. During the next half cycle S1

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becomes –ve and S2 becomes +ve and hence D1 gets reverse bias and D2 gets forward bias. Thus the current flows through D2 from C to D in load resistance. Hence the full wave rectifier, rectifies the both halves of a.c. The output d.c. is continuous but pulsating. To reduce the fluctuations, filter circits are used in output circits. Electrolytic condenser and zener diodes are use to reduce the fluctuations of d.c. Different types of junction diode :(I) Zener diode:- A specially designed diode in which P and N region are heavily dopped due to which the depelation layer junctioin width is small and the junction field ie potential barrier is high and it can operate continuously, with out being damaged in the region of reverse breakdown voltage, is called zener diode.

Output Voltage (VO)

An important application of zener diode is that it can be used as voltage regulator. The regulating action takes place because of the fact that in reverse breakdown region, a very small change in voltage produces large change in current. This causes a sufficient increase in voltage drop across the resistance to lower voltage back to normal. Similarly, when the voltage across the diode tends to decrease, the current through diode goes down out of proportion so that voltage drop across the resistor is much less and it raises voltage back to normal. Hence the output voltage remains constant. Regulated Output Voltage VZ

VZ Input Voltage (Vi)

Question-What is a photo diode? Explain its working principle. Also give some uses. Photo diode: - A junction diode made from light sensitive semiconductor is called a photo diode. mA LIGHT

LIGHT P

Reverse bias

RL N

Volts I1 I2 I3 I4 I4 > I3 > I2 > I1

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µA

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Photo diode is always reverse bias. When no light falls on it, a small reverse current flows through the junction. This current is due to the thermally generated electron-hole pairs and is called as dark current. When the photodiode is illuminated with light photons of energy hν>E g then it ejects the valence electrons due to which the current increases and becomes maximum. This current is called as saturation current. On increasing the light intensity, the saturation current increases. A photodiode can turn its current ON and OFF in nanoseconds. So it can be used as a fastest photo detector. Uses: 1. In detection of optical signals. 2. In demodulation of optical signals 3. In light-operated switches 4. In speed reading of computer punched cards. 5. In electronic counters. Light Emitting Diode (LED): - A light emitting diode is simply a forward biased p-n junction made of gallium arsenide or indium phosphide and emits spontaneous light radiation. When a LED is made forward bias then the energy is released due to the recombination of electrons and holes, falls in visible region or infrared region of EM spectrum. Advantages over conventional incandescent lamps: LIGHT 1. Low operational voltage and less power consumption. 2. Fast action and no warm up time required. 3. Long life and ruggedness. 4. Light emitted is nearly monochromatic Uses: 1. Infrared LED’s are used in burglar alarm systems. 2. In optical communication system. 3. LED’s are used in numeric displays (in watches and calculators). 4. In optical mouses for the computers. 5. In remote controls Solar cell: - It is a junction diode which converts solar energy into electrical energy and is based on photovoltaic effect (generation of voltage due to bombardment of photons). It consists of a p-n junction made of Si or GaAs. A very thin layer of n-type semiconductor is grown over a p-type semiconductor by using diffusion method. (So that the energy falling on the diode not greatly absorbed before reaching to junction) Working: When light is incident on p-n junction each photon absorbed creates an electron and a hole. If is because the electron acquires sufficient energy to move from valence to the conduction band. Due to barrier voltage electrons moves towards n region and holes towards the p region. As a result the two regions gets opposite potential and emf is developed across the terminals of the diode. This photovoltaic emf can be used as ordinary cell in the electrical circuits. Applications: [1] Solar cells are used in wrist watches and calculators. [2] They are used to produce power in artificial satellites and space craft. Transistor: - When a thin layer of one type of semiconductor is sandwiched between the two thick blocks of another type of semi conductor then obtained structure is called a transistor. These are used as an amplifier as well as an oscillator. These are of two types: PHYSICS DEPARTMENT, V.B.P.S., NOIDA

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SEMICONDUCTOR DEVICES (I)

E

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NPN transistor: - A junction transistor in which a thin layer of p-type semiconductor is sandwiched between two layers of n-type semiconductor is known as NPN transistor. N P N C E C Emitter

Collector B

(II)

B PNP Transistor: - A junction transistor in which a thin layer of N-type semiconductor is sandwiched between two layers of P-type semiconductors is known as PNP transistor. P N P E C E C Emitter

Collector

B

B In a transistor base is lightly doped and very thin. The region, which is lightly doped and very thin, is called as Base. The region, which is highly doped, is called emitter while the remaining one is called collector. When a transistor is used in a circuit, base emitter junction is always forward bias while the collector base junction is reverse bias. Action of Transistor: (a) Action of n-p-n Transistor: - The emitter base junction is made forward bias by using a battery VEE while the collector base junction is made reversed bias by using the VCC. The –ve N

P

N

E

C

IE

Emitter

IB

B

VEE

Collector

IC

VCC

pole of battery VEE repels the electrons in emitter region (as majority carrier in n-region) towards base. Since the base is very thin and lightly doped, hence about 95% electrons cross over the base region and entered the collection region where they are attracted by the +ve pole of the battery VCC. As soon as an electron enters the +ve pole of the battery VCC, at the same time an electron enters the emitter region from the –ve pole of the battery VEE and this process is carried out continuously. About 5% electrons recombined with holes in base region. For each recombination a covalent bond breaks which creates the hole and electron in pair. Electron enters +ve pole of VEE through B and hence base current IB flows which is very small. If IE, IC and IB are the emitter, collector and base current then (According to Kirchhoff’s 1st law) IE = IB + IC It may note that in n-p-n transistor current flows due to the flow of electrons in and outside of transistor.

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Action of P-N-P transistors:P

N

Legends: -

P

E

HoleElectron-

C

IE

Emitter

IB

B

IC

Collector

Holes VEE

VCC

Characteristics of n-p-n transistor in Common Emitter configuration: - Common Emitter Electrons characteristics of a transistor are the graph plotted between the voltage and the current when emitter is earthed, base is used as input terminal and the collector as output terminal. Ic C

mA

IB n-p-n µA

B E

VCC VBB

+ +

VBE

VCE

_

_

N-P-N Transistor: -The base emitter circuit is made forward biased by using a battery VBB while the emitter, collector circuit is made reversed bias by using battery VCC. To draw the characteristic the circuit arrangement is shown in the above figure in which a n-p-n transistor is used. A transistor has two types of characteristics. D. C. Input characteristics: - Keeping VCE at constant voltage, charge VBE (Base emitter voltage) and note down the corresponding values of base current. Now for some other value of VCE , find out the change in base current for the corresponding change in V BE. Now plot the graph between VBE and IB at different constant value of VCE. The graphs so obtained are called as input characteristics. A.C.I input resistance:- The ratio of the change in the emitter base voltage (Δ VBE) to the change in base current (Δ IB) at the constant VCE is called as a.c. input resistance. It is denoted by Rin.

 V Rin   BE  I B

  VCE

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(2) Output characteristics:- The graphs plotted between emitter collector voltage and the collector current (IC) at different constant values of base current (IB). Following result may be drawn from the output characteristic curves(I) The collector current changes rapidly in beginning but soon it becomes saturated. (II) The saturation current increases on increasing the base current. (III) In audio frequency amplifiers the linear part of the output characteristics is used in order to obtain undistorted output. Output resistance :-The radio of the change in emitter collector voltage to the change in

 VCE Rout    I C

   IB

collector current at the constant base current. It is denoted by Rout. Transfer characteristics:-The graph plotted between collector current (IC) and the base current (IB) at different constant values of collector voltages (VCE). Current gain :- The ratio of change in collector current to the change in base current at constant collector – emitter voltage is called as current gain. It is also called as current transfer ratio. It

 I



   C   I B V

CE

is denoted by: -

(VCE) =3v

(VBE) INPUT CHARACTERISTICS

(VCE) 0UTPUT CHARACTERISTICS

(VCE) = 3V IC (mA)

IB = 250 IB = 200 IB = 150 IB = 100 IB = 50

IC (m A)

IB (mA)

(VCE) = 2v

IB (mA) TRANSFER CHARACTERISTICS

TRANSISTOR AS AN AMPLIFIER:- An amplifier is a device which is used for increasing the amplitude of variation of alternating voltage or current or power. A transistor can be used as an amplifier. There are three configurations1. Common base amplifier 2. Common emitter amplifier 3. Common Collector Amplifier Common Emitter Amplifier: - In common emitter configuration emitter is common to both the base and collector.

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Amplifier circuit using n-p-n transistor: - The emitter is common to both the input and output. The emitter is made forward bias by the battery VBB and collector emitter circuit is made IC C Amplified Voltage Signal

IB Input Voltage Signal

n-p-n

B

RL

E

ICRL

VCE

IE VBB

VCC

reversed bias by the battery VCC Thus the input resistance is low and the output resistance is high. The low input voltage signal is plied across emitter base circuit and amplified output voltage is obtained across collector emitter circuit. Let IE , IB and IC are the emitter base and collector current so according to Kirchoff’s lawIE = IB + IC ---------------------- (1) If RL is the load resistance then ICRL will be voltage drop across it. If VCE is the voltage across emitter collector then VCE =VCC – IC RL ---------------------(2) The variation in input signal voltage cause the variation in emitter current which produce the variation in collector current and hence in collector voltage. These variations in collector voltage appear as amplified output-voltage. The input signal and output signal are in opposite phase. Phase relation between input and output signals: - The input signal and the output signal are in opposite phase, which can be explained as belowWhen an a.c. signal is fed to the input circuit, the forward bias increases during positive half cycle of the input. This results in increase in IC and consequent decrease in VCE , thus during positive half cycle of the input, the collector becomes less positive. During negative half cycle of the input, forward bias decreases, therefore, the value of IE and IC also decreases and VCE would increase making the collector more positive. In common emitter amplifier, thus there is 180ºout of phase amplification.

Current Gain: - It is defined as the ratio of the change in collector current to the change in base

  I C

I B

current at constant emitter base voltage. It is denoted β.

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Voltage Gain: - It is defined the ratio of the change in the output voltage to the change in input voltage. It is denoted by AV. AV 

Vout I C  Rout  Vin I B  Rin

AV 

I C  Rout I B  Rin

A V   ac  Resistanc e Gain

Since β > α so the voltages gain in common emitter amplifier is very large as compared to that in common base amplifier. A.C. Power Gain:-It is defined as the ratio of change in output power to change in the input power. It is denoted by AP i.e.Change in output pow er Change in input pow er PO AP  Pi AP 

AP 

I c 2  RO I B 2  Ri

AP   2  resistancegain

β> α so the power gain in common emitter amplifier is very large as compared to that in common base amplifier. Trans conductance:- It is defined as the ratio of the change in the collector current (ΔIC ) to the change in emitter base voltage (ΔVBE) at constant collector voltage. It is denoted by gm i.e.  I c  gm     VBE  VCE gm 

I C I B  I B VBE

g m   ac 

1 Rin

Relation Between α and β: We know that I E  I B  IC or I E  I B  I C divide by I C on both sides I E I B  1 I C I C 1

 1

 1



  

1

 1



1 1

1



  

1 1

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Transistor as an oscillator: -An oscillator is a device which converts direct current into alternating current and produces high frequency undamped. A transistor can be used to produce undamped oscillations. The base oscillatory circuit consists of an inductance and capacitance called tank circuit. Due to resistance of circuit, a part of energy is dissipated, therefore, amplitude of oscillations goes on decreasing with time and damped oscillations are produced. In order to maintain these oscillations, energy is supplied to circuit at the right moment and in the right direction using a feedback arrangement. The feedback arrangement consists of primary P and secondary with variable capacitor C of suitable range. The secondary coil of inductance L. The inductance L and capacitance C constitute tank circuit. Working: - When the tapping key K is pressed for a moment, a small current starts flowing through the coil L1 due to the change of current, an emf is induced in inductor L. Due this induced voltage the emitter current and hence the collector current increases. Due to the increase in collector current the magnetic flux linked with L & L1 increases; thus the voltage induced in L also increases and hence forward bias is further increased which increases IC and IE. This process continues until the induced emf across the inductor attains a saturation value. During this process the upper plate of the capacitor gets +ve charge. When induced emf attains saturation value the induced emf becomes zero. Now the capacitor discharges through L; as a result emitter current decreases and hence collector current also decreases. The decreasing collector current will induced emf in inductor L in the reverse direction, which decrease the emitter current and hence collector current. This process continues till the collector current reduces to zero. Now the mutual induction stops playing its role. At this stage the lower plate of the capacitor C will get + ve charge and discharges through L. Thus the emitter current and hence the collector current again start to increasing i.e. the process gets repeated and the collector current oscillates between a maximum and zero value. The repeated process generates oscillations of constant amplitude and the relation gives freq. ʋ = 1/ 2∏√ LC By changing the value of C the freq. of the oscillations can be changed TRANSISTOR AS A SWITCH A transistor can be used as a switch; the following fig (1) shows the circuit diagram of a base biased n-p-n transistor in CE configuration states Here RB is a resistor in the input circuit and Rc in the output circuit. Applying Kirchhoff’s rule to the input and output circuits separately, we get VBB = IBRB + VBE = Vi -----------------------------(1) VCE=VCC—ICRC = Vo ------------------------------(2) The voltage VBB has been regarded as the dc input voltage Vi and VCE as the dc output voltage V0.

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Fig. 2 shows typical output voltage (V0) — input voltage (Vi) characteristic, called the transfer characteristic of the base biased transistor. It has three well-defined regions as follows: 1. Cutoff region: When Vi increases from zero to a low value (less than 0.6 V in case of a Si transistor), the forward bias of the emitter-base junction is insufficient to start a forward current i.e. IB = 0 and hence Ic = 0. The transistor is said to be in the cutoff region. From equation (1), the output voltage Vo = Vcc. 2. Active region: When Vi increases slightly above 0.6 V. a current Ic flows in the output circuit and the transistor said to be in the active state. 3. Saturation region: When Vi becomes very high , a large collector current Ic flows which produces such a large potential drop across load resistance Rc that the emitter-collector junction also gets forward biased and output voltage V0 decreases to almost zero. Now the transistor is said to be in the saturation state because it cannot pass any more collector current Ic. Switching action of a transistor: A transistor can be used as a switch if it is operated in its cutoff and saturation states only. A switch circuit is designed in such a manner that the transistor does not remain in the active state. As long as the input voltage is low and unable to forwardbias the transistor, the output voltage V0 (at Vcc) is high. If Vi is high enough to drive the transistor into saturation, then V0 is low, nearly zero. Thus when the transistor is not conducting (in cutoff region), it is said to be switched off and when it is driven into saturation, it is said to be switched on.

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DIGITAL ELECTRONICS In these electronics circuits, the current or voltages will have only two values, High (1) and Low (0). In digital circuits, the electrical pulses of two levels only are used as signal voltages. Logic Gates: A gate is a digital circuit which is used to perform certain specific function. The three basic logic gates are: a.

OR gate

b.

AND gate

c.

NOT gate

All other logic gates can be formed by combination of these three gates. Truth Table: It is table that indicate all possible combinations of input signals and their output. Boolean Algebra: This is the algebra which can be applied to logic gates based on Binary number system. OR Gate: It is a two input single output gate. The output is one if any of the two inputs or both the inputs are one. The truth table and symbol of OR gate are: A

B

Y

0

0

0

0

1

1

1

0

1

1

1

1

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The circuit diagram for OR gate is: The diodes used are considered to be ideal diodes ie. during forward bias they offer zero resistance and during reverse bias they offer infinite resistance. Case I: When A = 0, B = 0: Here both the diodes are in off state. There is no current that flows through R, thus output voltage is Y=0. Case II: When A = 0, B = 1: In this case D1 is in off state and D2 is forward biased. The current flows through D2 and sets up a potential difference of 5V across it, so Y = 1. Case III: When A = 1, B = 0: in this case diode D1 is forward biased and D2 is reverse biased. Diode D1 conducts and Y=1. Case IV: When A=1, B=1: Here both the diodes are forward biased and hence conduct perfectly. A potential difference of 5V appears across resistance. Thus, Y = 1.

AND Gate:

It is also a two input single output gate. The output is one if both the inputs are one. (a) Suppose A=0 and B=0: The potentials at A and B are forward biased and offers no resistance. The diode D1 conducts and net potential difference appears across R and Y=0. (b) Suppose A = 0 and B = 1: In this case also A is forward biased and B is in off state. The diode D1 conducts and net potential difference appears across R and Y = 0. (c) When A = 1 and B = 0: In this case also A is in off state and B is forward biased. The diode D2 conducts and Y = 0

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(d) When A = 1 and B = 1: Here both diodes are in off state, hence no potential drop occurs across R and Y = 1. NOT Gate: It is a single input single output gate. The truth table and symbol is A

Y  A

0

1

1

0

It is realised with the help of a transistor. Consider an pnp transistor to be used as NOT gate. If A = 0, the emitter base junction is reverse biased and no current flows through it. Correspondingly current through RC is also equal to zero. The potential Y = 1. On the other hand, if A is 5V i.e. A =1, the emitter base junction is forward biased. Potential drop occurs R and Y = 0 NAND Gate: It is AND gate followed by a NOT Gate. It is two input single output gate. The truth table and symbol are, A

B

X

0

0

0

1

0

1

0

1

1

0

0

1

1

1

1

0

YX

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NOR Gate: It is OR gate followed by a NOT gate. It is a two input single output gate. The truth table and symbol are, A

B

X

0

0

0

1

0

1

1

0

1

0

1

0

1

1

1

0

YX

Exclusive OR (XOR) Gate: It is also two input, single output gate. The output is one iff one of the inputs is one. The truth table and symbol are; Y  AB  A B

A

B

0

0

0

0

1

1

1

0

1

1

1

0

Exclusive NOR Gate: It is an exclusive OR gate followed by a NOT gate. Output is one either both the inputs are one or zero. The truth table is, A

B

0

0

1

0

1

0

1

0

0

1

1

1

Y  AB  A B

PHYSICS DEPARTMENT, V.B.P.S., NOIDA

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