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QUESTION: 1

Reverse bias applied to a junction diode [2003]

Solution:

In reverse biasing, the conduction across the

p-n junction does not take place due to

majority carriers but takes place due to

minority carriers if the voltage of external

battery is large. The size of the depletion

region increases thereby increasing the

potential barrier.

QUESTION: 2

In semiconductors, at room temperature [2004]

Solution:

In semiconductors, the conduction band is

empty and the valence band is completely

filled at 0 K. No electron from valence band

can cross over to conduction band at 0K.

But at room temperature some electrons in

the valence band jump over to the

conduction band due to the small forbidden

gap, i.e. 1 eV.

QUESTION: 3

The peak voltage in the output of a half-wavediode rectifier fed with a sinusoidal signalwithout filter is 10V. The d.c. component of theoutput voltage is [2004]

Solution:

QUESTION: 4

The output of OR gate is 1 [2004]

Solution:

Output will be one if A or B or both are one.

Y= A + B

QUESTION: 5

In a p-n junction photocell, the value of thephoto-electromotive force produced bymonochromatic light is proportional to [2004]

Solution:

Electromotive force depends upon intensity

of light falling, it does not depend on

frequency of barrier voltage

QUESTION: 6

Of the diodes shown in following diagrams, which one is reverse biased ? [2004]

Solution:

Positive terminal is at lower potential (0V)

and negative terminal is at higher potential

+5 V

QUESTION: 7

Choose the only false statement from thefollowing [2005]

Solution:

(a) is true as in case of conductors either the

conduction & valence band overlap or

conduction band is partially filled.

(b) is true as insulators have energy gap of

the order of 5 to 10 eV.

(c) is false as resistivity (opposite of

conductivity) decreases with increase in

temperature.

(d) is true as with increase in temperature

more and more electrons jump to the

conduction band. So, conductivity increases.

QUESTION: 8

Carbon, silicon and germanium atoms have fourvalence electrons each. Their valence andconduction bands are separated by energy bandgaps represented by (E_{g})_{C}, (E_{g})_{Si} and (E_{g})Gerespectively. Which one of the followingrelationships is true in their case? [2005]

Solution:

Due to strong electronegativity of carbon

QUESTION: 9

Application of a forward bias to a p–n junction [2005]

Solution:

Number of donor s is more because

electrons from –ve terminal of the cell

pushes (enters) the n side and decreases

the number of uncompensated pentavalent

ion due to which potential barrier is

reduced. The neutralised pentavalent atom

are again in position to donate electrons.

QUESTION: 10

Zener diode is used for [2005]

Solution:

At a certain reverse bias voltage, zener diode

allows current to flow through it and hence

maintains the voltage supplied to any load

Hence, it is used for stabilisation.

QUESTION: 11

The following figure shows a logic gate circuit with two inputs A and B and the output C. The voltage waveforms of A, B and C are as shown below

The logic circuit gate is [2006]

Solution:

On the basis of given graph, following table

is possible.

It is the truth table of AND gate.

QUESTION: 12

A transistor is operated in common emitterconfiguration at constant collector voltage Vc =1.5V such that a change in the base current from100 μA to 150 μA produces a change in thecollector current from 5 mA to 10 mA. The currentgain (β) is [2006]

Solution:

QUESTION: 13

A forward biased diode is [2006]

Solution:

In forward biasing of a diode, the emitted

should be at a higher potential, Here, only in

option (c) emitter is at higher potential with

bias voltage 0V compared to the collector,

which is at –2V. So, it is reverse biased.

QUESTION: 14

The radius of germanium (Ge) nuclide ismeasured to be twice the radius of ^{9}_{4}Be . Thenumber of nucleons in Ge are [2006]

Solution:

We use the formula,

R = R_{0} A^{1/3}

This represents relation between atomic mass

and radius of the nucleus.

QUESTION: 15

In the following circuit, the output Y for all possible inputs A and B is expressed by the truth table [2007]

Solution:

Therefore truth table :

QUESTION: 16

A common emitter amplifier has a voltage gainof 50, an input impedance of 100Ω and an outputimpedance of 200Ω. The power gain of theamplifier is [2007]

Solution:

QUESTION: 17

For a cubic crystal structure which one of thefollowing relations indicating the cellcharacteristics is correct? [2007]

Solution:

For a cubic crystal,

a = b = c and α = β = γ = 90°

QUESTION: 18

In the energy band diagram of a material shown below, the open circles and filled circles denote holes and electrons respectively. The material is [2007]

Solution:

For a p-type semiconductor, the acceptor

energy level, as shown in the diagram, is

slightly above the top E_{v} of the valence band.

With very small supply of energy an electron

from the valence band can jump to the level

E_{A} and ionise acceptor negatively.

QUESTION: 19

The voltage gain of an amplifier with 9% negativefeedback is 10. The voltage gain withoutfeedback will be [2008]

Solution:

Negative feedback is applied to reduce the

output voltage of an amplifier. If there is no

negative feedback, the value of output

voltage could be very high. In the options

given, the maximum value of voltage gain is

100. Hence it is the correct option.

QUESTION: 20

A p-n photodiode is made of a material with aband gap of 2.0 eV. The minimum frequency ofthe radiation that can be absorbed by the materialis nearly [2008]

Solution:

E_{g} = 2.0 eV = 2 × 1.6 × 10^{–19} J

E_{g} = h_{v}

QUESTION: 21

The circuit

is equivalent to

Solution:

Let A and B be inputs and Y the output

Then Y_{1}

Hence, the given circuit is equivalent to a

NOR gate.

QUESTION: 22

The symbolic representation of four logic gates are given below : [2009]

The logic symbols for OR, NOT and NAND gates are respectively:

Solution:

(iv), (ii), (i)

Option (iii) represents AND gate.

QUESTION: 23

A p-n photodiode is fabricated from a semiconductor with a band gap of 2.5 eV. It candetect a signal of wavelength [2009]

Solution:

The wavelength detected by photodiode

should be less than λ_{max}. Hence it can

detect a signal of wavelength 4000Å.

QUESTION: 24

A transistor is operated in common-emitterconfiguration at V_{c} = 2 V such that a change inthe base current from 100 μA to 200 μA producesa change in the collector current from 5 mA to10 mA. The current gain is [2009]

Solution:

QUESTION: 25

The number of beta particles emitted by aradioactive substance is twice the number ofalpha particles emitted by it. The resultingdaughter is an [2009]

Solution:

Isotopes of an element have the same atomic

number but different mass number. A

radioactive substance when emits one alpha

particles its mass number reduces

by 4 and charge no. reduces by 2 and after

emission of two β-particles its charge no.

increase by 2 thus the charge no. i.e. atomic

number remains the same.

### NCERT Solutions - Semiconductor Electronics

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