Solution

CHAPTERWISE SAMPLE PAPER: SEMICONDUCTOR ELECTRONICS

Class 12 - Physics
Section A
1.
(c) increase its electrical conductivity
Explanation:
Conductivity of intrinsic semiconductors is increased by adding an appropriate amount of suitable impurity. This process is
called doping.

2. (a) 50%
Explanation:
50%
3.
(c) electrons
Explanation:
The more abundant charge carriers are called majority carriers, which are primarily responsible for current transport in a piece
of semiconductor. In n-type semiconductors, they are electrons, while in p-type semiconductors, they are holes.

4.
(c) p-type crystal
Explanation:
When we connect p-type region of a junction with the positive terminal of a voltage source and n-type region with the negative
terminal of the voltage source, then the junction is said to be forward biased.

5.
(b) voltage remains constant while current increases sharply
Explanation:
When a Zener diode is operated in the reverse breakdown region, the voltage across it remains practically constant (equal to
VZ) for a large change in the reverse current.

6.
(b) just below the conduction band
Explanation:
In an n-type semiconductor, the donor energy level lies just below the conduction band near the Fermi-level of the
semiconductor.
Dnor energy level is 0.05 eV for Si and 0.01 eV for Ge. By giving this much amount of energy to the electron they become free
and so to the conduction band.

7.
(c) diffusion of both electrons and holes
Explanation:
diffusion of both electrons and holes

8.
(d) 3.6 × 109 m-3
Explanation:
3.6 × 109 m-3

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 9.
(b) a micron
Explanation:
a micron

10.
(d) 0 K
Explanation:
At very low temperatures, electrons cannot jump from the valence band to the conduction band.

11.
(d) Both A and R are false.
Explanation:
In a p-n junction with open ends, a depletion layer is created at the junction due to diffusion of majority carriers from one side
to another and a constant electric field is set up across the junction and therefore, there is motion of charge carriers in p-n
junction due to which depletion layer is created.

12.
(b) Both A and R are true but R is not the correct explanation of A.
Explanation:
Both A and R are true but R is not the correct explanation of A.

13.
(d) A is false but R is true.
Explanation:
The direction of diffusion current is that when positively charged particles move from p-type to n-type of diode.

14.
(b)

Explanation:
When the input level is -5 V, the diode gets reverse biased. No output is obtained across RL. When the input level becomes +5
V, the diode gets forward biased and the current flows through RL. The diode is ideal, the output across RL will be exactly 5V.

15. (a) 283 V

Explanation:
A diode conducts only during the positive half cycle of a.c. Accordingly, the capacitor charges and discharges. During
charging, the p.d. across capacitor
–
= 200 × √2 = 283 V
16.

(c)

Explanation:
The p-n junction is said to be reverse biased, when the positive terminal of the external battery in the circuit is connected to n-
section and the negative terminal to p-section of the junction diode.

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17.
(d) 2.0 A
Explanation:
Diode D1 is reverse biased, so it offers an infinite resistance. So no current flows in the branch of diode D1.
Diode D2 is forward biased, and offers no resistance in the circuit. So current in the branch.
V 12
I = = = 2A
Req 2+4

18.
(c) C and A
Explanation:
In both figures (A) and (C), p-side is at higher potential than the n-side.

19.
(b) 3 corresponds to forward bias of junction and 1 corresponds to reverse bias of junction
Explanation:
When p-n junction is forward biased, it opposes the potential barrier across junction. When p-n junction is reverse biased, it
supports the potential barrier junction, resulting increase in potential barrier across the junction.

20.
(c) p-type semiconductor
Explanation:
One can see in the figure that number of holes are greater than number of electrons. Hence it is p−type semi conductor.

Section B
21. Diffusion current is set up across the junction due to the concentration difference of the majority charge carriers on the two sides
of the junction.
This diffusion develops an electric field from n- side to p- side across the junction which creates a drift current in the opposite
direction.
22. The size of the dopant atom should be equivalent to the size of Si or Ge. So that the symmetry of pure Si or Ge, does not disturb
and dopants can contribute the charge carrier on forming covalent bonds with Si or Germanium atoms. As the silicon and
germanium belongs to XIVth group so similar size of atom will be in XIII and XV group of modern periodic table.
23. i. When the input level is - 5V, the diode gets reverse biased. No output is obtained across R. When the input level becomes +
5V, the diode gets forward biased, and current flows through R. As the diode is ideal, the output across R will be exactly 5V,
as shown in the figure.

ii. When the input level is - 5V, the diode remains to reverse biased. It does not conduct current. This part of the input wave
appears across the diode, as shown in the figure. When the input level is +5V, the diode gets forward biased and conducts
currents. As the diode is ideal, no voltage appears across it.

24. Energy band diagrams of n-type and p-type semiconductors are shown below:

In the case of n-type semiconductors, The Donor energy level decreases the energy gap between the conduction band and the
valence band, electrons from donor impurity atoms will move into the conduction band with a very small supply of energy. Hence,

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 the conduction band will, therefore, have electrons as majority charge carriers. In the case of p-type semiconductor, a very small
supply of energy can cause an electron from its valence band to jump to the acceptor energy level. Hence, the valence band will
have a dominant density of holes which shows that holes are the majority charge carriers in p-type semiconductors.

25.

The figure shows a reverse-biased p-n junction diode in which the p-side is connected to the -ve terminal and the n-side is
connected to the +ve terminal of the battery and shows its voltage-current graph.

26. The d.c. resistance is just equal to the voltage divided by current.
VB 0.3 V
∴ rdc = IB
=
−3
= 66.67 Ω
4.5× 10 A

Consider two points A and C around the point of operation B. Then,

V −V 0.35−0.25
rac = ΔV

ΔI
=
C

IC − IA
A
=
−3
= 33.33 Ω
(6−3)×10

Section C

27.

The circuit diagram of full wave rectifier is as shown above. During first half cycle of the input a.c. signal, only diode 1 is forward
biased and conducts.
During the 2nd half cycle of the input ac signal only diode 2 is forward biased and conducts.
However, due to the use of the centre tapped transformer, the current in the load flows in the same direction during both these half
cycles. The current through the load is therefore unidirectional.

28. a.

During the formation of p - n junction diode; due to the concentration gradient across p and n sides of a diode, holes diffuse

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 from p side to n side and electrons diffuse from n side to p side giving rise to development of immobile positive charges on the
n side and the negative charges on the p side across the junction. Thus a potential barrier is formed at the junction.
b. The VI characteristics are obtained by connecting the battery, to the diode, through a potentiometer the battery, to the diode,
through a (or, rheostat). The applied voltage to the diode is changed. The applied voltage to the diode is changed. The values
of current, for different values of voltage, are noted and a graph between V and I is plotted. The V-I characteristics of a diode,
have the form the form shown here.

29. In an n-type semiconductor, the donor energy level ED is slightly below the bottom EC of the conduction band and electrons from
this level move into the conduction band with a very small supply of energy. Fermi-level shifts towards the conduction band
where a higher number of electrons are available for conduction. In an n-type semiconductor, the energy gap decreases.

In a p-type semiconductor, the acceptor energy level EA is above the top EV of the valence band, therefore with a small supply of
energy, the electrons can jump from the valence band to the acceptor energy level. Fermi levels shift closer to the valence bond
because holes are the majority carriers. In a p-type semiconductor, the energy band increases.

30.

Conduction band determines electrical conductivity

As temperature of a semiconductor rises, the carrier concentration (electron-hole pair) increases due to breaking of covalent bonds
and the conductivity of the semiconductor increases.

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31. A p-n juntion is a basic semiconductor device.

When a p-type crystal is placed in contact with n-type crystal so as to form one piece, the assembly so obtained is called p-n
junction or junction diode or crystal diode. The surface of contact of p and n-type crystals is called junction. In the p-section, holes
are the majority carriers; while in n-section, the majority carriers are electrons. Due to the high concentration of different types of
charge carriers in the two sections, holes from p-region diffuse into n-region and electrons from n-region diffuse into p-region. In
both cases, when an electron meets a hole, the two cancel the effect of each other and as a result, a thin layer at the junction
becomes devoid of charge carriers. This is called the depletion layer as shown in Fig.
i. When a p-n junction is forward biased, the width of the depletion layer decreases. As a result, it offers low resistance during
forward bias.
ii. When a p-n junction is reverse biased, the width of the depletion layer increases. As a result, it offers high resistance during
reverse bias.
OR
Diffusion and drift processes take place during the formation of the p-n junction.
The region near the junction which is free from the charge carriers is called the depletion layer. It is formed due to the
combination of electrons and holes during the diffusion process due to the difference in the density of charge carriers in p and n
semiconductors. It develops a layer of positive ions on n-side and negative ions on the p-side.

The positive ions and negative ions develops potential difference across the junction and an internal electric field E, directed from
n-side top-side. The potential difference gets developed due to positive and negative ions across the depletion layer is called the
potential barrier. It stops the diffusion process. This potential barrier reduces in forward biased mode and increases in reverse
biased mode.

32. a.

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 b. Threshold Voltage: Forward bias voltage at which the current increases significantly (exponentially) even for a very small
increase in voltage.
Break down voltage: Reverse bias voltage at which current increases suddenly.
c. Junction Diode: conducts when it is forward biased and does not conduct when reverse biased.
OR
Potential barrier: The potential barrier is the fictitious battery, which seems to be connected across the p-n junction with its
positive terminal in the n-region and the negative terminal in the p-region.
Depletion region: The region around the junction, which is devoid of any mobile charge carriers, is called the depletion layer or
region.
i. The forward-bias connections of a p-n junction are as shown in Fig.

When the p-n junction is forward biased, the depletion layer becomes thin. It is because, the polarity of the external d.c. source
opposes the fictitious battery developed across the junction. As a result, the potential drop across the junction decreases
making the depletion layer thin. It leads to the low resistance of the junction diode during forward bias.

ii. The reverse-bias connections of a p-n junction are as shown in Fig.

When the p-n junction is reverse biased, the depletion layer becomes thick. It is because, the external d.c. source aids the
fictitious battery. It results in the increase of potential drop across the junction and the depletion layer appears thick. Because
of the increased thickness of the depletion layer, the p-n junction offers high resistance during reverse bias.

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