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151.

Discuss the effect of external voltage on the width of depletion region of a p-n junction.

Answer»

1. A p-n junction can be connected to an external voltage supply in two possible ways.

2. A p-n junction is said to be connected in a forward bias when the p-region connected to the positive terminal and the n-region is connected to the negative terminal of an external voltage source. 

3. In forward bias connection, the external voltage effectively opposes the built-in potential of the junction. The width of depletion region is thus reduced. 

4. The second possibility of connecting pn junction is in reverse biased electric circuit. 

5. In reverse bias connection, the p-region is connected to the negative terminal and the n-region is connected to the positive terminal of the external voltage source. This external voltage effectively adds to the built-in potential of the junction. The width of potential barrier is thus increased.
Discussion
152.

A pn junction diode in which light in allowed to fall in its junction isA. zener diodeB. LEDC. solar cellD. photo diode

Answer» Correct Answer - D
Discussion
153.

Explain the importance of the depletion region in a p-n junction diode.

Answer»

i. The region across the p-n junction where there are no charges is called the depletion layer or the depletion region. 

ii. During diffusion of charge carriers across the junction, electrons migrate from the nside to the p-side of the junction. At the same time, holes are transported from p-side to nside of the junction. 

iii. As a result, in the p-type region near the junction there are negatively charged acceptor ions, and in the n-type region near the junction there are positively charged donor ions.

iv. The potential barrier thus developed, prevents continuous flow of charges across the junction. A state of electrostatic equilibrium is thus reached across the junction.

v. Free charge carriers cannot be present in a region where there is a potential barrier. This creates the depletion region.

vi. In absence of depletion region, all the majority charge carriers from n-region (i.e., electron) will get transferred to the p-region and will get combined with the holes present in that region. This will result in the decreased efficiency of p-n junction.

vii. Hence, formation of depletion layer across the junction is important to limit the number of majority carriers crossing the junction.
Discussion
154.

Distinguish between intrinsic semiconductors and extrinsic semiconductors

Answer»
Intrinsic semiconductorsExtrinsic semiconductors
1. A pure semiconductor is known as intrinsic semiconductors.The semiconductor, resulting
2. Their conductivity is lowTheir conductivity is high even at room temperature.
3. Its electrical conductivity is a function of temperature alone.Its electrical conductivity depends upon the temperature as well as on the quantity of impurity atoms doped in the structure.
4. The number density of holes (nh) is same as the number density of free electron (ne) (nh = ne)The number density of free electrons and number density of holes are unequal.
Discussion
155.

Intrinsic semiconductor is electrically neutral. Extrinsic semiconductor having large number of current carriers would beA. Positively chargedB. Negatively chargedC. Positively charged or negatively charged depending upon the type of impurity that has been addedD. Electrically neutral

Answer» Correct Answer - D
Both n-type and p-type semiconductors are electrically neutral.
Discussion
156.

In Boolean algebra, which of the following is not equal to zero :-A. `A.bar(A)`B. A.0C. `bar(A+bar(A))`D. `bar(bar(A).0)`

Answer» Correct Answer - D
Discussion
157.

Which of the following relation is valid in Boolean algebra :-A. A + A=AB. A + 1+ 1C. `A. bar(A) = 0`D. All

Answer» Correct Answer - D
Discussion
158.

Which of the following relation is valid in Boolean algebra :-A. `A + bar(A) = 0`B. `A+A = 2A`C. `A + bar(A) = 1`D. `A + bar(A)=A`

Answer» Correct Answer - C
Discussion
159.

In the Boolean algebra, which gate is expressed as `Y = overline(A + B)`.A. OR gateB. NAND gateC. AND gateD. NOR gate

Answer» Correct Answer - D
Discussion
160.

Which of the following relations is valid for Boolean algebra :-A. `A(B+bar(B))=A`B. `A+AB =A`C. `A+0=A`D. all

Answer» Correct Answer - D
Discussion
161.

The value of `overline A + A` in the Boolean algebra is

Answer» Correct Answer - B
`A+barA=1`
Discussion
162.

In boolean algebra, if `A=1` and `B=0` then the value of `A+barB` isA. AB. BC. `barA`D. `bar(A+B)`

Answer» Correct Answer - A
`A+barB=1+baro=1+1=1`
`therefore A+barB=A`
Discussion
163.

A light emitting diode is shown as A. B. C. D.

Answer» Correct Answer - B
A light emitting diode is represented by the symbol given in (b). (a) is a photodiode, (c) is a junction diode and ( d) is a zener diode.
Discussion
164.

A light emitting diode is shown as A. 3B. 4C. 2D. 1

Answer» Correct Answer - B
Discussion
165.

Statement-I : To be used as amplifier, the transistor in the common emitter confuration is preferred to the common base configuration. Statement-II : In the common emitter, the signal is applied between emitter and base.A. Statetment-I is True, Statement-II is True , Statement-II is a correct explanation for Statement-IB. Statement-I is True, Statement-II is True , Statement-II is NOT a correct explanation for Statement-IC. Statement-I is True, Statement-II is FalseD. Statement-I is False, Statement-II is True

Answer» Correct Answer - B
Discussion
166.

A transistor is used in common emitter mode as an amplifier. Then (1) the base-emitter junction is forward biased (2) the base emitter junction is reverse biased (3) the input signal is connected in series with the voltage applied to the base-emitter junction. (4) the input signal is connected in series with the voltage applied to the base collector junction.A. A,BB. A,DC. A,CD. only C

Answer» Correct Answer - C
Discussion
167.

The on/off time or switching time of light emitting diodes is the order ofA. a micro secondB. a nano secondC. a milli secondD. one second

Answer» Correct Answer - B
Discussion
168.

Barrier potential of Ga-As-P LED isA. 0.7 VB. 1.0 VC. 0.3 VD. 1.5 V

Answer» Correct Answer - D
Discussion
169.

in common emitter amplifier, the emitter base junction isA. forward biasB. reverse biasC. insulatorD. none of these

Answer» Correct Answer - A
Discussion
170.

Reverse bias applied to a junction diodeA. lowers the potential barrierB. decreases the majority charge carriersC. raises the potential barrierD. changes the mass of p-n junction diode

Answer» Correct Answer - C
Reverse bias applied to a p-n junction diode raises the potential barrier because p-type material connected to the negative terminal and pulles the holes away from the junction. Similarly, n-type material connected to positive terminal and pulles the electrons. Therefore, the depletion region widens.
Discussion
171.

A solar cell is p-n junction operating inA. reverse bias conditionB. unbiased conditionC. forward bias conditionD. in both forward and reverse bias condition

Answer» Correct Answer - B
Discussion
172.

The p-n junction which generates an emf when solar radiation falls on it, with no external bias applied, is aA. light emitting diodeB. photodiodeC. solar cellD. zener diode

Answer» Correct Answer - B
The p-n junction which generates an emf when solar radiation falls on it, with no external bias applied is a photodiode.
Discussion
173.

Silicon and Germanium `p`-`n` junction diodes are not used for making `LEDs`A. Silicon dioxideB. Gallium arsenide [Ga As]C. Gallium phosphide (Ga P]D. Gallium arsenide phosphide [Ga As P]

Answer» Correct Answer - B
Discussion
174.

Two junction diodes, one of germanium `(Ge)` and other of silicon `(Si)` are connected as shown in fig to a battery of `12 V` and a load resistance `10 kOmega`. The germanium diode conducts at `0.3 V` and silicon diode at `0.7 V`. When current flows in the circuit, the potential of terminal `Y` will be .A. 12 VB. 11 VC. 11.3 VD. 11.7 V

Answer» Correct Answer - D
Discussion
175.

In the middle of the depletion layer of a reverse - biased `p - n ` junction , theA. potential is zeroB. electric field is maximumC. potential is maximumD. electric field is very very small

Answer» Correct Answer - D
When the p-njunction is reversed biased, the width of the depletion region becomes very large and hence the electric field `[E=V/d]` becomes very small.
In forward bias , `E="dV"/"dx"` is very large
Discussion
176.

In p-n-p transistor the base current is 1 mA and the collector current is 10 mA . The emitter current isA. 9 mAB. 10 mAC. 11 mAD. 12 mA

Answer» Correct Answer - C
`I_(e)=I_(c)+I_(b)=10+1=11 mA`
Discussion
177.

The difference in the working of an amplifier and a step up transformer isA. amplifier increases the power where as the transformer does notB. amplifier decreases the power where as the transformer does notC. amplifier keeps the power constant where as the transformer decreases the powerD. amplifier keeps the power constant where as the transformer increases the power

Answer» Correct Answer - A
Discussion
178.

For a transistor, working as common emitter amplifier , the current gain is 45. What will be the current gain when the same transistor is worked as common base amplifier ?A. 9.8B. 0.98C. 0.098D. 0.0098

Answer» Correct Answer - B
`alpha=(beta)/(beta+1)=(45)/(45+1)=(45)/(46)=0.98`
Discussion
179.

To obtain the current gain (`beta`) of a transistor, when it is in CE mode, we useA. its input characteristicsB. its current transfer characteristicsC. its output characteristicsD. any one of the above three characteristics

Answer» Correct Answer - B
Discussion
180.

The difference in the working of a step up transformer and an amplifier isA. the transformer decreases the power whereas the amplifier keeps the power constantB. the transformer increases the power but the amplifier decreases the powerC. the ampIifier increases the power but the transformer cannot increase the powerD. the amplifier decreases the power but the transformer keeps the power constant

Answer» Correct Answer - C
Discussion
181.

In an oscillator, for sustained oscillations, Barkhausen criterion is `Abeta` equal to (A = voltage gain without feedback and `beta` = feedback factor)A. increases the input voltageB. is always in phase with the input voltageC. is always in antiphase or `180^@` out of phase with the input voltageD. transfers a part of the output energy of the amplifier to the resonating L-C circuit

Answer» Correct Answer - C
Discussion
182.

In an oscillator, for sustained oscillations, Barkhausen criterion is `Abeta` equal to (A = voltage gain without feedback and `beta` = feedback factor)A. zeroB. `(1)/(2)`C. 1D. 2

Answer» Correct Answer - C
According to question, Barklhausen criterion states that if A is the gain of the amplifying element in the circuit and B is the gain of the amplifying element in the circuit and B is the transfer function of the feedback path, then condition of sostained oscillation is given by `|betaA|=1`
So, option (c) is correct.
Discussion
183.

In an `NPN` transistor the collector current is `24 mA`. If `80%` of electrons reach collector it base current in `mA` isA. 36 mAB. 26mAC. 16 mAD. 6mA

Answer» Correct Answer - D
Discussion
184.

Which one of the following is an amorphous solid?A. RubberB. plasticC. GlassD. All of these

Answer» Correct Answer - D
Discussion
185.

The energy gap in a semiconductor is of the order of :-A. 1 eVB. 5 eVC. 10 eVD. 15 eV

Answer» Correct Answer - A
Discussion
186.

The depletion region of a p - n junction is formedA. when it is forward biasedB. when it is reversed biasedC. during the process of its manufactureD. when its temperature is decreased

Answer» Correct Answer - C
Discussion
187.

A p-type semiconductor isA. positively chargedB. made by mixing of impurity of boron in germaniumC. made by mixing of impurity of phosphorus in siliconD. made by mixing of carbon in silicon

Answer» Correct Answer - B
Discussion
188.

A p-n junction diode can not be usedA. as a rectifierB. for amplifying an A.C. signalC. for getting radiation of lightD. as a detection of light intensity

Answer» Correct Answer - B
Discussion
189.

A solar cell works on the principle ofA. photoelectricityB. photographic cameraC. photovoltaic conversionD. photosynthesis

Answer» Correct Answer - C
Discussion
190.

The depletion region of a p-n junction contains :-A. electrons onlyB. electrons and holes bothC. holes onlyD. neither electrons nor holes

Answer» Correct Answer - D
Discussion
191.

A diode converts A.C. voltage intoA. an A.C. voltage with a different peak value .B. a D.C. voltage with a constant valueC. a two directional pulsating voltage with a constant r.m.s. valueD. an unidirectional pulsating voltage that keeps on dropping to zero in between

Answer» Correct Answer - D
Discussion
192.

The most commonly used semiconducting material used to prepare a solar cell isA. Gallium arsenideB. Indium arsenideC. Cadmium arsenideD. Silicon

Answer» Correct Answer - D
Discussion
193.

The p-n junction is a :-A. ohmic resistanceB. non ohmic resistanceC. positive resistanceD. negative resistance

Answer» Correct Answer - B
Discussion
194.

Choose only the wrong statement from the following :A. In conductors the valence and conduction bands may overlapB. Substances withenergy gap of the order of 10 eV are insulatorsC. The resistivity of a semiconductor increases with increase in temperatureD. The conductivity of a semiconductor increases with increase in temperature

Answer» Correct Answer - C
(a),(b) and (d) are correct. (c ) is wrong.
Resistivity of a semiconductor decreases with increase in temperature .
Discussion
195.

Choose the wrong statementA. p-type semiconductor is positively chargedB. n-type semiconductor is negatively chargedC. both p-type and n-type are electrically neutralD. Both A & C

Answer» Correct Answer - A::B::C
Discussion
196.

When value of current increase in P-N junction, then the value of contact potential,A. decreasesB. IncreasesC. remains unchangedD. depends on temperature

Answer» Correct Answer - C
Discussion
197.

An LED is aA. forward biased p-n junction diodeB. reverse biased p-n junction diodeC. photodiodeD. pin diode

Answer» Correct Answer - A
Discussion
198.

A solar cell converts solar energy intoA. heat energyB. chemical energyC. electric energyD. light energy

Answer» Correct Answer - C
Discussion
199.

The contact potential at the junction site in a P-N junction is-A. positive on P side and negative on N sideB. negative on P side and positive on N sideC. remain unchangedD. depends on temperature

Answer» Correct Answer - B
Discussion
200.

Colour of light emitted by LED depends uponA. its forward biasB. its reverse biasC. the material of semiconductorD. the amount of forward or reverse current

Answer» Correct Answer - C
Discussion