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Question Bank No: 2

1. An N-type and a P-type silicon can be obtained by

a)		Boron and phosphorus respectively
b)		Sodium and radium respectively
c)		Phosphorus and boron respectively
d)		Sodium and manganese respectively

2. Germanium an Copper are cooled to 70 K from room temperature, then resistance of

a)		Copper increases while that of Germanium decreases
b)		Copper decreases while that of Germanium increases
c)		Both decreases
d)		Both increases

3. What is the voltage gain in common emitter where input resistance is 5 ohm, load resistance is 40 ohm and current gain is 60?

a)		240
b)		48
c)		840
d)		480

4. Some donor atoms are added to a pure germanium semiconductor to produce a N-type semiconductor of conductivity 5 siemen/cm. The number density of the donor atoms will be (given $μ_{e} = 3900 {cm}^{2} / V - S$ and neglect the contribution of holes to the conductivity)

a)		${5 \times 10}^{15} / {cm}^{3}$
b)		${6 \times 10}^{15} / {cm}^{3}$
c)		${8 \times 10}^{15} / {cm}^{3}$
d)		None of these

5. Relation between lattice parameters and terfacial angles in a triclinic system

a)		a $\neq$ b $\neq$ c
b)		a = b = c, $α$ = $β$ = $γ$ = $90^{0}$
c)		a = b $\neq$ c, $α$ = $β$ = $γ$ $90^{0}$
d)		a $\neq$ b $\neq$ c and $α \neq β \neq γ$ = $90^{0}$

6. P-type semiconductor has acceptor level 57eV above the valance band. The maximum wave length of light required to create a hole is

a)		217.1 $\overset{\circ}{A}$
b)		57 $\times 10^{20} \overset{\circ}{A}$
c)		57 $\times 10^{- 3} \overset{\circ}{A}$
d)		11.6 $\times 10^{- 33} \overset{\circ}{A}$

7. A semiconductor is cooled from $θ_{1} {}^{\circ}C$ to $θ_{2} {}^{\circ}C$ . Its resistance will

a)		First decreases and then increase
b)		Will not change
c)		Decrease
d)		Increase

8. Which one of the following is best conductor of electricity?

a)		Silver
b)		Iron
c)		Copper
d)		Gold

9. When we add two binary numbers 1010110 and 010101, we get a binary number

a)		110111
b)		100000
c)		111111
d)		010000

10. A common emitter transistor has current gain 80. What is the change in collector current, when the change in base current is 250 $μ A$ ?

a)		250/80 $μ A$
b)		(250+80) $μ A$
c)		(250-80) $μ A$
d)		(80 $\times$ 250) $μ A$

11. In a PN - junction diode not connected to any circuit

a)		There is an electric field at the junction from N-side to P-side
b)		Potential on P-side is higher than on N-side
c)		Potential is same everywhere
d)		Electric field at the junction is from P-side to N-side

12. A solar cell can be made from

a)		a thin wafer of Si doped with As
b)		a thin wafer of germanium
c)		a thin wafer of pure gallium arsenide
d)		None of these

13. Transfer ratio $β$ of a transistor is 50. Input resistance of the transistor when used in common emitter configuration is 1 k ohm. Peak value of the collector ac current for an ac input voltage of 0.01 V peak is

a)		1000 $μ A$
b)		500 $μ A$
c)		0.250 $μ A$
d)		0.01 $μ A$

14. For triode which one is correct relation?

a)		$μ$ = ${2 g}_{m} {Xr}_{p}$
b)		$μ$ = $g_{m} / r_{p}$
c)		$μ$ = $g_{m} {Xr}_{p}$
d)		None of these

15. Electrical conductivity of a semiconductor increases when electromagnetic radiations of wavelength shorter than 2480 nm is incident on it. Band gap for the semiconductor is

a)		1.1 eV
b)		0.9 eV
c)		0.7 eV
d)		0.5 eV

16. Number of atoms in a unit cell of a simple cubic crystal is

a)		1
b)		2
c)		3
d)		4

17. Thermionic current is proportional to

a)		$T^{1 / 2}$
b)		$T^{3 / 2}$
c)		$T^{2}$
d)		T

18. Resistivity of a semiconductor is given by

a)		$e^{- 1}$ ( $n_{e} μ_{e} + n_{h} μ_{h}$ )
b)		$e$ ${(n_{e} μ_{e} + n_{h} μ_{h})}^{- 1}$
c)		$e^{- 1} {(n_{e} μ_{e} + n_{h} μ_{h})}^{- 1}$
d)		e ( $n_{e} μ_{e} + n_{h} μ_{h}$ )

19. The thickness P-N junction is of the order of

a)		$10^{- 8}$ cm
b)		$10^{- 6}$ cm
c)		1mm
d)		$10^{- 12}$ cm

20. In a P-type silicon semiconductor, doping is done at an average of one indium atom per ${5 \times 10}^{7}$ silicon atoms. If the number density of atoms in silicon specimen is ${5 \times 10}^{18}$ atom/ $m^{3}$ , then number of indium atoms in silicon per ${cm}^{3}$ will be

a)		${2.5 \times 10}^{30} atoms / {cm}^{3}$
b)		${2.5 \times 10}^{36} atoms / {cm}^{3}$
c)		${1.0 \times 10}^{15} atoms / {cm}^{3}$
d)		${1.0 \times 10}^{13} atoms / {cm}^{3}$

21. Holes exists in

a)		Semiconductors
b)		Insulators
c)		Conductors
d)		None of these

22. A transistor has an $\infty = 0.95$ then $β$ as equal to

a)		0.95
b)		19
c)		1.5
d)		1/19

23. A valanche break down in a semiconductor diode occurs when

a)		forward current exceeds a certain value
b)		reverse bias exceeds a certain value
c)		forward bias exceeds a certain value
d)		the potential barrier is reduced to zero

24. The process of superimposing audio frequency signal on high frequency carries wave is called

a)		amplification
b)		modulation
c)		detection
d)		rectification

25. A LED is used in

a)		forward bias
b)		reverse bias
c)		an unbiased state
d)		none of the above

26. In atransistor circuit the emitter current is 1mA and the collector current is 0.95mA. The base current is

a)		1.95mA
b)		0.05mA
c)		0.95mA
d)		1mA

27. The current gain of a transistor in CE configuration is 100. If the base current changes by 100 $μ A$ , the change in collector current is

a)		20mA
b)		100 $μ A$
c)		2mA
d)		200mA

28. In a common emitter amplifier $β$ = 100, the input resistance $r_{i}$ = 1k $Ω$ and the output resistance $r_{o}$ = 10k $Ω$ . The voltage gain of the amplifier is

a)		10
b)		100
c)		1000
d)		10000

29. The symbolic arrangement given in the figure performs logic function of
phssd18

a)		AND gate
b)		OR gate
c)		XOR gate
d)		NAND gate

30. For use of transistor as an amplifier

a)		the emitter base junction should be forward biased and the collector base junction should be reverse biased
b)		the emitter base junction should be reverse biased and the collector base junction should be forward biased
c)		both collector and emitter junctions should be forward biased
d)		both collector and emitter junctions should be reverse biased

31. The base of a transistor is made thin and is lightly dopped in order to

a)		decrease the base current
b)		increase the base current
c)		save the transmitter from high current
d)		increase the emitter current

32. An oscillator is

a)		like a generator
b)		an amplifier with positive feed back
c)		an amplifier with negative feed back
d)		rectifier

33. In a full wave rectifier circuit operating from 50Hz main, the fundamental frequency of ripple would be

a)		100 Hz
b)		50 Hz
c)		25 Hz
d)		zero

34. Majority carriers in a semiconductor are

a)		holes in n type and electons p -type
b)		electrons in n type and holes in p type
c)		electron both p type and n type
d)		holes in both n and p type

35. With increase in temperature the electric conductivity of intrinsic semiconductor

a)		increases
b)		decreases
c)		first increases and then decreases
d)		first decreases and then increases modulation

36. Zener diode is used for

a)		rectification
b)		amplification
c)		voltage stabilisation
d)		modulation

37. To make a p-type semiconductor from germanium crystal it must br doped with foreign atom where valency is

a)		1
b)		2
c)		3
d)		4

38. We prefer common emitter configuration of a transistor for amplification because

a)		of large current gain and hence power gain
b)		of small current gain
c)		safety in operation
d)		of low power loss

39. When an electric field is applied across a crystal of a semiconductor, the hole drifts

a)		in the direction of the electric field
b)		in the direction opposite to the electric field
c)		in a direction perpendicular to the electric field
d)		does not drift

40. The current obtained from a rectifier circuit is

a)		varying direct current
b)		constant direct current
c)		half current
d)		eddy current

41. The truth table shown is for
phssd6

a)		AND
b)		NAND
c)		XOR
d)		NOT

42. The forbidden energy gap is maximum for a

a)		conductor
b)		semiconductor
c)		insulator
d)		metals

43. The relation between $α$ and $β$ is

a)		$α$ = $β$
b)		$β$ = $\frac{α}{1 - α}$
c)		$β$ = $\frac{1 - α}{α}$
d)		$β$ = $α (1 - α)$

44. In a forward bias V-I characterestic curve rises linearly

a)		after the barrier voltage is overcome
b)		after the barrier voltage reduces to zero
c)		after the break down voltage
d)		after the barrier voltage rises to infinity

45. A semiconductor diode is used to

a)		convert d.c into a.c
b)		convert a.c into d.c
c)		increase the voltage
d)		decrease the voltage

46. When the conductivity of a semiconductor is only due to the breaking of covalent bonds, the semiconductor is called

a)		acceptor
b)		donor
c)		extrinsic
d)		intrinsic