TPS Physics Papers with Solution
19
252
Semiconductor - Paper with Solution
Section A - (4 Marks)
1.
Semiconductor
Sr.
Intrinsic
Extrinsic
No.
Semiconductor
Semiconductor
1.
It is pure
semiconductor
and no
impurity atoms
are added to it.
It is prepared by
doping a small
quantity of
impurity atoms to
the pure
semiconducting
material.
2.
Its electrical
conductivity is
low.
Its electrical
conductivity is
high.
Carriers in base region of p-n-p
transistor are
[1]
a) minority carriers
b) majority carriers
c) both a and b
d) electrons
Soln.:
2.
Soln.:
3.
d) electrons
To obtain n-type semiconductor the
impurity introduced is
[1]
a) Arsenic
b) Aluminum
c) Silicon
d) Indium
6.
a) Arsenic
What is zener breakdown voltage ? [1]
What is doping ?
Draw the schematic symbols for
AND, OR, Not and NAND gate
[2]
Soln.:
Soln.: In reverse bias initially a small
reverse saturated correct flows, but when
reverse bias reaches a particular value the
current increases suddenly. This voltage is
called as zener breakdown voltage.
4.
[Each 1 Mark]
AND
[½]
OR
[1]
Soln.: The process of introduction of
impurity in semiconductor is called as
doping.
[½]
NOT
Section B – (12 Marks)
5.
Soln.:
Distinguish between intrinsic and
extrinsic semiconductor
[2]
[½]
NAND
[½]
TPS Physics Papers with Solution
7.
253
State any two applications of solar
cell
[2]
Soln.:
9.
Semiconductor
Draw the block diagram of oscillator.
[2]
Soln.:
1)
Solar cells are used in remote places,
satellites, sensors and equipment kept
in deep forest
2)
Solar cells are used in calculators,
watches, portable TV sets.
[1 Mark each]
8.
Explain the P-N junction diode as a
forward biased mode
[2]
10
State any four applications of LED’s
Soln.:
1.
[2]
Forward biasing : In this case, the
positive terminal of a battery is
connected to p-side and negative
terminal to n-side.
–
The e from the negative side terminal
of the battery enter the n-side and
lower the positive potential of the nside. The electrons from are attracted
to the positive terminal of battery
creating fresh holes on p side.
[½]
Hence p side becomes less negative.
Therefore the potential barrier at the junction
–
is lowered and e which constitute the
majority carriers cross over easily from the nside to the p-side. Therefore a current from pside to n-side. A resistance is used to limit the
strength of current.
[1]
Soln.:
1)
LEDs are used as status indicators on
various instruments
2)
LEDs are used in traffic control light
system
3)
LEDs are used as a light source in
optical fiber communications
4)
They are used as brake indicators in
bikes and vehicles
[½ mark each]
OR
10.
State advantages of LEDs
[2]
Soln.:
1)
LEDs are cheap
2)
LEDs require low operating voltages
and low power
3)
They have longer life and light weight
4)
They have high operating speed
[½ mark each]
Section C – (9 Marks)
11.
Fig. Q. 8
[Diagram - ½ Mark]
Explain the working of a transistor as
a switch
[3]
Soln.: Both PNP or NPN transistor can be
made to operate as a switch for controlling
TPS Physics Papers with Solution
254
higher power device. For using the transistor
as a switch it is to be operated in cut off
region or saturation region.
The main idea is that, load RL is
connected or disconnected from the source
VCC by transistor switch without any make or
brake of circuit. Also the switch is operated
by low base voltage (0.7 V) and requires very
low base current (few A). The transistor as
switch is used in logic circuits in power
electronics. Transistor switches have the
advantage of high speed of operation.
[1]
12.
Fig. Q. 11
[Diagram - 1 Mark]
Applying Kirchoff’s voltage law to
output part we get,
Semiconductor
Explain the formation of energy band
diagram in case of conductor and
semiconductor
[3]
Soln.: In conductors the valence band is
completely filled with electrons and the
conduction band is empty but the two bands
are overlapped.
Forbidden energy gap is zero.
[½]
VCC = IC RL + VCE
VCE = VCC – IC RL
When Vi = 0, IB = 0 and hence IC = 0
 VCE = VCC source voltage
That is when Vi = 0 no current flows
through transistor and load RL. Entire voltage
develops across transistor (VCE = VCC). This
condition is similar to open switch.
i.e. IC = 0, V0 = VCC = source voltage
When Vi is positive and greater than
0.7 V the base current flows that forces
transistor into saturation and large current
flows through transistor and load RL.
The entire source voltage gets
developed across the load RL and very low
voltage drops across transistor VCE = 0.2 V.
This condition is similar to closed switch
IC =
VCC
RL
and V0 = VCE = 0.2 V
[1]
Fig. Q. 12
[Diagram - ½ Mark]
Due to the overlapping of the valence
and conduction band no extra energy is
required to move the electron from valence
band to conduction band.
When electric field is applied across
such conductors large number of electrons
are available for conduction so conductivity
is very high for conductors.
Example : Mg, Zn etc.
[½]
In some conductors the valence band
may be completely filled and conduction
band is partially filled but the energy gap is
very small. e.g. Na, K etc.
TPS Physics Papers with Solution
Semiconductors :
255
Semiconductor
Half wave rectifier:
The electronic circuit which convert
half cycle of a.c. voltage into d.c. voltage is
called half wave rectifier.
Diagram :
Fig. Q. 12(a)
[Diagram - ½ Mark]
The energy band diagram for
semiconductors is shown as in Fig. 12(a).
The forbidden energy gap is small as
compared to that of insulators (Eg < 3 eV).
For germanium energy gap is 0.6 eV and for
silicon it is 1.1 eV.
Due to small energy gap, some of the
valence band electrons make transition to the
conduction band by acquiring thermal
energy. Those electrons leave an equal
number of vacant states or holes in the
valence band.
[½]
At room temperature and higher
temperature the electron acquire sufficient
thermal energy and jump to the conduction
band. But at low temp the electron does not
acquire energy so it remains in valence band.
At low temp the semiconductors behave as
insulators. When electric field is applied
across such semiconductors (at room
temperature and higher temperature) a few
electrons are available in the conduction
band. So the conductivity of semiconductors
is low.
[½]
13.
Explain with a neat diagram how a pn junction diode is used as a half
wave rectifier
[3]
Soln.:
P-n junction diode:
Fig. Q. 13
[Circuit diagram with direction of current - 1 Mark]
During every positive half cycle of the
a.c. input point A becomes positive w.r.t.
point B and diode becomes forward biased.
When forward voltage crosses 0.7 volt
(for Si diode) the diode works as closed
switch and large current Id flows through RL
from M to N.
This current produces d.c. output
voltage
V0 = Vdc
[½]
= Id RL across the load resistance
During every negative half cycle of a.c.
input the point A becomes negative w.r.t. B
and diode gets reverse biased and it works as
an open switch.
No current flows through the load
resistance in negative half cycle and hence no
output voltage is obtained.
The current always flows from M to N
(uni-directional) which produces d.c. voltage
with M positive and N negative.
Since current flows through RL only for
half cycle of applied a.c. it is called half wave
TPS Physics Papers with Solution
rectifier. The output voltage is uni-directional
256
Semiconductor
pulsating and intermittent.
[1 ½]
Fig. Q. 13(a): Input output waveforms for half wave rectifier
OR
13.
With the help of neat circuit diagram
explain the working of a photodiode.
State its any two uses
[3]
The photo current depends upon
intensity of incident radiation and is
independent of reverse bias.
[1]
Soln.: The photodiode is special purpose
semiconductor diode. While fabricating, a
transparent window is kept to expose its
junction to light radiation.
Working of photodiode :
When light in the form of photons
enters the depletion region and when photon
energy h > Eg electron hole pairs are
generated. Photodiode always operated in
reverse bias. These generated electron-holes
flow under the reverse bias and current in A
flows in the circuit.
Fig. Q. 13(b)
[Diagram - 1 Mark]
Uses :
1.
Photo diode are used to detect optical
signal.
[½]
TPS Physics Papers with Solution
in
object
257
257
2.
It is used
optocouplers.
counters,
[½]
3.
It is used sensor in remote controlled
receivers.
2)
Truth table – The table that lists all
possible levels on inputs and
corresponding output is called truth
table.
[1]
15.
A) Draw a neat circuit diagram to
study the characteristics of common
emitter n-p-n transistor with the help
of a graph explain the output
characteristics of this transistor
[3]
Section D – (10 Marks)
14.
A) What are ∝ and β parameters for a
transistor ? Obtain a relation between
them
[3]
Soln.: For transistor, the current ratios are
defined as –
(i)
Semiconductor
Soln.: Circuit diagram to study transistor
characteristic in C.E. mode
dc (alpha) : It is defined as the ratio of
collector current to emitter current.
dc =
(ii)
IC
…(1)
IE
[½]
dc (beta, current gain) : It is defined as
the ratio of collector current to base
current.
dc =
IC
IB
…(2)
[½]
…(3)
[½]
We have
IE = IB + IC
Diagram - 1 Mark
(Dividing by IC throughout)
IE
IC
=
IB
IC
+1
[½]
1
1
=
+1
dc
dc
dc =
  dc =
dc
1 + dc
dc
1 – dc
VBB and VCC – DC supply
[½]
milliammeter Rh1 and Rh2 along with source
provide voltage divider arrangement.
Output characteristics :
…(4)
[½]
…(5)
B) Define i) logic gate ii) truth table [2]
Soln.:
1)
IC collector current is measured by
logic gate- It is an electronic circuit
which has two or more inputs and one
output
[1]
It is a graph of output current versus
output voltage. For C – E mode, it is a graph
of collector current IC versus collector voltage
VCE, for the fixed value of the base current. To
study
output
characteristics
IB
is
kept
constant VCE is increased in small steps and IC
is recorded at every step. When IC versus VCE
is plotted we get output characteristics as
shown in diagram.
[½]
TPS Physics Papers with Solution
258
Semiconductor
B) What is Zener diode ? Give its
symbol
[1]
Soln.: Zener diode is a p-n junction diode
which works in reverse biased condition. It
works in breakdown region

Diagram - ½ Mark
From output characteristics it is clear that –
1.
When IB = 0, IC is nearly zero and we
say that the transistor is in cut off
region.
2.
For VCE < 0.3 V, IC increases with VCE
and can be controlled by VCE.
3.
When VCE > VBE (0.7 V), CE junction
gets
reverse
biased
and
IC
gets
saturated i.e. IC doesn’t depend on VCE
but can be controlled by IB and we say
that the transistor is in active region. It
has large current in this region.
4.
When VE < VB > VC both the junctions
get forward biased and IC can no
longer be controlled by VCE. We say
that the transistor is in saturation
region.
The dynamic output resistance of
transistor is given by,
ro =
  VCE 
  I  constant
C I

C
The output resistance in this mode
varies from few k to 100 k.
[1]
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