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DMPS public school

Chapter - 14 Semiconductor Electronics

CLASS 12 - PHYSICS

Section A a) Forward biasing, 5 mA

b) Forward biasing, 0 A
1) To produce n - type crystal, Ge or Si may be doped c) Reverse biasing, 2 mA
with a substance that is [1] d) Reverse biasing, 0 A
a) Tetravalent b) Divalent
c) Pentavalent d) Trivalent 10) The formation of depletion region in a p - n junction
diode is due to [1]
2) Ge is doped with As. Due to doping, [1]
a) Movement of dopant atoms
a) The number of conduction electrons increases. b) Drift of electrons only
b) The number of conduction electrons decreases. c) Diffusion of both electrons and holes
c) The number of holes increases. d) Drift of holes only
d) The structure of Ge lattice is distorted.
11) Current through the ideal diode is:
3) In the energy - band diagram of n - type Si, the gap
between the bottom of the conduction band EC and the
donor energy level ED is of the order of: [1]
a) 0.01 eV b) 10 eV
c) 1 eV d) 0.1 eV [1]
a) 20 A b) ( 20
1
) A
4) Application of a forward bias to p - n junction [1]
c) Zero d) ( 50
1
) A
a) Increases the number of donors on the n side
b) Increases the potential difference across the depletion 12) Which one of the following elements will require the
A K-
zone highest energy to take out an electron from them?
c) Widens the depletion zone Pb, Ge, C and Si [1]
a) C b) Ge
d) Decreases the electric field in the depletion zone
c) Pb d) Si
5) In an extrinsic semiconductor, the number density of holes
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is 4× 1020 m - 3 . If the number density of intrinsic car- 13) At 0 K, the resistivity of an intrinsic semiconductor is:
riers is 1.2 × 1015 m - 3 , the number density of electrons [1]
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in it is [1] a) Same as that at 0o C

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a) 2.4× 1010 m - 3 b) 3.2× 1010 m - 3

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b) Zero
c) 1.8× 10 m 9 - 3
d) 3.6× 109 m - 3 c) Infinite
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d) Same as that at 300 K

6) In forward bias the width of depletion layer [1]
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14) In n - type semiconductor majority carriers and minority

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a) Independent of V
carriers are respectively [1]
b) Dependent of V
c) Decreases with increase in V a) Germanium and silicon
d) Increases with increase in V b) Aluminum and boron
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c) Holes and electrons

7) At a certain temperature in an intrinsic semiconductor, d) Electrons and holes
A

the electrons and holes concentration is 1.5× 1016 m - 3 .

When it is doped with a trivalent dopant, hole concen- 15) An intrinsic semiconductor, at the absolute zero tempera-
tration increases to 4.5 × 1022 m - 3 . In the doped ture, behaves like a/an [1]
semiconductor, the concentration of electrons (ne ) will be: a) Insulator b) Superconductor
[1] c) N - type semiconductor d) P - type semiconductor
a) 5× 109 m - 3 b) 3× 106 m - 3 16) For the forward biasing of a p - n junction diode, which
c) 5× 107 m - 3 d) 6.75× 1038 m - 3
of the following statements isnot correct? [1]
8) The example of p - type semiconductor is [1] a) Forward current is due to the diffusion of both holes
a) Germanium doped with arsenic and electrons.
b) Pure germanium b) Minority carrier injection occurs.
c) Germanium doped with boron c) The potential barrier decreases.
d) Pure silicon d) Width of depletion layer increases.
17) A forward biased diode is [1]
9) The threshold voltage for a p - n junction diode used in
the circuit is 0.7 V. The type of biasing and current in a)
the circuit are:
b)
c)
d)
18) At equilibrium, in a p - n junction diode the net current
[1] is [1]
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a) Due to drift of minority charge carriers [1]

b) Due to diffusion of majority charge carriers
c) Zero as diffusion and drift currents are equal and a)
opposite
d) Zero as no charge carriers across the junction b)
19) In an unbiased p - n junction, holes diffuse from the p
- region to n - region because: [1] c)
a) All of these
b) They move across the junction by the potential dif-
ference d)
c) Hole concentration in p - region is more as compared 24) The current in the circuit will be
to n - region
d) Free electrons in the n - region attract them
20) The conductivity of a metal decreases with the increase
in temperature on account of [1]
a) Decrease in relaxation time [1]
b) Decrease in resistivity a) 5
10 A b) 5
50 A
c) Increase in mean free path c) 5
20 A d) 5
40 A
d) Decrease in number density of electrons
25) The given circuit has two ideal diodes connected as
21) In the following figure, the diodes which are forward shown in the figure below. The current flowing through
biased, are

i. the resistance R1 will be

[1]
a) 2.5 A b) 10.0 A
c) 1.43 A d) 3.13 A
A K-
ii.
26) Assertion (A): In n type semiconductor, number density
of electrons is greater than the number density of holes
iii. but the crystal maintains an overall charge neutrality.
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Reason (R): The charge of electrons donated by donor
atoms is just equal and opposite to that of the ionised
donor. [1]
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iv.
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R
[1] a) Both Assertion (A) and Reason (R) are true and Rea-
a) A, C and D b) B and C son (R) is the correct explanation of the Assertion
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c) C and A d) C only (A).

b) Both Assertion (A) and Reason (R) are true, but
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22) If in a p - n junction, a square input signal of 10 V is

Reason (R) isnot the correct explanation of the As-
applied as shown,
sertion (A).
c) Assertion (A) is true, but Reason (R) is false.
d) Assertion (A) is false and Reason (R) is also false.
D

27) Assertion (A):Silicon is preferred over germanium for

A

then the output across RL will be [1] making semiconductor devices.

Reason (R):The energy gap for germanium is more than
the energy gap for silicon. [1]
a)
a) Both Assertion (A) and Reason (R) are true and Rea-
b) son (R) is the correct explanation of the Assertion
(A).
b) Both Assertion (A) and Reason (R) are true, but
Reason (R) is not the correct explanation of the
c)
Assertion (A).
c) Assertion (A) is true, but Reason (R) is false.
d) d) Assertion (A) is false and Reason (R) is also false.
23) In the half wave rectifier circuit shown which one of the 28) Assertion (A): The resistance of an intrinsic semiconduc-
following wave forms is true for VCD , the output across tor decreases with increase in its temperature.
C and D? Reason (R): The number of conduction electrons as well
as hole increase in an intrinsic semiconductor with rise
in its temperature. [1]
a) Both Assertion (A) and Reason (R) are true and (R)
is the correct explanation of (A).
b) Both Assertion (A) and Reason (R) are true and (R)
is NOT the correct explanation of (A).
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c) Assertion (A) is true and Reason (R) is false.

d) Assertion (A) is false and Reason (R) is also false.

29) Assertion (A): A p - n junction cannot be formed by

joining a slab of p - type semiconductor and a slab of
n - type semiconductor.
Reason (R): There will not be continuous contact at the
[2]
atomic level between the p - type slab and the n - type
slab. [1] 38) A semiconductor has equal electron and hole concentration
of 2× 108 m - 3 . On doping with a certain impurity, the
hole concentration increases to 4 × 1010 m - 3 .
a) Both A and R are true and R is the correct expla-
i. What type of semiconductor is obtained on doping?
nation of A.
ii. Calculate the new electron concentration of the semi-
b) Both A and R are true but R is not the correct
conductor.
explanation of A.
iii. How does the energy gap vary with doping?
c) A is true but R is false.
[2]
d) A is false but R is true.
39) Distinguish between intrinsic and extrinsic semiconductors.
Although in an extrinsic semiconductor ne ̸= nh , yet it
30) Assertion (A): The direction of diffusion current in a is electrically neutral. Why? [2]
junction diode is from n - region to p - region. 40) A semiconductor has the electron concentration of 8×
Reason (R): The majority current carriers diffuse from 1013 cm - 3 and hole concentration of 4 × 1013 cm - 3 .
a region of higher concentration to a region of lower Is the semiconductor p - type or n - type? Also calcu-
concentration. [1] late the resistivity of this semiconductor. Given electron
mobility = 24,000 cm2 V - 1 s - 1 and hole mobility =
a) Both A and R are true and R is the correct expla- 200 cm2 V - 1 s - 1 . [2]
nation of A.
b) Both A and R are true but R is not the correct Section C
explanation of A. 41) Draw V - I characteristics of a p - n junction diode.
A K-
c) A is true but R is false. Answer the following giving reasons:
d) A is false but R is true. i. Why is the reverse bias current almost independent
of applied voltage up to breakdown voltage?
Section B ii. Why does the reverse current show a sudden increase
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at breakdown voltage?
[3]
31) Explain the variation of resistivity with temperature in 42) Draw the energy band diagrams (at T > 0K) for n - type
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pure - semiconductors. [2]

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and p - type semiconductors. Using diagram, explain

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32) Two crystals C1 and C2 , made of pure silicon, are doped why in n - type semiconductor the conduction band has
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with arsenic and aluminium respectively. most electrons from the donor impurities. [3]
i. Identify the extrinsic semiconductors so formed. 43) Determine the currents through the resistances of the cir-
ii. Why is doping of intrinsic semiconductors necessary?
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cuits shown in figure.

[2]
33) Briefly explain how a potential barrier is set up across a
p - n junction as a result of diffusion and drift of the
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charge carriers. [2]

34) Draw the energy band diagram of (i) n - type, and (ii)
A

p - type semiconductors at temperature T > 0 K.

In the case of n - type Si - semiconductor, the donor
energy level is slightly below the bottom of conduction
band whereas in p - type semiconductor, the acceptor
energy level is slightly above the top of valence band.
Explain, giving examples, what role do these energy levels
play in conduction and valence bands. [2] [3]
35) Explain the termsdepletion layer and potential barrier in a 44) The black box, shown here, converts the input voltage
p - n junction diode. How are the (a) width of depletion waveform into the output voltage waveform as is shown
layer, and (b) value of potential barrier affected when the in the figure:
p - n junction is forward biased? [2]
36) The (ratios) of number density of free electron to
holes, nh , for two different materials A and B, are
ne
Draw the circuit diagram of the circuit present in the
equal to one and less than one respectively. Name the black box and give a brief description of its working. [3]
type of semiconductors to which A and B belong. Draw 45) A battery of V may be connected across points A and
energy level diagram for A and B. [2] B, as shown in the figure. Find the current drawn from
37) The V - I characteristic of a silicon diode is as shown the battery if the positive terminal is connected to
in the figure. Calculate the resistance of the diode at i. The point A and
i. ID = 15 mA and ii. The point B
ii. VD = - 10V Assume that the resistance of each diode is zero in for-
ward bias and infinity in reverse bias.
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51) i. With the help of a circuit diagram, briefly explain

the working of a full - wave rectifier using p - n
junction diodes.
ii. Draw V - I characteristics of a p - n junction
diode. Explain how these characteristics make a
diode suitable for rectification.
iii. Carbon and silicon have the same lattice structure.
Then why is carbon an insulator but silicon a semi-
conductor?
[5]
[3]
46) The following figure shows the V - I characteristics of
a semiconductor diode. Section E
i. Identify the semiconductor diode used.
ii. Draw the circuit diagram to obtain the given char- 52) Read the text carefully and answer the questions: Semi-
acteristics of this device. conductor diodes in logic gates: These are also used to
perform logic operations. Both small and large states of
logic gates are comparable with forward and reverse the
biased state of semiconductor diodes. Thus diodes help in
OR, NAND, AND gates. Classification of various diodes
are given below: -

iii.
[3]
47) An a.c. the signal is fed into two circuits X and Y
and the corresponding output in the two cases have the
waveforms shown in the figure. Name the circuits X and
A K-
Y. Also draw their detailed circuit diagrams. [4]

(a) A crystal diode has .

a) Two PN junction b) None of these
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[3] c) One PN junction d) Three PN junction

Section D
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(b) If the temperature of a crystal diode increases, then

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leakage current?
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48) i. Draw the circuit diagram used to study I - V char-
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acteristics of a p - n junction diode in conducting a) None of these b) Increase

mode. Mark on the graph the threshold voltage of c) Decreases d) Same
the diode. Explain the significance of this voltage.
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ii. In the circuit shown in the figure, the forward volt- (c) Semiconductor diode is used as?
age drop across the diode is 0.3 V. Find the voltage a) All of these b) Rectifier
difference between A and B. c) Oscillator d) Modulator
D

(d) When a junction diode is reverse biased, what causes

A

current across the junction?

a) Drift of charges
b) Both diffusion and drift of charges
c) None of these
d) Diffusion of charges
[5]
49) i. In the following diagram, is the junction diode for-
OR
ward biased or reverse biased?
(e) Diode is a device.
ii. Draw the circuit diagram of a full wave rectifier and a) Bidirectional b) None of these
state how it works? c) Non - linear d) Unidirectional
[5]
50) i. Briefly describe the classification of solids into met- 53) Read the text carefully and answer the questions: There
als, insulators and semi - conductors on the basis are various types of diodes and these diodes are used
of energy level diagrams. in numerous ways. The most basic function would be
ii. In a silicon diode, the current increases from 10 changing AC current to DC current by removing some
mA to 20 mA when the voltage changes from 0.6 part of the signal. This functionality would make them
V to 0.7 V. Calculate the dynamic resistance of the rectifiers. They are used in electrical switches and are
diode. used in surge protectors because they can prevent a spike
[5] in the voltage.
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with a resistor of resistance R, ideal millimeter and key

K as shown in the figure. When the key is pressed, a
current of 20 mA passes through the diode.

[4]
(a) How many types of diode rectifiers are used in [4]
electronics?
a) 4 b) 3 (a) The intensity of the electric field in the depletion
c) 5 d) 2 region when p - n junction is unbiased is
a) 1.5 × 106 V m−1
(b) The maximum efficiency of half wave rectifier diode b) 1.0× 106 V m- 1
is? c) 2.0 × 106 V m−1
a) 40.6% b) 60.6% d) 0.5× 106 V m- 1
c) 50.6% d) 70.6%
(b) The resistance of resistor R is
(c) The average value of full wave rectifier is a) 300Ω b) 130Ω
rather than half wave rectifier. c) 150Ω d) 180Ω
a) None of these b) Half
c) Same d) Full (c) In a p - n junction, the potential barrier is due
to the charges on either side of the junction, these
(d) In full wave rectification, if input frequency is 50 charges are
Hz then output frequency is?
a) 200 Hz b) 150 Hz a) Minority carriers
c) 100 Hz d) 50 Hz b) Both majority carriers and minority carriers
c) Majority carriers
d) Fixed donor and acceptor ions
OR
A K- (d) If the voltage of the potential barrier is V0 . A
(e) Ripple factor of full wave rectifier is? voltage V is applied to the input, at what moment
a) 0.48 b) 0.38 will the barrier disappear?
c) 0.28 d) 0.58 a) V « V0 b) V < V0
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c) V > V0 d) V = V0
54) Read the text carefully and answer the questions: The
potential barrier in the p - n junction diode is the barrier
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in which the charge requires additional force for crossing OR

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the region. In other words, the barrier in which the
(e) If an electron with speed4.0×105 m s−1 approaches
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charge carrier stopped by the obstructive force is known

the p - n junction from the n - side, the speed
as the potential barrier. When a p - type semiconductor
with which it will enter the p - side is
is brought into close contact with n - type semiconductor,
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we get a p - n junction with a barrier potential 0.4 V a) 1.39 × 106 m s−1

and the width of the depletion region is 4.0× 10 - 7 m. b) 2.78 × 106 m s−1
This p - n junction is forward biased with a battery of c) 2.78 × 105 m s−1
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voltage 3 V and negligible internal resistance, in series d) 1.39 × 105 m s−1

A