RAINBOW CLASSES BY ER WAKHATWER ALI

Nawabganj, Bareilly

SEMICONDUCTOR ELECTRONIC
Class 12 - Physics
Time Allowed: 3 hours Maximum Marks: 140

Section A
Question No. 1 to 5 are based on the given text. Read the text carefully and answer the questions: [5]
Rectifier is a device that is used for converting alternating current or voltage into direct current or voltage. Its working
is based on the fact that the resistance of p-n junction becomes low when forward biased and becomes high when
reverse biased. A half-wave rectifier uses only a single diode while a full wave rectifier uses two diodes as shown in
figures (a) and (b).

V0
1. If the rms value of sinusoidal input to a full wave rectifier is √2
then the rms value of the rectifier's output is
2 –
a) V
0
​ b) √2V
2
0
​
2

2 V0
c) V
0
​ d) ​
√2
√2

2. In the diagram, the input ac is across the terminals A and C. The output across B and D is

a) same b) half wave rectified

c) zero d) full wave rectifier

3. A bridge rectifier is shown in figure. Alternating input is given across A and C. If output is taken across BD, then it is

a) zero b) Full wave rectified

c) half wave rectified d) same as liquid

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4. A p-n junction (D) shown in the figure can act as a rectifier. An alternating current source (V) is connected in the circuit.
The current (I) in the resistor (R) can be shown by

a) b)

c) d)

5. With an ac input from 50 Hz power line, the ripple frequency is

a) 50 Hz in the dc output of half wave and 100 b) 50 Hz in the dc output of half wave as well
Hz in dc output of full wave rectifier as full wave rectifier

c) 100 Hz in the dc output of half wave and 50 d) 100 Hz in the dc output of half wave as well
Hz in the dc output of full wave rectifier. as full wave rectifier

Question No. 6 to 10 are based on the given text. Read the text carefully and answer the questions: [5]
From Bohr's atomic model, we know that the electrons have well defined energy levels in an isolated atom. But due to
interatomic interactions in a crystal, the electrons of the outer shells are forced to have energies different from those in
isolated atoms. Each energy level splits into a number of energy levels forming a continuous band. The gap between the
top of the valence band and the bottom of the conduction band in which no allowed energy levels for electrons can exist
is called the energy gap.

6. In an insulator energy band gap is

a) None of these b) Eg > 3eV

c) Eg < 3eV d) Eg = 0

7. In a semiconductor, separation between conduction and valence band is of the order of

a) 50 eV b) 10 eV

c) 1 eV d) 0 eV
8. Based on the band theory of conductors, insulators and semiconductors, the forbidden gap is smallest in

a) semiconductors b) conductors

c) insulators d) All of these

9. Carbon, silicon and germanium have four valence electrons each. At room temperature which one of the following
statements is most appropriate?

a) The number of free electrons for conduction b) The number of free conduction electrons is
is significant in all the three. significant in C but small in Si and Ge.

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 c) The number of free conduction electrons is d) The number of free electrons for conduction
negligibly small in all the three. is significant only in Si and Ge but small in
C.
10. Solids having highest energy level partially filled with electrons are

a) insulator b) conductor

c) semiconductor d) none of these

Section B
11. What accounts for the flow of charge carriers in forward and reverse biasing of silicon p-n diode? [1]

a) drift in reverse bias and diffusion in forward b) drift in forward bias and diffusion in reverse
bias bias

c) diffusion in both forward and reverse bias d) drift in both reverse and forward bias
12. The energy band gap is maximum in: [1]

a) Metals b) Insulators

c) Superconductors d) Semiconductors
13. If a small amount of antimony is added to germanium crystal [1]

a) it becomes a p-type semiconductor b) the antimony becomes an acceptor atom

c) its resistance is increased d) there will be more free electrons than holes
in the semiconductor
14. When a junction is reversed biased, the flow of current across the junction is mainly due to [1]

a) drift of charges b) none of these

c) both drift and diffusion of the charges d) diffusion of charge carriers

15. The output from a full wave rectifier is [1]

a) a pulsating unidirectional voltage b) a dc voltage

c) zero d) unidirectional voltage having ripples

16. In p-type semiconductor, [1]
A. major current carrier are electrons
B. major carrier are mobile negative ions
C. major carrier are mobile holes
D. the number of mobile holes exceeds the number of acceptor atoms

a) Option C b) Option A

c) Option D d) Option B
17. Current through the ideal diode is: [1]

a) 20 A b) ( 1

20
)A

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 c) zero d) ( 1

50
)A
18. P-N junction can be used as [1]

a) a rectifier b) an oscillator

c) a modulator d) an amplifier
19. The probability of finding an electron in Fermi energy level is: [1]

a) 50% b) 20%

c) 0% d) 100%
20. The unidirectional flow of current through p-n junction makes it ideal to be used as [1]

a) photoelectricity b) modulator

c) oscillator d) rectifier
21. If a p-n diode is reverse biased, then the resistance measured by an ohmmeter will be [1]

a) low b) infinite

c) zero d) high
22. Which of the following p-n junction is forward biased? [1]

a) b)

c) d)

23. A donor impurity results in [1]

a) increase of resistance of the semiconductor b) production of n-semiconductor

c) energy bands just above the filled valency d) production of p-semiconductor

24. Which one of the following represents forward bias diode? [1]

a) b)

c) d)

25. In the half-wave rectifier circuit operating from 50 Hz main frequency, the fundamental frequency in the ripple [1]
would be

a) 70.7 Hz b) 50 Hz

c) 100 Hz d) 25 Hz
Section C
26. Assertion (A): A pure semiconductor has a negative temperature coefficient of resistance. [1]
Reason (R): In a semiconductor on raising the temperature, more charge carriers are released, conductance
increases and resistance decreases.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.

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 c) A is true but R is false. d) A is false but R is true.
27. Assertion (A): The drift current in a p-n junction is from the n-side to the p-side. [1]
Reason (R): It is due to free electrons only.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.

c) A is true but R is false. d) A is false but R is true.

28. Assertion (A): The resistivity of a semi|conductor increases with temperature. [1]
Reason (R): The atoms of a semiconductor vibrate with larger amplitudes at higher temperatures thereby
increasing its resistivity.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.

c) A is true but R is false. d) A is false but R is true.

29. Assertion (A): A pure semiconductor has a negative temperature coefficient of resistance. [1]
Reason (R): On raising the temperature, more charge carriers are released, conductance increases and resistance
decreases.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.

c) A is true but R is false. d) A is false but R is true.

30. Assertion (A): If the temperature of a semiconductor is increased then its resistance decreases. [1]
Reason (R): The energy gap between conduction band and valence band is very small.

a) Both A and R are true and R is the correct b) Both A and R are true but R is not the
explanation of A. correct explanation of A.

c) A is true but R is false. d) A is false but R is true.

Section D
31. What elements other than indium and gallium can be used to form a p-type semiconductor? [1]
32. Why do Ge and Si behave as semiconductors? [1]
33. What type of charge carriers are there in a p-type semiconductor? [1]
34. How much is the resistance of a p-n junction when it is reverse biased? [1]
35. What is the difference between an n-type and a p-type extrinsic semiconductor? [1]
36. State two disadvantages of semiconductor devices. [1]
37. The forbidden energy band of silicon is 1.1 eV. What does it mean? [1]
38. What is an ideal junction diode? [1]
39. Distinguish between 'intrinsic' and 'extrinsic' semiconductors? [1]
40. What is a hole? Which type of doping creates a hole? [1]
41. Write two points of difference between intrinsic and extrinsic semiconductors. [2]
42. How does an increase in doping concentration affect the width of the depletion layer of a p-n junction diode? [2]
43. The V-I characteristic of a silicon diode is as shown in the figure. Calculate the resistance of the diode at [2]
a. ID = 15 mA and

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 b. VD = -10V

44. Two crystals C1 and C2, made of pure silicon, are doped with arsenic and aluminium respectively. [2]

a. Identify the extrinsic semiconductors so formed.

b. Why is doping of intrinsic semiconductors necessary?
45. C, Si, and Ge have the same lattice structure. Why is C insulator while Si and Ge intrinsic semiconductors? [2]
46. What are holes? Give their important characteristics. [2]
47. Mention two advantages and disadvantages of semiconductor devices. [2]
48. Why can't we take one slab of p-type semiconductor and physically join it to another slab of n-type [2]
semiconductor to get a p-n junction?
49. Assuming that the two diodes D1 and D2 used in the electric circuit shown in the figure are ideal, find out the [2]
value of the current flowing through 2.5 Ω resistor

50. In the circuits shown in figure, which one of the two diodes is forward biased and which is reverse biased? [2]

i.

ii.

51. In the following diagrams indicate which of the diodes are forward biased and which are reverse biased. [2]

i.

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 ii.

iii.

iv.

52. For the circuit shown here, find the current flowing through the 1Ω resistor. Assume that the two diodes D1 and [2]

D2 are ideal diodes.

53. With the help of the circuit diagram, explain the working of a diode as a half-wave rectifier. [2]
54. Write the characteristics of a p-n junction which make it suitable for rectification. [2]
55. The figure shows the characteristic curve of a junction diode. Determine the d.c. and a.c. resistance of the diode, [2]
when it operates at 0.3 V.

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

57. Determine the currents through the resistances of the circuits shown in figure. [3]

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58. With the help of a suitable diagram, describe briefly the two important processes involved in the formation of a [3]
p-n junction. Define the terms depletion region and potential barrier.
59. Draw the energy band diagrams for conductors, semiconductors and insulators. Which band determines the [3]
electrical conductivity of a solid? How is the electrical conductivity of a semiconductor affected with rise in its
temperature? Explain.
60. If a small voltage is applied to a p-n junction diode how will the barrier potential be affected when it is [3]
i. forward biased, and
ii. reverse biased? Briefly explain.
61. Write briefly the two important processes that occur during the formation of p-n junction. With the help of [3]
necessary diagrams, explain the terms depletion region and barrier potential.
62. Explain, with the help of a circuit diagram, how the thickness of depletion layer in a p-n junction diode changes [3]
when it is forward biased. In the following circuits which one of the two diodes is forward biased and which is
reverse biased?

i.

ii.

63. i. Explain how a potential barrier is developed in a p-n junction diode. [3]
ii. Draw the circuit arrangement for studying the V-I characteristics of a p-n junction diode in reverse bias. Plot
the V-I characteristics in this case.
64. Draw a circuit diagram of a full-wave rectifier. Explain its working principle. Draw the input/output, wave-forms [3]
indicating clearly the functions of the two diodes used.
65. i. Distinguish between n-type and p-type semiconductors on the basis of energy band diagrams. [3]
ii. Compare their conductivities at absolute zero temperature and at room temperature.
66. Write any two distinguishing features between conductors, semiconductors and insulators on the basis of energy [3]
band diagrams.
67. The following figure shows the V-I characteristics of a semiconductor diode. [3]
i. Identify the semiconductor diode used.
ii. Draw the circuit diagram to obtain the given characteristics of this device.

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 iii. Briefly explain how this diode can be used as a voltage regulator.

68. Draw V-I characteristics of a p-n junction diode. Answer the following questions, giving reasons: [3]
i. Why is the current under reverse bias almost independent of the applied potential upto a critical voltage?
ii. Why does the reverse current show a sudden increase at the critical voltage? Name any semiconductor device
which operates under the reverse bias in the breakdown region.
69. Explain the formation of depletion layer and barrier potential in a p-n junction diode. [3]
70. Draw V-I characteristics of a p-n junction diode. Answer the following giving reasons: [3]
a. Why is the reverse bias current almost independent of applied voltage up to breakdown voltage?
b. Why does the reverse current show a sudden increase at breakdown voltage?
71. Draw a circuit diagram of full-wave rectifier. Explain its working and draw input and output waveforms. [5]
72. An AC signal is fed into two circuits 'X' and 'Y' and the corresponding output in the two cases have the wave [5]
forms as shown.
i. Identify the circuits 'X' and 'Y'. Draw their labelled circuit diagrams.
ii. Briefly explain the working of circuit Y.
iii. How does the output waveform from circuit Y get modified when a capacitor is connected across the output
terminals parallel to the load resistor?

73. i. In the following diagram, is the junction diode forward biased or reverse biased? [5]

ii. Draw the circuit diagram of a full wave rectifier and state how it works?

74. i. Draw the circuit arrangement for studying the I - V characteristics of a p-n junction diode in (a) forward and [5]
(b) reverse bias. Briefly explain how the typical I - V characteristics of a diode are obtained and draw these
characteristics
ii. With the help of the necessary circuit diagram explain the working of a photodiode used for detecting optical
signals.
75. Assume that the silicon diode in the circuit shown in the figure requires a minimum current of 1mA to be above [5]
the knee point (0.7 V) of its I-V characteristics.

Also, assume that the voltage across the diode is independent of the current above the knee point.

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 i. If VB = 5 V, what should be the maximum value of R so that the voltage is above the knee point?
ii. If VB = 5 V, what should be the value of R to establish the current of 5 mA in the circuit?
iii. What is the power dissipated in the resistance R and in the diode, when a current of 5 mA flows in the circuit
at VB = 6 V?

iv. If R = 1 kΩ , what is the minimum voltage VB required to keep the diode above the knee point?

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