Semiconductor

ENERGY BANDS IN SOLIDS Considering example of sodium which has 11 electrons electronic configuration is 1S2 2S2 2P6 3S1 if we take N sodium atoms separated from each other by large distance. We will have 11 N electrons totally. For each atom there are 2, 1S energy state so altogether for N atoms there are 2N identical 1S state and are completely filled by 2N electrons. Similarly there are 2N, 2S identical completely filled levels and 6N identical 2P energy states which are also completely filled. There are 2N energy state of 3S of which only N are filled. If the atoms are separated by a large distance all the 2N state of is 1S level will have identical energy similarly for 2S, 2P, 3S etc. Now If the atoms are brought close to each other the outer 3S electron of one atom starts interacting with the 3S electrons of the neighboring atoms. Because of this interaction energy of all the 2N states of 3S level will not remain identical.

Some of the states are shifted up in energy and some of them are shifted down. As a result what was sharply defined 3S energy now becomes a compotation of several closely spaced energies called energy band of 3S. The inner electrons interact weakly with each other so that splitting is less in inner energy levels. The difference between the highest energy in a band and the lowest energy in the next higher band is called the band gap between the two bands. In all solids we have energy band separated by band gap. The energy band structure in solids may be classified in four broad types as shown in figure.

mk

SEMICONDUCTOR (T.N.)

PAGE # 1

(A)

The highest occupied band is partially filled at 0K. If outer most sub shell contains odd number of electrons, we get this type of band structure. They are good conductors of electricity because as an electric field is applied the electron in the partially filled band can receive energy and move in the same band.

(B)

The highest occupied energy level is completely filled at 0K and the next higher level is completely empty. But these two bands overlap each other zinc–30 (1S)2 (2S)2 (2P)6 (3S)2 (3P)6 (4S)2 In solid zinc, the 4P band overlaps with the 4S band. Hence electrons can take energy from electric field and can move in other energy levels. So these are also good conductor of electricity

(C)

Insulators:- The highest occupied energy band is completely filled and next higher band is completely empty. The energy gap is much and it cannot be given by electric field and cannot move to CB and are non-conductors.

(D)

The highest occupied band is completely filled at 0K and the next higher band is empty. But energy difference between the two bands is not so high as for example in silicon in which band gap is 1.1 ev. A ordinary battery still cannot provide this much of energy. But at temperature much above 0 K thermal collisions push some electrons from the highest occupied band to the next empty band. These few electrons can respond to even a weak battery and move in this empty band. Also vacancy is created in VB and promotes conduction. But have conductivity small as compared to conductors and called semi-conductors. The energy bands which are completely filled at 0K is called valance bands. The bands with higher energy are called conduction band.

SEMI CONDUCTORS Silicon has an atomic structure 1S2, 2S2, 2P6, 3S2, 3P2 Each atom has four outer most electrons and two silicon atoms contribute one electron each and two electrons are shared by two atoms. Both the electrons are in valance band due to collision one of the electrons may gain energy and start orbiting in larger radius and reaches the conduction band.

The electron from CA can easily shift between AB so we can consider a hole or e+ charge moving to right. Whenever a valance electron is shifted to conduction band a hole is created. Thus in a pure semiconductor number of conduction electrons and holes are equal. When electric field is applied holes moves in direction of electric field and conduction electron in opposite direction of electric field. Such pure semi conductor is called intrinsic semiconductors.

mk


PAGE # 2

P-Type and N-Type semiconductors EXTRINSIC SEMICONDUCTORS

N type Semiconductor: - when a small amount of phosphorus is diffused into silicon crystals. As it have five outer most electrons. Some of the phosphorus replaces silicon. Four of the outer electron of phosphorus is shared with Si but the fifth one moves with large radius. The energy level (called impurity) level for this electron of slightly less than the conduction band. Hence this extra electron participates in conduction. Such impurity which donate electrons for conduction, are called donor impurity. P type Semiconductor:

When silicon is doped by aluminium than three valance electron of aluminum are used to form covalent bond with neighboring three silicon atoms. But bond with forth is not created. This can be filled by another valance electron if energy of 0.057 e.v is given so holes move in direction of electric field and conduction can take place.

DENSITY OF CHARGE CARRIERS AND CONDUCTIVITY J V = ne   = ne E E Where n is no of electrons per unit volume and v is the drift velocity of the electron when electric field E is applied, is mobility. In case of semiconductors conduction is due to the conduction electron as well as due to the hole. Mobility of electrons and holes are also different.  = neee + nheh SEMICONDUCTOR (T.N.) PAGE # 3 The conductivity of a metal is given as

mk

=

P-N JUNCTION

When P type and N type semiconductors are brought together as there is large concentration of electrons on n side and hole towards left side electron will diffuse from right to left side. As two side were electrically neutral in beginning. Diffusion of holes towards the right and diffusion of electrons towards left make the right half positively charged and left half negatively charged. This creates an electric field near the junction from the right to the left.

Any hole near the junction is pushed by the electric field in left half and any conduction electron is pushed in right part. Thus no charge, charge carrier can remain in a small region near the junction. This region is called the depletion layer.

DIFFUSION CURRENT Due to concentration difference holes try to diffuse to right and electron try to diffuse to left side. But electric field in the depletion region opposes this diffusion. But holes hawing high energy can diffuse from P to N side and conduction electron diffuse from right to left hence a current from P side to N side called diffusion current starts flowing.

mk


PAGE # 4

DRIFT CURRENT If an electron hole pair is created in the depletion layer, the electron is quickly pushed by the electric field towards the N side and holes towards the P side. Hence there is a current from n side to P side called drift current. When P-N Junction is not connected to Battery Drift current and diffusion current are equal in magnitude and opposite in direction. The potential of N side is greater them potential of P side and potential difference between two sides is called potential barrier. P-N JUNCTION BIASING

If positive terminal is connected to P side and negative terminal is connected to N side. This connection is called forward biasing. Due to the forward bias connection, the potential of P side is raised and hence the height of the potential barrier decreases. This allows more diffusion to take place. The diffusion current thus increase and drift current remain almost same hence net current is from the P side to the N side. When the applied potential difference is so high that the potential barrier is reduced to zero or is reversed diffusion increase very rapidly. Current in the circuit changes non linearly.

Reversed - Biasing

If P side is connected to negative terminal and N side is connected to positive terminal of a battery, the Junction is said to be reversed biased. In this case the applied electric field and electric field in the depletion layer is in the same direction so net electric field increases. Hence potential barriers become higher. The diffusion current decreases and drift current not appreciably affected. So a net current having very small value flows from N side to P side.

mk


PAGE # 5

IV CHARACTERISTIC OF PN JUNCTION DIODE Any device which freely allows electric current in one direction but does not allow it in the opposite direction is called a diode. P-N Junction is very near to an Ideal diode as current flows in forwards biased condition but not in reverse biasing. Dynamic Resistance: As Resistance of P-N Junction is not constant

V I Where V denotes small change in applied potential difference and I denotes small change in corresponding current. It is reciprocal of I-V characteristic. P-N Junction as a Rectifier A rectifier is a device which converts A. C into D. C. R=

mk


PAGE # 6

•

•

Half wave rectifier: As Shown in diagram an a-c source having VB –VA = V0 sin (  t +  ) Is connected in series with PN junction diode and load Resistance R. In first half cycle VB - VA is positive and PN Junction is forwards biased. So we get current in resistance and we get potential difference across the resistor. In the next half cycle VB - VA is negative and PN Junction is Reversed biased and we get no current in Load Resistance no potential difference in out put, Full wave Rectifier:

A full wave Rectification is possible by using two diodes shown in figure. The AC potential difference is obtained across the secondary of transformer and connected as shown. In first half cycle VA > VC > VB so D1 Conducts as it is forwards biased but D2 not as it is reversed biased and we get P.D across R1. In second half cycle VB > VC > VA hence D2 becomes forward biased and D1 becomes reversed biased so D2 conducts and there in P.D across load resistor R. (i)

Photo diode: Its working is based on photoconduction of light. A photodiode is made up of

hc is sufficient so  break a valance bond. When such light falls on diode new hole electron pair are created. The number of charge carriers increases hence conductivity increases. If the Junction is connected in some circuit the current in the circuit can be controlled by intensity of light falling on diode. USES 1. In photo detection for optical signals 2. In demodulation of optical signals 3. In switching light on and off. 4. In logic circuits. photosensitive semiconductor material. When photon is such that its energy E =

mk


PAGE # 7

•

PHOTODIODE

Its working is based on photo conduction of light. A photodiode is made up of photosensitive hc is sufficient  to break a valance bond. When such lift falls on diode, new hole electron pair are created the number of charge carries increases hence the conductivity increases. If the junction is connected in same circuit the current in the circuit can be controlled by intensity of light falling on diode.

semiconductor material. When wave length of light photon is such the its energy E =

1. 2. 3. 4.

mk

Uses: In photo detection for optical signals In demodulation of optical signals In switching light on and off. In logic circuits.


PAGE # 8

•

LIGHT EMITTING DIODE It is a P-N junction diode working in forward biased.

When holes from P side and electron from N side diffuses in N side and P side respectively then at the junction they recombine and due to some energy difference energy is released and if this energy is in the visible light range it will be visible and colour of such light depends upon material used as for example In calculator In optical communication In image sensing circuit for picture phone.

1. 2. 3.

• TYPE OF JUNCTION DIODE (a) Zener diode:-

It is a specially defined heaviely doped P-N junction. It can work without being damaged in reverse break down voltage.

mk


PAGE # 9

In the reverse breakdown region the voltage across the zener diode remains constant, even if the current through zener diode in creases consideribely. It can be used to supply constant voltage can be obtained across the load resistor RL.

When P-N junction is highly doped the P-N junction depletion layer is very small and high value of electric field is there in the depletion layer and it breaks the covalent bond in the depletion layer and after that a step rise in current is there and potential difference across the P-N junction remain constant as resistance becomes almost zero •

SOLAR CELL It is a solar energy converter. It consist of a silicon P-N junction diode packed in a can with glass window on top. The upper layer is P type semi conductor. It is thin so that incident light photons may easily reach the P-N junction. On top of P layer the metal finger electrodes are prepared in order to have enough spacing so that light reaches the P-N junction. When photons of light reaches the P-N junction holes and electrons are created and holes moves towards P side and electrons moves towards N side and get collected there. This creates a potential difference between P and N side and when a load is connected a photo current flows.

1. Used for charging batteries 2. They are used in artificial satellites 3. Solar cells are used in calculators.

mk


PAGE # 10

JUNCTION TRANSISTOR A junction transistor is formed by sandwiching a thin layer of one type of semiconductor in thin layer of other two type of semiconductors (i) npn (ii) pnp

The middle part layer is called base and is very lightly doped one of the outer layer is hastily doped and called emitter and right most layer is moderately doped and called collector. In normal operation of transistor base junction is always forward biased and collector base Junction is reverse-biased

mk


PAGE # 11

Biasing: suitable Potential differences should be applied across the two Junctions to operate the transistor. It can be operated in three modes.

COMMON EMITTER The emitter in given zero potential and other two terminals is kept at appropriate potential. In common emitter circuit has input and output section. Input section contains the base emitter junction and the voltage source where output section contains base collector junction and voltage source. The current IB is called input current and IC is called output current. When the input current IB is plotted against the voltage VBE between base and emitters we get input characteristic This is like these of a forward biased P-N junction. If the basing is small as compared pared to height of the potential barrier at the junction the current IB in very small once the voltage is more than the barrier height the current rapidly increases.

Output characteristic: - are shown in figures The collector-base junction is reversed baised the current Ic is very small. As the electrons are forced from the emitter side the current Ic is still quite large as compared to a single reverse based p-N junction. At high VCE Ic becomes constant.

mk


PAGE # 12

Solved problems Q.1 [Sol.

Draw block diagram and logic symbol of p-n-p and n-p-n transistors and give its working. A transistor is a three terminal semiconductor device. A junction transistor is obtained by growing a thin layer of one type of semiconductor in between two thick layers of the similar type semiconductors. Two types of transistor are n-p-n junction transistor & p-n-p junction transistor. In n-p-n transistor, the p-section is sandwiched between n-section. In p-n-p transistor, the n-section is sandwiched between two p-sections. The three sections of the transistor are called emitter (E), base (B) and collector (C). Emitter: The left region of transistor is known as emitter. It supplies majority charge carriers to the base region. Emitter region is heavily doped. Collector: The right region of transistor is known as collector. It is also heavily doped. The collector collects the majority charge carriers. Base: The middle region of the transistor is known as base. The base is very thin as compared to the emitter and collector regions and it is lightly doped. npn C n p n E

E

C

B

p

n

B n-p-n transistor p E

E

pnp

C

C B

B

p-n-p transistor Here arrowhead points hole current i.e. conventional current. In n-p-n transistor the arrowhead on the emitter points away from the base, while in p-n-p transistor arrowhead points towards the base. Action of n-p-n transistor: The emitter base junction is forward biased i.e. positive pole of emitter base battery VEB is connected to base and negative pole to emitter. The resistance of emitter base junction is very low. The collector base junction is reverse biased i.e. positive pole of collector base battery VCB is connected to collector and negative pole to base. The resistance of this junction is very high, electrons which are majority carries in emitter are repelled towards base by negative potential of VEB resulting emitter current IE. The base being thin and lightly doped has low number density of holes. When electrons enter in the base region, then only a few electrons (say 5%) get neutralized by the electron-hole combination, resulting base current IB. The remaining 95% electrons pass over to the collector, due to positive potential of VCB, resulting collector current IC. n

p

n E

IC

IE

mk

C

IC

IE IB

IB VEB

n-p-n

VCB

VEB


VCB

PAGE # 13

As one electron reaches the collector, it flows to the positive terminal of VCB. At the same time one electron flows from negative terminal of VCB to positive terminal of VEB and one electron flows from negative terminal of VEB to emitter. Thus, is n-p-n transistor, the current is carried inside the transistor as well as in external circuit by electron. So the direction of conventional current is opposite to direction of flow of electrons. Thus, IE = IB + IC. Action of p-n-p transistor: The emitter base junction is forward biased i.e. positive pole of emitter base battery VEB is connected to emitter and negative pole to base. Collector base junction is reverse biased i.e. negative pole of collector base battery VCB is connected to collector and its positive pole to the base. The resistance of emitter base junction is very low and the resistance of collector base junction is very high. Holes which are majority carries in emitter are repelled by positive potential of VEB and move towards base resulting emitter current I1. The base being thin and lightly doped has low number density of electrons. When holes from emitter enter the base, only a few holes (say 5%) get neutralized by the electron hole combination, resulting base current IB. The remaining 95% holes pass to collector due to positive potential of VCB, resulting collector current IE. As one hole reaches the collector, one electron from negative terminal of VCB flows to the collector. At the same time a covalent bond in emitter is broken, electron released goes to positive terminal of VEB and hole moves towards the base. Thus, the current in p-n-p transistor is carried by holes and at the same time concentration is maintained. Thus, IE = IB + IC. p

p

n

p-n-p

E

B

IE

IC

IE

C

Q.2 [Sol.

]

IB

IB VEB

VCB

VEB

IC

VCB

Give transistor characteristics in common emitter configuration. In common emitter configuration, emitter terminal of transistor is common between input and output circuits. IB

IC + C

VBE + –

IB +

µA

B

+

n-p-n

–V

VCE + V

VBE

IE

+ V

Input characteristics: These characteristics represent the variation of base current IB with base emitter voltage VBE, keeping collector emitter voltage VCE constant. Input resistance: It is defined as the ratio of small charge in base emitter voltage to the small change produced in base current at constant collector emitter voltage.

mk


PAGE # 14

 VBE   Rinput =   IB  VCE  constant VCE=2V VCE=4V VCE=6V IB(µA)

VBE(volts)

Output characteristics: These characteristics represent the variation of collector current IC with collector emitter voltage VCE, keeping base current IB constant. Output resistance: It is defined as the ratio of small change in collector emitter voltage to small change produced in collector current at constant base current.

 VCE   Routput =   IC  IB  constant Saturation region IB=30µA IC(mA)

IB=20µA

Active region

IB=10µA IB=0 Cut off region VC(volts)

Output characteristics of a transistor in common emitter configuration are divided into three regions: (i) Active region: Active region lies above IB = 0. In this region, collector junction is reverse biased and emitter junction is forwarded biased. For a given value of IB, collector current increases as VCE increases. A transistor is operated in active region if it is used as an amplifier. (ii) Cut off region: Cut off region lies between IB = 0. The collector current has finite value. In order to cut off the transistor, the emitter junctions has to be made slightly reverse biased in addition to IB = 0. (iii) Saturation region: Saturation lies close to zero voltage axis where all the curves coincide. In this region, collector current is independent of the base current. ]

mk


PAGE # 15

Q.3 [Sol.

Explain p-n-p transistor as common amplifier. Emitter is common to both input and output circuit. The input (emitter base) circuit is forward biased with battery VBB and the output (collector emitter) circuit is reverse biased. Due to which the resistance of input circuit is low and that output circuit is high. A load resistance RL is connected in series in the collector circuit. C IB

IC

B p-n-p

ICRL RL

input singal

ouput singal

VCE VCC +

IE

+ VBB

IB

IC

When no ac signal is applied to the input circuit, then according to kirchhoff 's first law IE = IB + IC .......(1) If we assume that 5% of emitter current appears as base current due to electron hole combination in base, then 95% of the emitter current flows as collector current. IB = 5% of IE = 0.05 IE IC = 955 of IE = 0.95 IE IC flows through the load resistance RL which produces a potential drop ICRL which is in opposition to VCC net collector voltage VCE = VCC – ICRL .......(2) where VCE is the potential difference between collector & emitter. When the input signal is fed to the emitter base circuit, it will change the emitter voltage and hence the emitter current which in turn will change the collector current [from equation (1)]. Due to which the net collector voltage VCE varies in accordance with equation (2). These variations in collector voltage appear as amplified output. Phase relationship between input and output signals: Amplification of positive half of input signal: The positive half cycle of the input signal opposes the forward biasing of base emitter circuit. Due to the decreases in forward bias, the emitter current and hence the collector current decreases. This decreases the potential drop ICRL . From equation (2) VCE increases. In other words input signal, the negative half cycle of output signal is obtained. Amplification of negative half of input signal: The negative half cycle of input signal supports the forward biasing of base emitter circuit. Due to increase in forward bias, the emitter current & hence the collector current increases. This increases the potential drop ICRL. From equation (2) VCE decreases. In other words collector becomes less negative. So positive output signal is obtained. Thus, for negative half cycle of input signal, the positive half cycle of output signal is obtained. When p-n-p transistor is used as common emitter amplifier, the output & input signals are 180° out of phase. Various gains in common emitter amplifier: 1. (i)

mk

Current gain: It is of two types D.C. current gain: It is defined as the ratio of collector current (IC) to the base current (IB). It is denoted by .


PAGE # 16

=

IC IC = IB I E  IC

(ii) A.C. circuit gain: It is defined as the ratio of change in collector current (IC) to the change in base current (IB) at constant collector emitter voltage. It is denoted ac. 2.

3.

Trans-conductance (gm): It defined as the ratio of change in collector current (IC) to the change is base emitter voltage i.e. input voltage (Vi). gm =

I C I C I B  = Vi I B Vi

gm =

ac Ri

 Vi   R i  . I B  

A.C. voltage gain (Av) : It is defined as the ratio of change in output voltage (VC) to the change in input voltage (Vi) Av =

VC I C  R 0 R = = ac × 0 Vi I B  R i Ri

Av = ac × resistance gain Av = ac ×

R0 Ri

Av = gm × R0 4.

A.C. power gain: It is defined as the ratio of change in output power to change in input power. Power gain =

P0 I 2 R = C2  0 Pi I B R i

2 × resistance gain Power gain = ac Power gain = ac × Av. ]

Q.4 [Sol.

Explain working of transistor as oscillator. Tank circuit consisting of an inductance L and a capacitor C, connected in parallel is the simplest type electrical oscillating system. In this circuit electrical energy once given to the circuit, oscillates as magnetic energy in the inductance and electrostatic energy in the capacitance. Frequency of oscillations is given by 1 2 LC Here LC circuit is inserted in emitter-base circuit of transistor which is forward biased with battery B1. The collector-emitter circuit is reverse biased with battery B2. A coil L1 is inserted in collector emitter circuit. I is coupled with L in such a way that if increasing magnetic flux is linked with L it will support the forward bias of emitter base circuit and decreasing magnetic flux with oppose the forward bias. v=

mk


PAGE # 17

L1 +

C

B

B2

n-p-n E L

C

+

K

B2

Working : When K is closed, the current starts rising due to L1. As a result magnetic flux linked with L1 increases and hence increases with L. Due to mutual induction, an emf is induced in L which will change the upper plate of the capacitor with positive charge which supports the forward biasing of base emitter circuit. This results in an increase in the emitter current and hence an increase in collector current. In this way the collector current through L1 goes on increasing, till the induced emf across L attains a saturation value. ] Q.5 [Sol.

Describe transistor as a switch. To understand the operation of the transistor as a switch (i.e. a device for the on and off the current in the circuit), we use n-p-n transistor with common emitter transistor circuit as shown in the figure. IC

C RB

IB B p-n-p E

input singal V1

ouput V0 voltage

VBE VCC +

IE

IB

mk

RC

IC

Using Kirchhoff 's voltage law: for the input circuit we have – VBB + IB RB + VBE = 0 or VBB = IB RB + VBE .......(1) For the output circuit, we have – VCC + IC RC + VCE = 0 or VCE = VCC – IC RC .......(2) We shall assume VBB as the d.c. input voltage Vi and VCE as the output voltage V0. Then From (1), Vi = IB RB + VBE ........(3) From (2), V0 = VCC – IC RC ........(4) Now, we shall try to understand how V0 changes as Vi increases from zero onwards. In case of Si-transistor as long as Vi < 0. 7V, the transistor will be in cut off state and current IC will be zero. The from (4), V0 = VCC When Vi > 0.7 V, the transistor is in active state. There will be some current IC in the collector emitter circuit. From (3) we note that the value of output voltage V0 decreases as the term IC RC increases. With the increase of VBIC increases of V0 with Vi becomes non-linear and transistor goes to saturation state. With further increase in Vi, V0 is found to decrease further towards zero but never becomes zero. SEMICONDUCTOR (T.N.) PAGE # 18

cut off region V0

active region

saturation region

O

V1

1.0V

0.7V

From above, we note that as long as Vi is low (i.e. Vi < 0.7 V), V0 is high ( = VCC). The transistor is in cut off state. It means the transistor is said to be in switched on state. If Vi is high (i.e. Vi > 1.0 V), the transistor is in saturation state, the V0 is low and is very near to zero. It means the transistor is not conducting. In this saturation the transistor is said to be in the switched off state. If we define low and high states as below and above certain voltage levels corresponding to cut off and saturation states of transistor, then we can say that a low input state switches the transistor on and a high input state switches it off. It means the low input to transistor gives a high output and a high input gives a low output. It shows that a transistor acts as a switch. ] Q.6 [Sol.

Explain OR, AND and NOT gate used in digital circuit. Boolean algebra and logic operations: George Boole developed algebra to solve the logical problems. This algebra is known as Boolean Algebra. This logic is a binary or two valued logic. So this algebra allows only two values or states for a variable. These two values or states represent either 'true' or 'false'; 'ON' or 'OFF'; 'closed' or 'open' ; 'high' or 'low' etc. by 1 and 0 respectively. Logic gates: The digital circuit that can be analysed with the help of boolean algebra is called logic gate or logic circuit. A logic gate has one or two input but only one output. There are primarily three logic gates namely the OR gate, the AND gate and the NOT gate. Truth table: The operation of a logic gate or circuit can be represented in a table which contains all possible inputs and their corresponding output is called truth table. The OR gate: OR gate has two inputs (A and B) and only one output (Y). The relation between the output (Y) and the inputs (A and B) is given by the boolean expression. Y = A + B and is read as "Y equal A OR B" Logic symbol of OR gate is shown as A Y

B

OR gate used in digital circuit: OR gate consists of two diodes connected in such a way that their n-regions are connected at a common point. The input applied to A or B is either 0 or 1. These diodes are assumed to be ideal (zero resistance). A B 0

mk

D1 Y D2

5V


PAGE # 19

1. 2.

3.

4.

Following cases may happen: When A = 0 and B = 0, both the diodes are reverse biased. In this case, non of the diodes conducts and hence there is no output i.e. Y = 0. When A = 0 and B = 1. In this case, D2 is forward biased and D1 is reverse biased. So diode D2 conducts. The whole input voltage (i.e. 5V) appears at the output as diode D2 is ideal one. The high voltage (5V) is represented by 1, so the output Y = 1. When A = 1 and B = 0. In this case, diode D1 is forward biased and diode D2 is reverse biased. So diode D1 conducts. The whole input voltage (5V) appears at the output as diode D1 is ideal. The high voltage (i.e. 5V) is represented by 1. So output Y = 1. When A = 1 and B = 1. In this case, both the diodes are forward biased and hence both conduct. The output voltages of two diodes obtained across R are in parallel, so the net output Y = 1 (i.e. 5V). The truth table of OR gate is given below: Inputs Output A

B

Y

0

0

0

0

1

1

1

0

1

1

1

1

The operation OR can be understood with the help of the following example. Consider a circuit having two parallel switches A and B and an electric bulb in the circuit will glow only if either switch A or switch B or both are closed. If both the switches are open, the bulb will not low at all.

A

B

Y

+ The AND gate: AND gate has two inputs and only one output. The relation between inputs (A and B) and the output (Y) is given by the boolean expression. Y = A · B and is read as "Y equal A AND B" Logic symbol of the AND gate is shown as A Y

B

The AND gate used in digital circuit: The AND gate consists of two diodes connected in such a way that their p-regions are connected at common point. The diodes are assumed to be ideal.

A B

D1 Y D2

0 0

mk


PAGE # 20

1.

2. 3. 4.

Following cases may happen: When A = 0 and B = 0. In this case both the diodes are forward biased and hence they conduct. In this case, terminals A and B both are actually grounded because they are connected to grounded terminal of battery B1. The diodes being ideal, no voltage drop takes places across either diode and as such the output is zero. i.e. output Y = 0. When A = 0 and B = 1. In this case diode D1 is forward biased and diode D2 is reverse biased. So diode D1 conducts. As the diodes are ideal, so output voltage is again zero i.e. output Y = 0. When A = 1 and B = 0. In this case, diode D1 is reverse biased and diode D2 is forward biased. So diode D2 conducts. As such output voltage is again zero i.e. output Y = 0. When A = 1 and B = 1. In this case, both the diodes are reverse biased and hence do not conduct. So the output voltage (Y) is equal to the battery voltage connected to the resistance R. Hence Y = 1. The truth table of AND gate is given below: Inputs Output A

B

Y

0

0

0

0

1

0

1

0

0

1

1

1

The operation AND can be understood with the help of the following example. Consider a circuit having switches A and B in series with a bulb. The bulb will glow only if both the switches are closed. If any one of the switches is open, the bulb will not glow.

A

B Y

+

The NOT gate: The NOT has only one input and only one output. The relation between input (A) and output (Y) is given by boolean expression as Y = A and is read as Y NOT equal to A. This shows that if A = 0, Y = 1 and if A = 1, Y = 0. It means NOT gate inverts the input. Due to this reason, NOT gate is also known as inverter. NOT gate used in digital circuit: Since input signal is to be inverted so a transistor amplifier in common emitter configuration is used instead of a diode. This type of transistor configuration changes the phase of the signal or introduces a phase inversion. The NOT gate used in a digital circuit is shown in figure.

mk


PAGE # 21

RL

A 1

C

B

Y

E

0

1.

2.

Following cases may happen: When A = 0, the emitter base junction is reverse biased and hence there is no collector current. The transistor is cut off. So, there is no potential drop across RL. Thus, the output voltage (Y) is equal to voltage of the battery connected to the collector. Since high voltage is represented by 1, so Y = 1. When A = 1, the emitter base junction is forward biased and hence large collector current flows through RL. The transistor is saturated. The voltage drop across RL is equal to the voltage of the battery connected to the collector. Hence Y = 0. The truth table of NOT gate is given below: Output Inputs (Y) (A) Y= A ] 0 1 1

Q.7 [Sol.

1. 2. 3. 4. 5. 6. 1. 2. 3.

mk

0

What is integrated circuits (IC). Give advantage, limitation and its uses. Integrated circuit is an assembly of large number of transistor, capacitors and resistors joined on a simple piece (square or rectangle) of silicon, which may be very small in size. In other words, integrated circuit is a collection of interconnected transistors, diodes, resistors and capacitors fabricated onto a single piece of silicon, known as chip. Advantage of Integrated circuit's: They have low cost. They are the more reliable because there is no solder joint inside IC's. IC's are smaller and hence the device formed by IC's is very compact. A complex circuit of reasonable size can be designed by using IC's. They require low power to operate. Greater ability of operate at extreme values of temperature. Limitations: If any one component goes out of order, the whole IC has to be replaced. It is not possible to produce high power ( > 10 W) by IC's. It is not possible to fabricate inductors and transformers on the surface of the chip. Uses: IC technology is widely used in televisions, computers, amplifiers, radios, video recorders, telecommunication equipments etc. ]


PAGE # 22

Unsolved problems Q.1 What is the ratio of number of holes to number of conduction electrons in (i) n-type (ii) p-type extrinsic semiconductor? It is more, less or equal to one. Q.2

What is the ratio number of holes to number of conduction electrons in a intrinsic semiconductor?

Q.3

Why is the conductivity of n-type conductor greater than that of the p-type semiconductor even when both of these have same level of doping?

Q.4

How does the energy gap vary with doping in a pure semiconductor?

Q.5

Whey germanium is preferred over silicon for making semiconductor devices.

Q.6

Why does the conductivity of a semiconductor increase with rise of temperature?

Q.7

In a semiconductor the concentration of electrons is 8 × 1013 cm–3 and that of holes is 5 × 1012 cm– 3 . Is it p-type or n-type semiconductor?

Q.8

Carbon and silicon are known to have similar lattice structure. However, the four bonding electrons of C are present in second orbit while those Si are present in its third orbit. How does the difference result in a difference in their electrical conductivities?

A semiconductor has equal electron and hole concentration of 6 × 108 m–3. On doping with a certain impurity, electron concentration increases to 8 × 1012 m–3. (i) Identify the new semiconductor obtained after doping. (ii) Calculate the new hole concentration. (iii) How does the energy gap vary with doping?

Q.9

Q.10 What is the net charge on (i) p-type (ii) n-type semiconductor? Q.11

Which of the diodes are forward biased and which are reverse biased? Give reason.

+5V

+10V

R – 10V

+5V

(i)

(iii)

(ii)

– 12V

R

R (iv)

mk

– 5V

(v)


– 10V

PAGE # 23

Q.12

Two semiconductor materials A and B made by doping Ge crystals with As and In respectively. The two are joined end to end and connected to a battery as shown: (i) Will the junction be forward biased or reverse biased? (ii) Sketch V–I graph for this arrangement. A

B

Q.13

When a forward bias is applied to a p-n junction, it (A) raises the potential barrier. (B) reduces the majority carrier current to zero. (C) lowers the potential barrier. (D) none of the above.

Q.14

A p-n photodiode is fabricated from a semiconductor with band gap of 2.8 eV. Can it detect a wavelength of 600 nm?

Q.15

What is the phase relationship between collector and base voltage in common emitter configuration.

Q.16

Which type of biasing gives a semiconductor diode a very high resistance?

Q.17

In a CE transistor amplifier, the current gain is 100, input resistance is 1k, output resistance is 10k. Find the voltage gain of the circuit.

Q.18

A transistor has a current gain of 50. If the collector resistance is 5k and the input resistance is 1k. Calculate the output voltage if the input voltage is 0.01 V.

Q.19

In a CE circuit, if VCE is changed by 0.2 V, collector current changes by 0.004 mA. Calculate the output resistance.

Q.20

The current gain for CE amplifier is 59. If the emitter current is 6mA, find the base current and collector current. Board Problems

Q.1

Give the logic symbol of NOR gate.

[2009]

Q.2 (i)

With the help of circuit diagrams, distinguish between forward biasing and reverse biasing of a p-n junction diode. [2009] (ii) Draw V–I characteristics of a p-n junction diode in (a) forward bias (b) reverse bias.

Q.3

With the help of a suitable diagram, explain the formation of depletion region in a p-n junction. How does its width change when the junction is [2009] (i) forward biased and (ii) reverse biased?

mk


PAGE # 24

Q.4

Give a circuit diagram of a common emitter amplifier using an n-p-n transistor. Draw the input and output waveforms of the signal. Write the expression for its voltage gain. [2009]

Q.5

Explain with the help of a circuit diagram how a never diode works as a DC voltage regulator. Draw its I–V characteristics. [2009]

Q.6 (i)

Identify the logic gates marked P and Q in the given logic circuit. A B

P

[2010]

X

Q

(ii) Write down the output at X for the inputs A = 0, B = 0 and A = 1, B = 1. Q.7

Draw a schematic diagram of a step-up transformer. Explain its working principle. Deduce the expression for the secondary to primary voltage in terms of the number of turns in the coils. In an ideal transformer, how is this ratio related to the currents in the two coils? How is the transformer used in large scale transmission and distribution of electrical energy over long distances? [2010]

Q.8 (a) Draw the circuit diagrams of a n-n junction diode in (i) forward bias (ii) reverse bias. How are these circuit used to study the V–I characteristics of a silicon diode? Draw the typical V–I characteristics. (b) What is a light emitting diode (LED)? Mention two important advantage of LEDs over conventional lamps. [2010] Q.9 (a) Draw the circuit arrangement for studying the input and output characteristics of an n-p-n transistor in CE configuration. (b) Current amplification factor. (c) Describe briefly with the help of a circuit diagram how an n-p-n transistor is used to produce self-sustained oscillations. [2010] Q.10 (i)

Identify the logic gates marked P and Q in the given logic circuit.

P Q

X

(ii) Write down the output of x for inputs A = 0, B = 0 and A = 1, B = 1.

mk


[2010]

PAGE # 25

Q.11

Describe briefly with the help of a circuit diagram, how the flow of current carriers in a p-n-p transistor is regulated with emitter base junction forward biased and base collector junction reverse biased. [2012]

Q.12 (a) Explain briefly the principle on which a transistor amplifier works as an oscillator. Draw the necessary circuit diagram and explain its working. [2012] (b) Identify the equivalent gate for the following circuit and write its truth table. A A' Y B

mk

B'


PAGE # 26

Semiconductor

Recommend Documents