Practice Start Test

Semiconductor

NEET Physics · 93 questions · Page 8 of 10 · Click an option or "Show Solution" to reveal answer

Q71
A transistor is operated in common emitter configuration at constant collector voltage V C = 1.5 V such that a change in the base current from 100 μ\mu A to 150 μ\mu A profuces a change in the collector current from 5 mA to 10 mA. The current gain β\beta is
A 50
B 67
C 75
D 100
Correct Answer
Option D
Solution

Current gain, β\beta =

ΔICΔIB{{\Delta {I_C}} \over {\Delta {I_B}}}

=

(10−5)×10−3(150−100)×10−6{{\left( {10 - 5} \right) \times {{10}^{ - 3}}} \over {\left( {150 - 100} \right) \times {{10}^{ - 6}}}}

= 100

Q72
In a p-n junction photo cell, the value of the photo-electromotive force profuced by monochromatic light is proportional to
A The barrier voltage at the p-n junction.
B The intensity of the light falling on the cell.
C The frequency of the light falling on the cell.
D The voltage applied at the p-n junction.
Correct Answer
Option B
Solution

Electromotive force depends on intensity of light that falls on it and does not depend on the frequency.

Q73
Choose the only false statement from the following.
A In conductors the valence and conduction bands overlap.
B Substances with energy gap of the order of 10 eV are insulators.
C The resistivity of a semiconductor increases with increase in temperature.
D The conductivity of a semiconductor increases with increase in temperature.
Correct Answer
Option C
Solution

Option (a) is correct as in conductor conduction and valance band overlap and the conduction band is partially filled Option (b) is correct as insulators have energy gap of 5 - 10 eV.

Option (c) is incorrect as resistivity decreases with increase in temperature.

Option (d) is correct as with increase in temperature, more and more electrons jump to conduction band, hence conductivity increases.

Q74
Zener diode is used for
A amplification
B rectification
C stabilisation
D producing oscillations in an oscillator.
Correct Answer
Option C
Solution

Zener diode is used for stabilisation while p-n junction diode is used for rectification.

Q75
Copper has face centered cubic (fcc) lattice with interatomic spacing equal to 2.54 A∘\mathop A\limits^ \circ . The value of lattice constant for this lattice is
A 2.54 A∘\mathop A\limits^ \circ
B 3.59 A∘\mathop A\limits^ \circ
C 1.27 A∘\mathop A\limits^ \circ
D 5.08 A∘\mathop A\limits^ \circ
Correct Answer
Option B
Solution

Lattice constant for (f.c.c.) = a = interatomic spacing ×\times

2\sqrt 2

= 3.59

A∘\mathop A\limits^ \circ
Q76
Application of a forward bias to a p-n junction
A widens the depletion zone
B increases the potential difference across the depletion zone
C increases the number of donors on the n side
D decreases the electric field in the depletion zone.
Correct Answer
Option D
Solution

Number of donors 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.

Q77
Carbon, silicon and germanium atoms have four valence electrons each. Their valence and conduction bands are separated by energy band gaps represented by (E g ) C , (E g ) si and (E g ) Ge respectively. Which one of the following relationships is true in their case?
A (E g ) C > (E g ) Si
B (E g ) C < (E g ) Si
C (E g ) C = (E g ) Si
D (E g ) C < (E g ) Ge .
Correct Answer
Option A
Solution

Band gap of carbon is 5.5 eV while that of silicon is 1.1 eV ∴\therefore (E g ) C >> (E g ) Si

Q78
In semiconductors at a room temperature
A the valence band is partially empty and the conduction band is partially filled
B the valence band is completely filled and the conduction band is partially filled
C the valence band is completely filled
D the conduction band is completely empty
Correct Answer
Option A
Solution

In semiconductors at room temperature the electrons get enough energy so that they are able to over come the forbidden gap.

Thus at room temperature the valence band is partially empty and conduction band is partially filled.

Conduction band in semiconductor is completely empty only at 0 K.

Q79
The output of OR gate is 1
A if both inputs are zero
B if either or both inputs are 1
C only if both inputs are 1
D if either inputs is zero
Correct Answer
Option B
Solution

The truth table of OR gate is

ABY000011101111\begin{array}{lll}A & B & Y \\ 0 & 0 & 0 \\ 0 & 1 & 1 \\ 1 & 0 & 1 \\ 1 & 1 & 1 \end{array}

It is seen that output of OR gate is Y = A + B. If both or any one has 1 input then output will be 1.

Q80
The peak voltage in the output of a half wave diode rectifier fed with a sinusoidal signal without filter is 10 V. The d.c. component of the output voltage is
A (10/2\sqrt 2 ) V
B (10π)V\left( {{{10} \over \pi }} \right)V
C 10 V
D (20/π\pi ) V
Correct Answer
Option B
Solution

V =

V0π{{{V_0}} \over \pi }

⇒\Rightarrow V =

(10π)V\left( {{{10} \over \pi }} \right)V
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