IISER Physics - Electronic Devices
Exam Duration: 45 Mins Total Questions : 30
For a germanium sample, traces of gallium are added as an impurity. The resultant sample could behave like
- (a)
a conductor
- (b)
an n-type semiconductor
- (c)
a p-type semiconductor
- (d)
an insulator
The conductivity of a semiconductor increases with increase in temperature, because
- (a)
number density of free current carriers increases
- (b)
relaxation time increases
- (c)
both number density of carriers and relaxation time increase
- (d)
number density of current carriers increases, relaxation time decreases but effect of decrease relaxation time is much less than increase in number density.
For pure Ge semiconductor, quantity of electron and hole is 1019 e/m3 . If we doped a donor impurity in it with density 1023 e/m3, then quantity of hole (e/m3) in a semiconductor is
- (a)
1015
- (b)
1019
- (c)
1023
- (d)
1027
Mobility of electrons in n-type Ge is 5000 cm2 /V-s and conductivity is 5 mho/cm. If the effect of holes is negligible, then impurity concentration will be
- (a)
6.25 x 1015/cm3
- (b)
9.25 x 1014/cm3
- (c)
6 x 1013/cm3
- (d)
9 x 1013/cm3
In sample of pure silicon 1023 e/m3 is mixed to phosphorous. If all atoms are active, then what will be the resistivity at 20oC, if mobility of electron is 1200 cm2/V-s?
- (a)
0.5209 ohm-cm
- (b)
5.209 ohm-cm
- (c)
52.09 ohm-cm
- (d)
520.9 ohm-cm
What will be conductivity of pure silicon crystal at 300 K temperature, if electron-hole pairs per cm3 are 1.072 x 1010, \(\mu_e\) = 1350 cm2/V-s and \(\mu_e\) = 480 cm2/V-s ?
- (a)
3.14 x 10-6 mho/cm
- (b)
3 x 106 mho/cm
- (c)
10-6 mho/cm
- (d)
106 mho/cm
When a p-n junction diode is reverse biased
- (a)
electrons and holes are attracted towards each other and move towards the depletion region.
- (b)
electrons and hole move away from the junction depletion region.
- (c)
height of the potential barrier decreases.
- (d)
no change in the current takes place.
Ripple factor of half-wave rectifier is
- (a)
1.21
- (b)
1.6
- (c)
1.5
- (d)
1.4
In an n-p-n transistor circuit, the collector current is 10 mA. If 90% of the electrons emitted, reach the collector, the emitter current(IE) and base current (IB) are given by.
- (a)
IE = 1 mA; IB=11 mA
- (b)
IE = 11 mA; IB=1 mA
- (c)
IE = -1 mA; IB=9 mA
- (d)
IE = 9 mA; IB=-1 mA
A transistor has \(\beta = 40\). A change in base current of \(100 \ \mu A\), produce a change in collector current.
- (a)
\(40 \times 100 \ \mu A\)
- (b)
\(100 -40 \ \mu A\)
- (c)
\(40 + 100 \ \mu A\)
- (d)
\(40 \times 100 \ \mu A\)
The current gain of a transistor in a common emitter configuration is 40. If the emitter current is 8.2 mA, then base current is
- (a)
0.02 mA
- (b)
0.2 mA
- (c)
2.0 mA
- (d)
0.4 mA
Three amplifier stages, each with a gain of 10 are cascaded. The overall gain is
- (a)
10
- (b)
30
- (c)
1000
- (d)
100
The dominant mechanisms for motion of charge carriers in forward and reverse biased silicon p-n junction are
- (a)
drift in forward biased, diffusion in reverse biased
- (b)
diffusion
in forward biased, drift in reverse biased - (c)
diffusion
in both forward and reverse biased - (d)
drift in both forward and reverse biased
If copper and germanium are heated by 50oC from room temperature, then
- (a)
resistance of copper increases while that of germanium decreases
- (b)
resistance of copper decreases while that of germanium increases
- (c)
resistance of both decreases
- (d)
resistance of both increases
Match the categories of semiconductor given in column I with the words given in column I with the words given in column II and select the correct option from the choices given below
Column I | Column II |
(A) Intrinsic semiconductor | Si or Ge doped with trivalent impurity |
(B) p-type semiconductor | Conductivity due to internal charge carriers |
(C) n-type semiconductor | Si or Ge doped with pentavalent impurity |
- (a)
A-1, B-3, C-2
- (b)
A-1, B-2, C-3
- (c)
A-2, B-3, C-1
- (d)
A-2, B-1, C-3
Th current voltage relation of diode is given by, I=(e1000V/T - 1) mA, where the applied voltage V is in volt and the temperature T is in kelvin. If a student makes an error measuring \(\pm 0.01 \ V\) while measuring the current of 5 mA at 300 K, what will be the error in the value of current in mA?
- (a)
0.2 mA
- (b)
0.02 mA
- (c)
0.5 mA
- (d)
0.05 mA
The I-V characteristic of an LED is
- (a)
- (b)
- (c)
- (d)
Carbon, silicon and germanium have four valence electrons each. At room temperature, which one of the following statement is most appropriate?
- (a)
The number of free conduction electrons is significant in C but small in Si and Ge
- (b)
The number of free conduction electrons is negligibly small in all three
- (c)
The number of free electrons for conduction is significant in all the three
- (d)
The number of free electrons for conduction is significant only in Si and Ge but small in C
In a common-base mode of a transistor, the collector current is 5.60 mA. The value of the base current amplification factor \((\beta)\) will be,
- (a)
49
- (b)
50
- (c)
51
- (d)
48
A solid which is not transparent to visible light and whose conductivity increases with temperature is formed by,
- (a)
ionic bonding
- (b)
covalent bonding
- (c)
van der Waals bonding
- (d)
metallic bonding
If the ratio of the concentration of electrons to that of holes in a semiconductor is 7/5 and the ratio of currents is 7/4, then what is the ratio of their drift velocities?
- (a)
\(\frac{5}{8}\)
- (b)
\(\frac{4}{5}\)
- (c)
\(\frac{5}{4}\)
- (d)
\(\frac{4}{7}\)
In the following, which one of the diodes is reverse biased?
- (a)
- (b)
- (c)
- (d)
The electric conductivity of a semiconductor increases when electromagnetic radiation of wavelength shorter than 2480 nm, is incident on it. The band gap in (eV) for the semiconductor is
- (a)
1.1 eV
- (b)
2.5 eV
- (c)
0.5 eV
- (d)
0.7 eV
In a common base amplifier, the phase difference between the input signal voltage and output voltage is
- (a)
\(\pi/4\)
- (b)
\(\pi\)
- (c)
zero
- (d)
\(\pi/2\)
In a full-wave rectifier circuit operating from 50 Hz mains frequency, the fundamental frequency in the ripple would be,
- (a)
50 Hz
- (b)
25 Hz
- (c)
100 Hz
- (d)
70.7 Hz
When an n-p-n transistor is used as an amplifier,
- (a)
electrons move from base to collector
- (b)
holes move from emitter to base
- (c)
electrons move from collector to base
- (d)
holes move from base to emitter
For a transistor amplifier in common-emitter configuration for load impedance \(1 \ k \Omega (h_{fe}=50\) and \(h_{oe}=25 \ \ \mu A/V)\) , the current gain is
- (a)
- 5.2
- (b)
- 15.7
- (c)
- 24.8
- (d)
-48.78
A piece of copper and another of germanium are cooled from room temperature to 77 K, the resistance of
- (a)
each of them increases
- (b)
each of them decreases
- (c)
copper decreases and germanium increases
- (d)
copper increases and germanium decreases
When p-n junction diode is forward biased, then
- (a)
the depletion region is reduced and barrier height is increased
- (b)
the depletion region is widened and barrier height is reduced
- (c)
both the depletion region and barrier height are reduced
- (d)
both the depletion region and barrier height are increased
The difference in the variation of resistance with temperature in a metal and a semiconductor arises essentially due to the difference in the
- (a)
crystal structure
- (b)
variation of the number of charge carriers with temperature
- (c)
type of bonding
- (d)
variation of scattering mechanism with temperature