Chapter 14.

Semiconductor Electronics

Semiconductor, Diode and its

Application
1 Mark Questions
1.The graph shown in the figure represents a plot of current versus voltage for a given
semiconductor. Identify the region, if any over which the semiconductor has a negative
resistance. [All India 2013]

Ans.Resistance of a material can be found out by the slope of the curve V versus I. Part BC of
the curve shows the negative resistance as with the increase in current and decrease in
voltage.

2.What happens to the width of depletion layer of a p-n junction when it is

(i)forward biased?
(ii)reverse biased? [All India 2011]
Ans.(i) Width of depletion layer’s decreases in forward bias
(ii) Width depletion layer increases in reverse bias.

3.Why cannot we take one slab of p-type semiconductor and physically join it to another

slab of n-type semiconductor to get p-n junction?

Ans.In this way, continuous contact cannot be produced at atomic level and junction will
behave as a discontinuity for the flowing charge carrier.

4.What is the most common use of photo diode? [All India 2009]
Ans.The photodiode can be used as a photodetector to detect optical signals

5.State the relation between the frequency v of radiation emitted by LED and the band gap
energy E, of the semiconductor used to fabricate it.[All India 2009 C]
Ans.Zener diode, which is used as a DC voltage regulator.
6.Figure show the I-V characteristics of a given device. Name the device and write where it
is used.

Ans.

7.State the reason, why GaAs is most commonly used in making of a solar cell. [All India
2008]
Ans.solar radiation is nearly 1.5 eV. In order to have photo excitation, the energy of radiation
(hv) must be greater than energy band gap (Eg). Therefore, the semiconductor with energy
band gap about 1.5 eV or lower than it and with higher absorption coefficient is likely to give
better solar conversion efficiency. The energy band gap for Si is about 1.1 eV, while for GaAs,
it is about 1.53 eV. The GaAs is better inspite of its higher band gap than Si because it
absorbs relatively more energy from the incident solar radiations being of relatively higher
absorption coefficient.

8.At what temperature would an intrinsic semiconductor behave like a perfect insulator?
[Delhi 2008 C]
Ans.At O° K temperature, an intrinsic semiconductor behaves like a perfect insulator

9.Why should a photo diode be operated at reverse bias?[All India 2008]

Ans.The photodiodes are used to detect the optical signals. The fraction change in minority
carrier in case of reverse biased diode is easily measurable than that of forward biased. That
is why photodiode is operated at reverse bias.

2 Marks Questions
10.Explain, with the help of a circuit diagram, the working of a p-n junction diode as a half-
wave rectifier. [ah India 2014]
Ans.p-n Junction Diode as a Half-Wave Rectifier AC voltage to be rectified is connected to
the primary coil of a step-down transformer. Secondary coil is connected to the diode through
resistors Rj,, across which output is obtained.

Working During positive half cycle of the input AC, the p-n junction is forward biased. Thus,
the resistance in p-n junction becomes low and current flows. Hence, we get output in the
load.
During negative half cycle of the input AC, the p-n junction is reverse biased. Thus, the
resistance of p-n junction is high and current does not flow. Hence, no output in the load. So,
for complete cycle of AC, current flows through the load resistance in the same direction.

11.Draw energy band diagram of n-type and p-type semiconductor at temperature T > OK.
Mark the donar and acceptor energy level with their energies. [Foreign 2014]
Ans.

12.Distinguish between a metal and an insulator on the basis of energy band diagram.
[Foreign 2014]
Ans.(i) Metal For metals, the valence band is completely filled and the conduction band can
have two possibilities either it is partially filled with an extremely small energy gap between
the valence and conduction bands or it is empty, with two bands overlapping each other as
shown below:

(ii) On applying an small even electric field, metals can conduct electricity.
(i) Insulators : for insulator, the energy gap between the conduction and valence bands are
very large, also the conduction band is practically empty, as shown below:

(ii) When an electric field is applied across such a solid, the electrons find it difficult to acquire.
So, a large amount of energy is required to reach the conduction band.Thus, the conduction
band continues to be empty. That is why no current flows through insulators.

Ans.

14.Explain the term ‘drift velocity’ of electrons in conductor. Hence, obtain an expression
for the current through a conductor in terms of ‘drift velocity’.[All India 2013]
Ans.’Drift velocity’ of electrons in a conductor
Metals contain a large number of free electrons. These electrons are in continuous random
motion. Due to the random motion, the free electrons collide with positive metal ions with high
frequency and undergo change in direction at each collision. So, the average velocity for the
electrons in a conductor is zero.Now, when this conductor is connected to a source of emf, an
electric field is established in the conductor, such that E = V/L
where, V = potential difference across the conductor and L = length of the conductor. The
electric field exerts an electrostatic force -Ee on each free electron in the conductor. The
acceleration of each electron is given by

where, e = electric charge on electron and m = mass of electron.

The average velocity of all the free electrons in the conductor is called the drift velocity of free
electrons of the conductor.

15.Mention the important considerations required while fabricating a p-n junction diode to
be used as a Light Emitting Diode (LED). What should be the order of band gap of an LED, if
it is required to emit light in the visible range? [Delhi 2013]
Ans.
16.Write two characteristics features to distinguish between n-type and p-type
semiconductors. [All India 2012]
Ans.

17.Give two advantages of LED’s over the conventional incandescent lamps.[Foreign 2012]
Ans.When we apply sufficient voltage to LED, electron move across the junction into p-region
and get attracted to the holes there. Thus, electrons and holes recombine. During each
recombination, the electric potential energy is converted into the electromagnetic energy and
a photon of light with a characteristic frequency is emitted, this is how, LED works.
Advantages of LEDs over incandescent lamps
(i) Since, LEDs do not have a filament that can burn out, hence, they last longer.
They do not get hot during use.

Ans.When photo diode is illuminated with light due to breaking of covalent bonds, equal
number of additional electrons and holes comes into existence whereas fractional change in
minority charge carrier is much higher than fractional change in majority charge carrier. Since,
the fractional change of minority carrier current is measurable significantly in reverse bias
than that of forward bias. Therefore, photo diode are connected in reverse bias.

19.Name the semiconductor device that can be used to regulate an unregulated DC power
supply. With the help of 7-V characteristics of this device, explain its working principle.
[Delhi 2011]
Ans. Zener diode is used as voltage regulator.
Principle Zener diode is operated in the reverse breakdown region. The voltage across it
remains constant, equal to the breakdown voltage for large charge in reverse current

20.How is forward biasing different from reverse biasing in a p-n junction diode? [Delhi
2011]
Ans.Differences between forward and reverse biases are given below:
21.Explain, how a depletion region is formed in a junction diode?[Delhi 2011]
Ans. With the formation of p-n junction, the holes from p-region diffuse into the n-region and
electrons from n-region diffuse into p-region and electron-hole pair combine and get
annihilated.
This input produces potential barrier, VB across in junction which opposes the further diffusion
through the junction. Thus, small region forms in the vicinity of the junction which is depleted
of free charge carrier and has only immotile ions is called the depletion region.

22.Draw the circuit diagram showing how a p-n junction diode is

(i)forward biased
(ii)reverse biased.
How is the width of depletion layer affected in the two cases?[All India 2011 C]
Ans.Circuit diagram of forward biased and reverse biased p-n junction diode is shown

The width of depletion layer (i) decreases in forward bias.

(ii) increases in reverse bias.

23.Carbon and silicon both have four valence electrons each, then how are they
distinguished? [Delhi 2011 c]
Ans.The four valence electrons of carbon are present in second orbit while that of silicon in
third orbit. So, energy required to extricate an electron from silicon is much smaller than
carbon.
Therefore, the number of free electrons for conduction in silicon is significant on contrary of
carbon. This makes silicon conductivity much higher than carbon. This is the main
distinguishable property.
Ans.

25.Draw the circuit diagram of an illuminated photo diode in reverse bias. How is photo
diode used to measure light intensity? [Delhi 2010]26.Write the main use of the
(i)photo diode
(ii)Zener diode. [All India 2010 c]
Ans.Circuit diagram of illuminated photo diode in reverse bias is shown below:

26.Write the main use of the (i) photo diode (ii) Zener diode [All India 2010 c]
Ans.Main use of photo diode In demodulation of optical signal and detection of optical
signal.
Main use of Zener diode As DC voltage regulator.
Ans.Differences between intrinsic and p-type semiconductors are given below:

In p-type semiconductor, trivalent impurity is doped with tetravalent pure semiconductor. Both
type of atom (impurity and host semiconductor) are electrically neutral and hence, so
produced p-type semiconductor is electrically neutral

28.Draw a circuit diagram showing the biasing of an LED. State the factor which controls
(i)wavelength of light.
(ii) intensity of light emitted by the diode. [Foreign 2008]
Ans.Circuit diagram of a forward biased LED is shown below:

29.How is that the reverse current in a Zener diode starts increasing suddenly at a
relatively low breakdowns voltage of 5 V or so?[All India 2008 C]
Ans.

30.Draw the (i) symbol and (ii) the reverse I-V characteristics of a Zener diode. Explain
briefly, which property of the characteristics enables us to use Zener diode as voltage
regulator. [All India 2008 C]
Ans. Zener diode is used as voltage regulator.
Principle Zener diode is operated in the reverse breakdown region. The voltage across it
remains constant, equal to the breakdown voltage for large charge in reverse current

3 Marks Questions
31.Write any two distinguishing features between conductors, semiconductors and
insulators on the basis of energy band diagrams. [All India 2014]32.Draw the circuit
diagram of a full-wave rectifier using p-n junction diode. Explain its working and show the
output input waveforms. [Delhi 2012]
Ans.

32.Draw the circuit diagram of a full-wave rectifier using p-n junction diode. Explain its
working and show the output input wave forms. [Delhi 2012]
Ans.
33.Draw V-I characteristics of a p-n junction diode. Answer the following questions, giving
reasons.
(i)Why is the current under reverse bias almost independent of the applied potential up to a
critical voltage?
(ii)Why does the reverse current show a sudden increase at the critical voltage?
Name any semiconductor device which operates under the reverse bias in the breakdown
region.
[All India 2012]
Ans.

(i)Under the reverse bias condition, the holes of p-side are attracted towards the negative
terminal of the battery and the electrons of the n-side are attracted towards the positive
terminal of the battery. This increases the depletion layer and the potential barrier. However,
the minority charge carriers are drifted across the junction producing a small current.
At any temperature, the number of minority carriers is constant, so there is the small current
at any applied potential. This is the reason for the current under reverse bias to be
almost’independent of applied potential. At the critical voltage, avalanche breakdown takes
place which results in a sudden flow of large current.
(ii) At the critical voltage, the holes in the n-side and conduction electrons in the p-side are
accelerated due to the reverse bias voltage. These minority carriers acquire sufficient kinetic
energy from the electric field and collide with a valence electron.
Thus, the bond is finally broken and the valence electrons move into the conduction band
resulting in enormous flow of electrons and thus, formation of hole-electron pairs. Thus, there
is a sudden increase in the current at the critical voltage.
Zener diode is a semiconductor device which operates under the reverse bias in the
breakdown region.

34.Draw a labelled diagram of a full -wave rectifier circuit. State its working principle.
Show the input-output wave forms.[All India 2011]
Ans.
35.Name the important processes that occurs during the formation of a p-n
junction.Explain briefly, with the help of asuitable diagram, how a p-n junction is
formed.Define the term ‘barrier potential’?[Foreign 2011]
Ans.

During formation of p-n junction, diffusion of charge takes place. As, soon as p-type
semiconductor is joined with n-type semiconductor, diffusion of free charges across the
junction starts.
For explanation of formation p-n junction
With the formation of p-n junction, the holes from p-region diffuse into the n-region and
electrons from n-region diffuse into p-region and electron-hole pair combine and get
annihilated.
This input produces potential barrier, VB across in junction which opposes the further diffusion
through the junction. Thus, small region forms in the vicinity of the junction which is depleted
of free charge carrier and has only immotile ions is called the depletion region.
Potential barrier The potential distribution near the p-n junction is known as potential barrier

36.(i) Why is a photo diode operated in reverse bias mode?

(ii)For what purpose is a photo diode used?
(iii)Draw its I-V characteristics for different intensities of illumination. [HOTS; All India
2011 C]
Ans.

(i) Photo diode is connected in reverse bias and feeble reverse current flows due to thermally
generated electron-hole pair, known as dark current. When light of

(ii)Main use of photo diode In demodulation of optical signal and detection of optical signal.
(iii)Circuit diagram of illuminated photo diode in reverse bias is shown below:
37.(i)Why are Si and GaAs preferred materials for solar cells?
(ii)Describe briefly with the help of a necessary circuit diagram, the working principle of a
solar cell.[All India 2011 C]
Ans. (i) solar radiation is nearly 1.5 eV. In order to have photo excitation, the energy of
radiation (hv) must be greater than energy band gap (Eg). Therefore, the semiconductor with
energy band gap about 1.5 eV or lower than it and with higher absorption coefficient is likely
to give better solar conversion efficiency. The energy band gap for Si is about 1.1 eV, while for
GaAs, it is about 1.53 eV. The GaAs is better inspite of its higher band gap than Si because it
absorbs relatively more energy from the incident solar radiations being of relatively higher
absorption coefficient.

38.(i) Describe the working of Light Emitting Diodes (LEDs).

(ii)Which semiconductors are preferred to make LEDs and why?
(iii)Give two advantages of using LEDs over conventional incandescent low power lamps.
[All India 2011]
Ans.(i) Working of LED LED is a forward biased p-n junction which converts electrical energy
into optical energy of infrared and visible light region.
Being in forward bias, thin depletion layer and low potential barrier facilitate diffusion of
electron and hole through the junction when high energy electron of conduction band
combines with the low energy holes in valence band, then energy is released in the form of
photon, may be seen in the form of light.
(ii) Semiconductors with appropriate band gap (Eg) close to 1.5 eV are preferred to make LED
size GaAs, CdTe, etc.the other reasons to select these materials e high optical absorption,
availability of raw material and low cost
(iii) Uses of LEDs (a) LED can operate at very low voltage and consumes less power in
comparison to incandescent lamps
(b) Unlike the lamps, they take very less operational time and have long life.

39.With the help of a suitable diagram, explain the formation of depletion region in a p-n
junction. How does its width change when the junction is
(i)forward biased and
(ii)reverse biased? [All India 2009]
Ans. With the formation of p-n junction, the holes from p-region diffuse into the n-region and
electrons from n-region diffuse into p-region and electron-hole pair combine and get
annihilated.
This input produces potential barrier, VB across in junction which opposes the further diffusion
through the junction. Thus, small region forms in the vicinity of the junction which is depleted
of free charge carrier and has only immotile ions is called the depletion region.
Circuit diagram of forward biased and reverse biased p-n junction diode is shown

The width of depletion layer (i) decreases in forward bias.

(ii) increases in reverse bias.

40.(i) With the help of circuit diagrams, distinguish between forward biasing and reverse
biasing of p-n junction diode.
(ii)Draw V-I characteristics of a p-n junction diode in
(a)forward bias.
(b)reverse bias. [AH India 2009]

Ans.(i)Circuit diagram of forward biased and reverse biased p-n junction diode is shown

The width of depletion layer (a) decreases in forward bias.

(b) increases in reverse bias.
(ii)(a)Differences between forward and reverse biases are given below:
At the critical voltage, the holes in the n-side and conduction electrons in the p-side are
accelerated due to the reverse bias voltage. These minority carriers acquire sufficient kinetic
energy from the electric field and collide with a valence electron.
Thus, the bond is finally broken and the valence electrons move into the conduction band
resulting in enormous flow of electrons and thus, formation of hole-electron pairs. Thus, there
is a sudden increase in the current at the critical voltage.
Zener diode is a semiconductor device which operates under the reverse bias in the
breakdown region.

41.Explain with the help of a circuit diagram how a Zener diode works as a DC voltage
regulator? Draw its I-V characteristic s.[All India 2009]
Ans. Zener diode is used as voltage regulator.
Principle Zener diode is operated in the reverse breakdown region. The voltage across it
remains constant, equal to the breakdown voltage for large charge in reverse current
42.How is a Zener diode fabricated so as to make it a special purpose semiconductor
diode? Draw the circuit diagram of a Zener diode as a voltage regulator and explain its
working. [All India 2009 C]
Ans.Zener diode fabrication Zener diode is made by heavily doping of both p and n-type
semiconductors and hence, the width of depletion layer becomes thin which lead to produce
large electric field to increase the current even on applying reverse voltage of 4 or 5 V.
(i)Zener diode is used as voltage regulator.
Principle Zener diode is operated in the reverse breakdown region. The voltage across it
remains constant, equal to the breakdown voltage for large charge in reverse current

(ii) From the figure, it is clear that the device, X is a full-wave rectifier. Circuit diagram as
shown in figure below:

Zener diode connected with unregulated DC voltage in reverse bias. When the input voltage
increases, then current through R, increase and hence, voltage drop across Rp increases while
voltage across the Zener diode remains constant. The voltage across Zener diode remains
constant beyond Zener voltage and hence, same/constant regulated voltage is obtained
across RL.

Thus, fractional change in minority charge carrier (hole) is much higher than fraction change
in majority charge carrier (electron). Also, minority charge carrier contribute in drift current in
reverse bias.
Thus, with incidence of light, fractional change in minority charge carrier is significant.
Therefore, photo diode should be connected in reverse bias for measuring light intensity.

43.The figure below, shows the V-I characteristics of a semiconductor diode.

(i)Identify the semiconductor diode used.
(ii) Draw the circuit diagram to obtain the given characteristic of this device.
(iii) Briefly explain, how this diode can be used as a voltage regulator.[Delhi 2008]
Ans.(i) Zener diode
(ii) From the figure, it is clear that the device, X is a full-wave rectifier. Circuit diagram as
shown in figure below:

Zener diode connected with unregulated DC voltage in reverse bias. When the input voltage
increases, then current through R, increase and hence, voltage drop across Rp increases while
voltage across the Zener diode remains constant. The voltage across Zener diode remains
constant beyond Zener voltage and hence, same/constant regulated voltage is obtained
across RL.
(iii) Zener diode is used as voltage regulator.
Principle Zener diode is operated in the reverse breakdown region. The voltage across it
remains constant, equal to the breakdown voltage for large charge in reverse current

5Marks Questions
44.(i)State briefly the processes involved in the formation of p-n junction, explaining
clearly how the
depletion region is formed.
(ii)Using the necessary circuit diagrams, show how the V-I characteristics of a p-n junction
are obtained in (a) forward biasing (b) reverse biasing How are these characteristics made
use of in rectification? [Delhi 2014]
Ans.(i) p-n Junction A p-n junction is an arrangement made by a close contact of n-type
semiconductor and p-type semiconductor.There are various methods of forming p-n junction
diode. In one method, an n-type germanium crystal is cut into thin slices called wafers. An
aluminium film is laid on an n-type wafer which is then heated in an oven at a temperature of
about 600°C. Aluminium then diffuses into the surface of wafer. In this way, a p-type
semiconductor is formed on n-type semiconductor.
Formation of Depletion Region in p-n Junction In an n-type semiconductor, the
concentration of electrons is more than concentration of holes. Similarly, in a p-type
semiconductor, the concentration of holes is more than that of concentration of electrons.
During formation of p-n junction and due to the concentration gradient across p and n-sides,
holes diffuse from p-side to n-side ( p—> n) and electrons diffuse from n-side to p-side (n —>
p).

The diffused charge carriers combine with their counterparts in the immediate vicinity of the
junction and neutralise each other, (l) Thus, near the junction, positive charge is built on n-side
and negative charge on p-side This sets up potential difference across the junction and an
internal electric field Ej directed from n-side to p-side. The equilibrium is established when the
field £, becomes strong enough to stop further diffusion of the majority charge carriers
(however, it helps the minority charge carriers to diffuse across the junction).The region on
either side of the junction which becomes depleted (free) from the mobile charge carriers is
called depletion region or depletion layer. The width of depletion region is of the order of 10 -6
m.The potential difference developed across the depletion region is called the potential
barrier. Potential barrier depends on dopant concentration in the semiconductor and
temperature of the junction.
(ii) (a) Forward Biased Characteristics
The circuit diagram for studying forward biased characteristics is shown in the figure.
Starting from a low value, forward bias voltage is increased step by step (measured by
voltmeter) and forward current is noted (by ammeter). A graph is plotted between voltage and
current. The curve so obtained is the forward characteristic of the diode.
At the start when applied voltage is low, the current through the diode is almost zero. It is
because of the potential barrier, which opposes the applied voltage. Till the applied voltage
exceeds the potential barrier, the current increases very slowly with increase in applied voltage
(OA portion of the graph).With further increase in applied voltage, the current increases very
rapidly (AB portion of the graph), in this situation, the diode behaves like a conductor. The
forward voltage beyond which the current through the junction starts increasing rapidly with
voltage is called knee voltage. If line AB is extended back, it cuts the voltage axis at potential
barrier voltage.
(b) Reverse Biased Characteristics
The circuit diagram for studying reverse biased characteristics is shown in the figure.

In reverse biased, the applied voltage supports the flow of minority charge carriers across the
junction. So, a very small current flows across the junction due to minority charge carriers.
Motion of minority charge carriers is also supported by internal potential barrier, so all the
minority carriers cross over the junction.Therefore, the small reverse current remains almost
constant over a sufficiently long range of reverse bias, increasing very little with increasing
voltage (OC portion of the graph). This reverse current is voltage independent upto certain
voltage known as breakdown voltage and this voltage independent current – is called reverse
saturation current.
Use of p-n Junction Characteristics in Rectification
From forward and reverse characteristics, it is clear that current flows through the junction
diode only in forward bias not in reverse bias i.e. current flows only in one direction

45.(a) Draw the circuit arrangement for studying the V-I characteristics of a p-n junction
diode in (i) forward and (ii) reverse bias. Briefly explain how the typical V- I characteristics
of a diode are obtained and draw these characteristics.
(b)With the help of necessary circuit diagram, explain the working of a photo diode used
for detecting optical signals. [All India 2014 C]
Ans. (a) Forward Biased Characteristics
The circuit diagram for studying forward biased characteristics is shown in the figure.
Starting from a low value, forward bias voltage is increased step by step (measured by
voltmeter) and forward current is noted (by ammeter). A graph is plotted between voltage and
current. The curve so obtained is the forward characteristic of the diode.

At the start when applied voltage is low, the current through the diode is almost zero. It is
because of the potential barrier, which opposes the applied voltage. Till the applied voltage
exceeds the potential barrier, the current increases very slowly with increase in applied voltage
(OA portion of the graph).With further increase in applied voltage, the current increases very
rapidly (AB portion of the graph), in this situation, the diode behaves like a conductor. The
forward voltage beyond which the current through the junction starts increasing rapidly with
voltage is called knee voltage. If line AB is extended back, it cuts the voltage axis at potential
barrier voltage.
(b) Reverse Biased Characteristics
The circuit diagram for studying reverse biased characteristics is shown in the figure.
In reverse biased, the applied voltage supports the flow of minority charge carriers across the
junction. So, a very small current flows across the junction due to minority charge carriers.
Motion of minority charge carriers is also supported by internal potential barrier, so all the
minority carriers cross over the junction.Therefore, the small reverse current remains almost
constant over a sufficiently long range of reverse bias, increasing very little with increasing
voltage (OC portion of the graph). This reverse current is voltage independent upto certain
voltage known as breakdown voltage and this voltage independent current – is called reverse
saturation current.
Use of p-n Junction Characteristics in Rectification
From forward and reverse characteristics, it is clear that current flows through the junction
diode only in forward bias not in reverse bias i.e. current flows only in one direction
(b)Circuit diagram of illuminated photo diode in reverse bias is shown below:
46.(a) Explain with the help of diagram, how a depletion layer and barrier potential are
formed in a junction diode.
(b) Draw a circuit diagram of a full-wave rectifier. Explain its working and draw input and
output waveforms. [Delhi 2014 c]
Ans.(a)

During formation of p-n junction, diffusion of charge takes place. As, soon as p-type
semiconductor is joined with n-type semiconductor, diffusion of free charges across the
junction starts.
For explanation of formation p-n junction
With the formation of p-n junction, the holes from p-region diffuse into the n-region and
electrons from n-region diffuse into p-region and electron-hole pair combine and get
annihilated.
This input produces potential barrier, VB across in junction which opposes the further diffusion
through the junction. Thus, small region forms in the vicinity of the junction which is depleted
of free charge carrier and has only immotile ions is called the depletion region.
Potential barrier The potential distribution near the p-n junction is known as potential barrier
(b)
47.(i) How is a depletion region formed in p-n junction?
(ii)With the help of a labelled circuit diagram. Explain how a junction diode is used as a
full-wave rectifier. Draw its input, output wave forms.
(iii)How do you obtain steady DC output from the pulsating voltage?[Delhi 2013 C]
Ans.(i) p-n Junction A p-n junction is an arrangement made by a close contact of n-type
semiconductor and p-type semiconductor.There are various methods of forming p-n junction
diode. In one method, an n-type germanium crystal is cut into thin slices called wafers. An
aluminium film is laid on an n-type wafer which is then heated in an oven at a temperature of
about 600°C. Aluminium then diffuses into the surface of wafer. In this way, a p-type
semiconductor is formed on n-type semiconductor.
Formation of Depletion Region in p-n Junction In an n-type semiconductor, the
concentration of electrons is more than concentration of holes. Similarly, in a p-type
semiconductor, the concentration of holes is more than that of concentration of electrons.
During formation of p-n junction and due to the concentration gradient across p and n-sides,
holes diffuse from p-side to n-side ( p—> n) and electrons diffuse from n-side to p-side (n —>
p).
The diffused charge carriers combine with their counterparts in the immediate vicinity of the
junction and neutralise each other, (l) Thus, near the junction, positive charge is built on n-side
and negative charge on p-side This sets up potential difference across the junction and an
internal electric field Ej directed from n-side to p-side. The equilibrium is established when the
field £, becomes strong enough to stop further diffusion of the majority charge carriers
(however, it helps the minority charge carriers to diffuse across the junction).The region on
either side of the junction which becomes depleted (free) from the mobile charge carriers is
called depletion region or depletion layer. The width of depletion region is of the order of 10 -6
m.The potential difference developed across the depletion region is called the potential
barrier. Potential barrier depends on dopant concentration in the semiconductor and
temperature of the junction.
(ii)

(iii) A full-wave bridge rectifier using four diodes (full-wave bridge rectifier) gives a continuous,
unidirectional but pulsating output voltage or current.
The rectified output is passed through a filter circuit which removes the ripple and an almost
steady DC voltage (or current) is obtained.

48.Why is a Zener diode considered as a special purpose semiconductor diode? Draw the
I-V characteristics of Zener diode and explain briefly, how reverse current suddenly
increase at the breakdown voltage? Describe briefly with the help of a circuit diagram, how
a Zener diode work’s to obtain a constant DC voltage from the unregulated DC output of a
rectifier.[Delhi 2009 C; Foreign 2012]
Ans.Zener diode works only in reverse breakdown region that is why it is considered as a
special purpose semiconductor

Reverse current is due to the flow of electrons from n —> p and holes from p —» n. As, the
reverse biased voltage increase the elecHic field across the junction, increases signiffcantly
and when reverse bias voltage V – Vz, then the electric field strength is high enough to pull the
electrons from p-side and accelerated it to n-side.
These electrons are responsible for the high current at the breakdown.

Voltage regulator converts an unregulated DC output of rectifier into a constant regulated DC

voltage, using Zener diode. The unregulated voltage is connected to the Zener diode through a
series resistance Rs such that the Zener diode is reverse biased. If the input voltage increases,
then current through Rs apd Zener diode increases. Thus, the voltage drop across R^
increases without any change in the voltage drop across Zener diode. This is because of the
breakdown region, Zener voltage remain constant even though the current through Zener
diode changes.
Similarly, if the input voltage decreases, the current through Rs and Zener diode decreases.
The voltage drop across Rs, decreases without any change in the voltage across the Zener
diode.
Now, any change in input voltage results the change in voltage drop across Rs, without any
change in voltage across the Zener diode. Thus, Zener diode acts as a voltage regulator

49.(i) Describe briefly, with the help of a diagram, the role of the two important processes
involved in the formation of a p-n junction.
(ii) Name the device which is used as a voltage regulator. Draw the necessary circuit
diagram and explain its working.[HOTS; All India 2012]
Ans.
The two process involved in the formation of p-n junction.
(a)Diffusion
(b)Drift
Holes and electrons diffuse from p to n and n to p respectively.
The majority charge carrier drifts under the influence of applied electric field such that
(a)holes along applied E and
(b)electron opposite to E.
(ii) Zener diode is used as voltage regulator.
Principle Zener diode is operated in the reverse breakdown region. The voltage across it
remains constant, equal to the breakdown voltage for large charge in reverse current

50.(i) Draw the circuit diagram of a p-n junction diode in

(a)forward bias.
(b)reverse bias.
How are these circuits used to study the V-1 characteristics of a silicon diode? Draw the
typical V-I characteristics.
(ii) What is a Light Emitting Diode (LED)? Mention two important advantage of LEDs over
conventional lamps.[Delhi 2010 C; All India 2010]
Ans.(i) p-n Junction A p-n junction is an arrangement made by a close contact of n-type
semiconductor and p-type semiconductor.There are various methods of forming p-n junction
diode. In one method, an n-type germanium crystal is cut into thin slices called wafers. An
aluminium film is laid on an n-type wafer which is then heated in an oven at a temperature of
about 600°C. Aluminium then diffuses into the surface of wafer. In this way, a p-type
semiconductor is formed on n-type semiconductor.
Formation of Depletion Region in p-n Junction In an n-type semiconductor, the
concentration of electrons is more than concentration of holes. Similarly, in a p-type
semiconductor, the concentration of holes is more than that of concentration of electrons.
During formation of p-n junction and due to the concentration gradient across p and n-sides,
holes diffuse from p-side to n-side ( p—> n) and electrons diffuse from n-side to p-side (n —>
p).

The diffused charge carriers combine with their counterparts in the immediate vicinity of the
junction and neutralise each other, (l) Thus, near the junction, positive charge is built on n-side
and negative charge on p-side This sets up potential difference across the junction and an
internal electric field Ej directed from n-side to p-side. The equilibrium is established when the
field £, becomes strong enough to stop further diffusion of the majority charge carriers
(however, it helps the minority charge carriers to diffuse across the junction).The region on
either side of the junction which becomes depleted (free) from the mobile charge carriers is
called depletion region or depletion layer. The width of depletion region is of the order of 10 -6
m.The potential difference developed across the depletion region is called the potential
barrier. Potential barrier depends on dopant concentration in the semiconductor and
temperature of the junction.
(a) Forward Biased Characteristics
The circuit diagram for studying forward biased characteristics is shown in the figure.
Starting from a low value, forward bias voltage is increased step by step (measured by
voltmeter) and forward current is noted (by ammeter). A graph is plotted between voltage and
current. The curve so obtained is the forward characteristic of the diode.

At the start when applied voltage is low, the current through the diode is almost zero. It is
because of the potential barrier, which opposes the applied voltage. Till the applied voltage
exceeds the potential barrier, the current increases very slowly with increase in applied voltage
(OA portion of the graph).With further increase in applied voltage, the current increases very
rapidly (AB portion of the graph), in this situation, the diode behaves like a conductor. The
forward voltage beyond which the current through the junction starts increasing rapidly with
voltage is called knee voltage. If line AB is extended back, it cuts the voltage axis at potential
barrier voltage.
(b) Reverse Biased Characteristics
The circuit diagram for studying reverse biased characteristics is shown in the figure.

In reverse biased, the applied voltage supports the flow of minority charge carriers across the
junction. So, a very small current flows across the junction due to minority charge carriers.
Motion of minority charge carriers is also supported by internal potential barrier, so all the
minority carriers cross over the junction.Therefore, the small reverse current remains almost
constant over a sufficiently long range of reverse bias, increasing very little with increasing
voltage (OC portion of the graph). This reverse current is voltage independent upto certain
voltage known as breakdown voltage and this voltage independent current – is called reverse
saturation current.
Use of p-n Junction Characteristics in Rectification
From forward and reverse characteristics, it is clear that current flows through the junction
diode only in forward bias not in reverse bias i.e. current flows only in one direction
(ii) (a) Working of LED LED is a forward biased p-n junction which converts electrical energy
into optical energy of infrared and visible light region.
Being in forward bias, thin depletion layer and low potential barrier facilitate diffusion of
electron and hole through the junction when high energy electron of conduction band
combines with the low energy holes in valence band, then energy is released in the form of
photon, may be seen in the form of light.
(b)Uses of LEDs (a) LED can operate at very low voltage and consumes less power in
comparison to incandescent lamps. Unlike the lamps, they take very less operational time and
have long life.

51.(i) Draw I-V characteristics of a Zener diode.

(ii)Explain with the help of a circuit diagram, the use of a Zener diode as a voltage
regulator.
(iii)A photo diode is operated under reverse bias although in the forward bias, the current is
known to be more than the current in the reverse bias. Explain, giving reason. [HOTS;
Foreign 2010]
Ans. (i)Zener diode is used as voltage regulator.
Principle Zener diode is operated in the reverse breakdown region. The voltage across it
remains constant, equal to the breakdown voltage for large charge in reverse current

(ii) From the figure, it is clear that the device, X is a full-wave rectifier. Circuit diagram as
shown in figure below:

Zener diode connected with unregulated DC voltage in reverse bias. When the input voltage
increases, then current through R, increase and hence, voltage drop across Rp increases while
voltage across the Zener diode remains constant. The voltage across Zener diode remains
constant beyond Zener voltage and hence, same/constant regulated voltage is obtained
across RL.

Thus, fractional change in minority charge carrier (hole) is much higher than fraction change
in majority charge carrier (electron). Also, minority charge carrier contribute in drift current in
reverse bias.
Thus, with incidence of light, fractional change in minority charge carrier is significant.
Therefore, photo diode should be connected in reverse bias for measuring light intensity.

52.(i) Draw a circuit arrangement for studying V-I characteristics of a p-n junction diode
in(a)forward bias and (b) reverse bias.
Show typical V-I characteristics of a silicon diode.
(ii) State the main practical application of LED. Explain, giving reason, why the
semiconductor used for
fabrication of visible light LEDs must have a band gap of at least (nearly) 1.8 eV. [Delhi
2010 C]
Ans.(i) p-n Junction A p-n junction is an arrangement made by a close contact of n-type
semiconductor and p-type semiconductor.There are various methods of forming p-n junction
diode. In one method, an n-type germanium crystal is cut into thin slices called wafers. An
aluminium film is laid on an n-type wafer which is then heated in an oven at a temperature of
about 600°C. Aluminium then diffuses into the surface of wafer. In this way, a p-type
semiconductor is formed on n-type semiconductor.
Formation of Depletion Region in p-n Junction In an n-type semiconductor, the
concentration of electrons is more than concentration of holes. Similarly, in a p-type
semiconductor, the concentration of holes is more than that of concentration of electrons.
During formation of p-n junction and due to the concentration gradient across p and n-sides,
holes diffuse from p-side to n-side ( p—> n) and electrons diffuse from n-side to p-side (n —>
p).

The diffused charge carriers combine with their counterparts in the immediate vicinity of the
junction and neutralise each other, (l) Thus, near the junction, positive charge is built on n-side
and negative charge on p-side This sets up potential difference across the junction and an
internal electric field Ej directed from n-side to p-side. The equilibrium is established when the
field £, becomes strong enough to stop further diffusion of the majority charge carriers
(however, it helps the minority charge carriers to diffuse across the junction).The region on
either side of the junction which becomes depleted (free) from the mobile charge carriers is
called depletion region or depletion layer. The width of depletion region is of the order of 10 -6
m.The potential difference developed across the depletion region is called the potential
barrier. Potential barrier depends on dopant concentration in the semiconductor and
temperature of the junction.
(a) Forward Biased Characteristics
The circuit diagram for studying forward biased characteristics is shown in the figure.
Starting from a low value, forward bias voltage is increased step by step (measured by
voltmeter) and forward current is noted (by ammeter). A graph is plotted between voltage and
current. The curve so obtained is the forward characteristic of the diode.
At the start when applied voltage is low, the current through the diode is almost zero. It is
because of the potential barrier, which opposes the applied voltage. Till the applied voltage
exceeds the potential barrier, the current increases very slowly with increase in applied voltage
(OA portion of the graph).With further increase in applied voltage, the current increases very
rapidly (AB portion of the graph), in this situation, the diode behaves like a conductor. The
forward voltage beyond which the current through the junction starts increasing rapidly with
voltage is called knee voltage. If line AB is extended back, it cuts the voltage axis at potential
barrier voltage.
(b) Reverse Biased Characteristics
The circuit diagram for studying reverse biased characteristics is shown in the figure.

In reverse biased, the applied voltage supports the flow of minority charge carriers across the
junction. So, a very small current flows across the junction due to minority charge carriers.
Motion of minority charge carriers is also supported by internal potential barrier, so all the
minority carriers cross over the junction.Therefore, the small reverse current remains almost
constant over a sufficiently long range of reverse bias, increasing very little with increasing
voltage (OC portion of the graph). This reverse current is voltage independent upto certain
voltage known as breakdown voltage and this voltage independent current – is called reverse
saturation current.
Use of p-n Junction Characteristics in Rectification
From forward and reverse characteristics, it is clear that current flows through the junction
diode only in forward bias not in reverse bias i.e. current flows only in one direction
(ii) Working of LED LED is a forward biased p-n junction which converts electrical energy into
optical energy of infrared and visible light region.
Being in forward bias, thin depletion layer and low potential barrier facilitate diffusion of
electron and hole through the junction when high energy electron of conduction band
combines with the low energy holes in valence band, then energy is released in the form of
photon, may be seen in the form of light.
(a) Semiconductors with appropriate band gap (Eg) close to 1.5 eV are preferred to make LED
size GaAs, CdTe, etc.the other reasons to select these materials e high optical absorption,
availability of raw material and low cost
(b) Uses of LEDs
(i) LED can operate at very low voltage and consumes less power in comparison to
incandescent lamps
(ii) Unlike the lamps, they take very less operational time and have long life.

53.How is a Zener diode fabricated so as to make it a special purpose diode? Draw 7-V
characteristics of Zener diode and explain the significance of breakdown voltage, (ii)
Explain briefly, with the help of a circuit diagram, how a p-n junction diode works as a half-
wave rectifier. [Delhi 2009c]
Ans.(i)Zener diode works only in reverse breakdown region that is why it is considered as a
special purpose semiconductor

Reverse current is due to the flow of electrons from n —> p and holes from p —» n. As, the
reverse biased voltage increase the elecHic field across the junction, increases signiffcantly
and when reverse bias voltage V – Vz, then the electric field strength is high enough to pull the
electrons from p-side and accelerated it to n-side.
These electrons are responsible for the high current at the breakdown.

Voltage regulator converts an unregulated DC output of rectifier into a constant regulated DC

voltage, using Zener diode. The unregulated voltage is connected to the Zener diode through a
series resistance Rs such that the Zener diode is reverse biased. If the input voltage increases,
then current through Rs apd Zener diode increases. Thus, the voltage drop across R^
increases without any change in the voltage drop across Zener diode. This is because of the
breakdown region, Zener voltage remain constant even though the current through Zener
diode changes.
Similarly, if the input voltage decreases, the current through Rs and Zener diode decreases.
The voltage drop across Rs, decreases without any change in the voltage across the Zener
diode.
Now, any change in input voltage results the change in voltage drop across Rs, without any
change in voltage across the Zener diode. Thus, Zener diode acts as a voltage regulator
(ii) Circuit diagram of p-n junction diode as half-wave rectifier is shown below

Diode conducts corresponding to positive half cycle and does not conduct during negative
half cycle hence, AC is converted by diode into unidirectional pulsating DC. This action is
known as half-wave rectification.

54.(i) Draw the typical shape of the V-I characteristics of a p-n junction diode both in (a)
forward(b)reverse bias configuration. How do we infer, from these characteristics that a
diode can be used to rectify alternating voltages?
(ii) Draw the circuit diagram of a full-wave rectifier using a centre-tap transformer and two
p-n junction diodes. Give a brief description of the working of this circuit. [Delhi 2009 C]

Ans.(i) p-n Junction A p-n junction is an arrangement made by a close contact of n-type
semiconductor and p-type semiconductor.There are various methods of forming p-n junction
diode. In one method, an n-type germanium crystal is cut into thin slices called wafers. An
aluminium film is laid on an n-type wafer which is then heated in an oven at a temperature of
about 600°C. Aluminium then diffuses into the surface of wafer. In this way, a p-type
semiconductor is formed on n-type semiconductor.
Formation of Depletion Region in p-n Junction In an n-type semiconductor, the
concentration of electrons is more than concentration of holes. Similarly, in a p-type
semiconductor, the concentration of holes is more than that of concentration of electrons.
During formation of p-n junction and due to the concentration gradient across p and n-sides,
holes diffuse from p-side to n-side ( p—> n) and electrons diffuse from n-side to p-side (n —>
p).
The diffused charge carriers combine with their counterparts in the immediate vicinity of the
junction and neutralise each other, (l) Thus, near the junction, positive charge is built on n-side
and negative charge on p-side This sets up potential difference across the junction and an
internal electric field Ej directed from n-side to p-side. The equilibrium is established when the
field £, becomes strong enough to stop further diffusion of the majority charge carriers
(however, it helps the minority charge carriers to diffuse across the junction).The region on
either side of the junction which becomes depleted (free) from the mobile charge carriers is
called depletion region or depletion layer. The width of depletion region is of the order of 10 -6
m.The potential difference developed across the depletion region is called the potential
barrier. Potential barrier depends on dopant concentration in the semiconductor and
temperature of the junction.
(a) Forward Biased Characteristics
The circuit diagram for studying forward biased characteristics is shown in the figure.
Starting from a low value, forward bias voltage is increased step by step (measured by
voltmeter) and forward current is noted (by ammeter). A graph is plotted between voltage and
current. The curve so obtained is the forward characteristic of the diode.

At the start when applied voltage is low, the current through the diode is almost zero. It is
because of the potential barrier, which opposes the applied voltage. Till the applied voltage
exceeds the potential barrier, the current increases very slowly with increase in applied voltage
(OA portion of the graph).With further increase in applied voltage, the current increases very
rapidly (AB portion of the graph), in this situation, the diode behaves like a conductor. The
forward voltage beyond which the current through the junction starts increasing rapidly with
voltage is called knee voltage. If line AB is extended back, it cuts the voltage axis at potential
barrier voltage.
(b) Reverse Biased Characteristics
The circuit diagram for studying reverse biased characteristics is shown in the figure.

In reverse biased, the applied voltage supports the flow of minority charge carriers across the
junction. So, a very small current flows across the junction due to minority charge carriers.
Motion of minority charge carriers is also supported by internal potential barrier, so all the
minority carriers cross over the junction.Therefore, the small reverse current remains almost
constant over a sufficiently long range of reverse bias, increasing very little with increasing
voltage (OC portion of the graph). This reverse current is voltage independent upto certain
voltage known as breakdown voltage and this voltage independent current – is called reverse
saturation current.
Use of p-n Junction Characteristics in Rectification
From forward and reverse characteristics, it is clear that current flows through the junction
diode only in forward bias not in reverse bias i.e. current flows only in one direction

(ii)
 LogicGates, Transistors and its
Applications
1 Mark Questions
1.In a transistor, doping level in base is increased slightly. How will it affect
(i)collector current and
(ii)base current? [Delhi 2011]
Ans.(i) Collector current decreases.
(ii) Base current increases

2.Draw the logic circuit of a NAND gate and write its truth table.[Foreign 20113]
Ans.

3.Draw the logic circuit of AND gate and write its truth table.[Foreign 2011]
Ans.

4.Draw the logic circuit of NOT gate and write its truth table.[Foreign 2011]
Ans.

Ans.
Ans .

Ans.

8.Give the logic symbol of NOR gate.[All India 2009]

Ans.
9.Give the logic symbol of NAND gate. [All India 2009]
Ans.

10.Give the logic symbol of AND gate.[All India 2009]

Ans.

11.Define current amplification factor in common-emitter mode of transistor. [Delhi 2009 C,

All India 2010 C]
Ans.

2 Marks Questions
12.Draw a circuit diagram of n-p-n transistor amplifier in CE configuration. Under what
condition does the transistor act as an amplifier? [All India 2014]
Ans.Circuit diagram of n-p-n transistor amplifier in CE configuration is given below
The condition for the amplifier to work is that the base-emitter junction should be forward
biased and collector-base junction should be reversed biased

Ans.

Ans.
The logic gates are Pis NAND gate and Q is OR gate.
The truth table is given as shown in below:

15.The outputs of two NOT gates are fed to a NOR gate. Draw the logic circuit of the
combination of gates. Give its truth table. Identify the gate represented by this combination.
[Delhi 2014 C]
Ans.

16.The input wave forms A and B and the output waveform Y of a gate are shown below.
Name the gate it represents, write its truth table and draw the logic symbol of this gate.

Ans.

17. Identify the equivalent gate represented by the circuit shown in the figure. Draw its logic
symbol and write the truth table.

Ans.

18.In the given circuit diagram, a voltmeter V is connected across a lamp L. How would
(i)the brightness of the lamp and
(ii)voltmeter reading V be affected if the value of resistance R is decreased? Justify your
answer.
Ans.

19.Draw a typical output characteristics of an n-p-n transistor in CE Show how these

characteristics can be used to determine output resistance?[All India 2013]
Ans.
Ans.

Ans.
Ans.

23.Describe briefly with the help of a circuit diagram, how the flow of current carriers in a p-
n-p transistor is regulated with emitter-base junction in forward biased and base-collector
junction in reverse biased. [All India 2012]
Ans.

Heavily doped emitter is subjected to electric field by emitter-base battery and consequently,
holes gets drifted towards collector through thin and lightly doped base region. Nearly 5%
hole, which drifted from emitter combined with electron in base region and remaining nearly
95% hole reaches to collector under the influence VCE.
 CE

24.Distinguish between analog signal and digital signal. [All India 2012]
Ans .A signal in which current or voltage changes continuously with the time is called analog
signals. A signal in which current or voltage can take only two discrete values is called a digital
signal.

25.Draw the output waveform at X using the given inputs, A and B for the logic circuit shown
below. Also, identify the logic operation performed by this circuit

Ans.Equivalent gate is OR gate. If input A or B or both are 1, then the output of OR gate is 1.
Boolean expression of OR gate is given as A + B = X
Logic symbol of OR gate and the output waveform as shown below

26.In the given circuit, a voltmeter V is connected across lamp L What changes would you
observe in the lamp L and the voltmeter V if the value of resistor R is reduced?
Ans.Lamp glows brighter and voltmeter reading increases with the decrease of R. Input
current increase which in turn by transistor action lead to increase collector current. This
makes lamp brighter and hence, voltmeter reading goes up.

27.Draw the transfer characteristic curve of a base-biased transistor in CE Explain clearly

how the active region of the V 0 versus Vi. curve, in a transistor is used as an amplifier? [Delhi
2011]
Ans .The transfer characteristic curve of base biased transistor in CE configuration as shown
below:
28.Draw the output waveform at X using the given inputs, A and B for the logic circuit shown
below. Also, identify the logic operation performed by this circuit.[Delhi 2011; 2008]

Ans.

29.Write the truth table for the logic circuit shown below and identify the logic operation
performed by this Circuit. [Delhi 2011]

Ans.
30.Identify the logic gates X and Y in the figure. Write down the truth table of output Z for all
possible inputs A and [All India 2011 c]

Ans.

(ii) Show output waveform for all possible inputs of A and B.[All India 2011 C]
Ans.
Ans.

Ans.

Ans.
Ans.

36.The output of a 2-input AND gate is fed to a NOT gate. Give the name of the combination
and its logic symbol. Write down its truth table.[Foreign 2008, Delhi 2009]
Ans.
Ans.
If this, logic gate is connected to NOT gate, what will be the output when
(i)A = 0, B = 0 and
(ii)A = 1 B = 1?
Draw the logic symbol of the Combination.[Foreign 2009]
Ans.

39.A logic gate is obtained by applying output of OR gate to a NOT gate.Name the gate so
formed. Write the symbol and truth table of this gate.[Foreign 2009]
Ans .

40.A logic gate is obtained by applying output of AND gate to a NOT gate. Name the gate so
formed. Write the symbol and truth table of this gate.[Foreign 2009]
Ans.

41.The two circuits shown here are a combination

(i)Three NAND gates.
(ii)Three NOR gates.
Write truth tables for each of these combinations. [Delhi 2009 c]
Ans.

42.The given inputs A, B are fed to a 2-input NAND gate. Draw the output waveform of the
gate.

Ans.

43.In the output of a 2-input NOR gate is fed as both inputs, A and B to another NOR gate,
write down a truth table to find the final output, for all combinations of A, B.[Delhi 2008]
Ans.

44.Write the truth table and draw the logic symbol of the gate for the circuit given as below:
[Foreign 2008]

Ans.

45. Write the truth table and draw the logic symbol of the gate for the circuit given as below:
[Foreign 2008]

Ans.

3 Marks Questions
46.Output characteristics of an n-p-n transistor in CE configuration is -shown in the figure.
Determine
Ans.

47.You are given a circuit below. Write its truth table. Hence, identify the logic operation
carried out by this circuit. Draw the logic symbol of the gate which Corresponds to [All India
2011]

Ans.
48.You are given a circuit below. Write its truth table. Hence, identify the logic operation
carried out by this circuit. Draw the logic symbol of the gate which corresponds to[All India
2011]

Ans.

49.Draw transfer characteristics of a common-emitter n-p-n Point out the region in which the
transistor operates as an amplifier.
Define the following terms used in transistor amplifiers:
(i)Input resistance
(ii)Output resistance
(iii)Current amplification factor.[Foreign 2011]
Ans.
50.Draw the general shape of the transfer characteristics of a transistor in its CE Which
regions of this characteristic of a transistor are used when it works as an amplifier?[All India
2010 C]
Ans.
51.Give the circuit diagram of a common-emitter amplifier using an n-p-n transistor. Draw
the input and output wave forms of the signal. Write the expression for its voltage gain.
[HOTS, All India 2010]
Ans.
52.The inputs A and B shown here are used as the inputs for three different gates G 1, G2 and
G3. The outputs obtained in the three cases have the forms shown. Identify the three gates
and write their truth tables. [All India 2009 c]

Ans.
53.The inputs A and B shown here are used as the inputs for’ three different gates G 1, G2
and G3 one by one. The outputs obtained in the three cases have the forms shown. Identify
the three gates and write their symbols. [All India 2009 C]

Ans.

54.Draw the labelled circuit diagram of a common-emitter transistor amplifier. Explain

clearly, how the input and output signals are in opposite phase?[All India 2009]
Ans.
Thus, corresponding to positive half cycle of input AC, a negative amplified cycle is obtained
at collector and vice-versa. This shows that output and input signals are in opposite phase

55.The inputs A and B are inverted by using two NOT gates and their outputs are fed to the
NOR gate as shown below:
Analyse the action of the gates (1) and (2) and identify the logic gate of the complete circuit
so obtained. Give its symbol and the truth table.[All India 2009]
Ans.

56.Identify the gate equivalent to the dotted box shown here and give its symbol and truth
table The input A shown here is used with another unknown input B in this set up. If the
output Y has the form shown, give the intervals over which the input B is in its high state.
[Delhi 2008 C]

Ans.
B is in high state in the interval 3 to 4, 4 to 5 and 7 to 8.

5 Marks Questions
57.(i) Differentiate between three segments of a transistor on the basis of their size and
level of doping.
(ii)How is a transistor biased to be in active state?
(iii)With the help of necessary circuit diagram, describe briefly, how n-p-n transistor in CE
configuration amplifies a small sinusoidal input voltage. Write the expression for the AC
current gain. [Delhi 2014]
Ans .(i) The base region of the transistor is physically located between the emitter and the
collector region and is made from lightly doped high resistivity material. The emitter and
collector regions are heavily doped. But the doping level in emitter is slightly greater than that
of collector and the area of collector region is slightly more than that of emitter.
In term of doping level,
Emitter region > collector region > base region
In term of area of the region,
Collector region > emitter region > base region.The area of the collector region is greater than
that of emitter. This is because the collector region has to handle more power than the
emitter and also it has to collect more number of charge carriers to constituent the current.
Emitter is heavily doped to provide large member of majority charge carriers, while base and
collector are lightly doped to accept these charge carriers from emitter.
(ii) The conditions of a transistor for to be in active state are below:
(a)The input circuit should be forward biased by using a low voltage battery.
(b)The output circuit should be reverse biased by using a high voltage battery.
(iii) CE configuration While finding gain for CE configuration we should mind that it will
depend upon the load resistance, input resistance as well as output will be inverted.
Working In the circuit, emitter is forward biased and collector is reversed biased. This makes
input resistance (R in) very low and output resistance (R out) high. During the positive half cycle
of input AC decrease the forward bias.
58.(i)Explain briefly with the help of a circuit diagram, how an n-p-n transistor in CE
configuration is used to study input and output characteristics.
(ii) Describe briefly the underlying principle of a transistor amplifier working as an oscillator.
Hence, use the necessary circuit diagram to explain how self sustained oscillations are
achieved in the oscillator. [Delhi 2014 C]
Ans .(i) Common-emitter Transistor Characteristics
To study the characteristics of an n-p-n transistor in common-emitter mode, required circuit is
shown in the figure. Here, base-emitter circuit is forward biased with battery VBE and emitter-
collector circuit is reverse biased with battery Vcc .

From circuit diagram, we come across to know that it is made up of two sections, i.e. input
and output.
These two characteristics can be studied as shown below:
(a) Emitter or Input Characteristics
A graphical relation between the emitter voltage and the emitter current by keeping collector
voltage constant is called input characteristics of the transistor.
Adjust collector-emitter voltage at a suitable high value VCE (say = + 10 V). It is necessary so
as to make the base-collector junction reverse biased.
Now, with the help of rheostat gradually increases, the value of base-emitter voltage VBE in
small steps and note the corresponding values of base current lB.
(ii) Feedback When a portion of the output power is returned back to the input in phase this is
termed as positive feedback.

Feedback network The phenomenon of mutual inductance is used to take a part of output in
coil L’ back into input coil L. When the switch K is closed, collector current begin to flow
through L’, which in turn increases the magnetic flux linked with L’ and hence with L This leads
to produce an induce emf in L, which increases the forward bias. This also increases the base
current and hence collector current along with the charging of capacitor takes place with
upper plate as positive. This phenomenon is repeated again and again till the collector current
reaches to its maximum value.

At maximum value of lc, current through L’ does not change and therefore flux remains
unchanged and emf in L’ and L reduces to zero. Now, the discharging of capacitor begins
through L. The positivity of upper plate decreases and forward bias decrease, which results in
the form of decrease in base current and hence, decrease in collector current. This
phenomenon repeats till collector current reduces to zero and emf in the coil L also reduces
to zero.
Thus, the time duration in which collector current grows from zero to maximum, the current in
coil L of tank circuit complete its half cycle. The duration in which collector current reduces
from maximum to zero, the current in L’ completes its next half cycle

Thus, the AC of desired frequency and amplitude can be obtained by taking appropriate
value of inductance,capacitance and strength of battery B

Ans.(i) In this transistor, the emitter-base junction is forward biased and its resistance is very
low. So, the voltage of VEE is quite small.
The collector-base junction is reverse biased. The resistance of this junction is very high. So,
the voltage of Vcc (VCB) is quite large (= 45 V). Electrons in emitter are repelled towards base
by negative potential of VEE on emitter, resulting emitter current l E. The base being thin and
lightly doped has low density of holes, thus when electrons enter the base region, then only a
few holes get neutralised by electron hole combination, resulting in base current (IB). The
remaining electrons pass over to the collector, due to high positive potential of collector,
resulting in collector current (Ic) . As,’ e electron reaches to collector, it gets neutialised by the
flow of one electron from the negative terminal of the battery Vcc to collector through
connecting wire. Then, one electron flow from negative terminal of battery Vcc to positive
terminal of battery VEE and one electron flow from negative terminal of V EE to emitter.
When the electron coming from emitter combines with the holes in base, then deficiency of
hole in the base is compensated by the breaking of covalent bond there. The electron, so
released flows to the positive terminal of battery VEE, through connecting wire.
Thus, in n-p-n transistor, the current is carried inside as well as in external circuit by the
electrons
(ii) Common-emitter Transistor Characteristics
To study the characteristics of an n-p-n transistor in common-emitter mode, required circuit is
shown in the figure. Here, base-emitter circuit is forward biased with battery VBE and emitter-
collector circuit is reverse biased with battery Vcc .

From circuit diagram, we come across to know that it is made up of two sections, i.e. input
and output.
These two characteristics can be studied as shown below:
(a) Emitter or Input Characteristics
A graphical relation between the emitter voltage and the emitter current by keeping collector
voltage constant is called input characteristics of the transistor.
Adjust collector-emitter voltage at a suitable high value VCE (say = + 10 V). It is necessary so
as to make the base-collector junction reverse biased.
Now, with the help of rheostat gradually increases, the value of base-emitter voltage VBE in
small steps and note the corresponding values of base current lB.
(c) Feedback When a portion of the output power is returned back to the input in phase this is
termed as positive feedback.

Feedback network The phenomenon of mutual inductance is used to take a part of output in
coil L’ back into input coil L. When the switch K is closed, collector current begin to flow
throughL’, which in turn increases the magnetic flux linked with L’ and hence with L This leads
to produce an induce emf in L, which increases the forward bias. This also increases the base
current and hence collector current along with the charging of capacitor takes place with
upper plate as positive. This phenomenon is repeated again and again till the collector current
reaches to its maximum value.

At maximum value of lc, current through L’ does not change and therefore flux remains
unchanged and emf in L’ and L reduces to zero. Now, the discharging of capacitor begins
through L. The positivity of upper plate decreases and forward bias decrease, which results in
the form of decrease in base current and hence, decrease in collector current. This
phenomenon repeats till collector current reduces to zero and emf in the coil L also reduces
to zero.
Thus, the time duration in which collector current grows from zero to maximum, the current in
coil L of tank circuit complete its half cycle. The duration in which collector current reduces
from maximum to zero, the current in L’ completes its next half cycle

Thus, the AC of desired frequency and amplitude can be obtained by taking appropriate
value of inductance,capacitance and strength of battery B

60.(i) Why is the base region of a transistor thin and lightly doped?
(ii) Draw the circuit diagram for studying the characteristics of an n-p-n transistor in
common-emitter
configuration.Sketch the typical (a) input and (b) output characteristics in this configuration.
(iii)Describe briefly, how the output characteristics can be used to obtain the current gain in
the transistor? [Delhi 2013 C]

Ans.(i) In this transistor, the emitter-base junction is forward biased and its resistance is very
low. So, the voltage of VEE is quite small.
The collector-base junction is reverse biased. The resistance of this junction is very high. So,
the voltage of Vcc (VCB) is quite large (= 45 V). Electrons in emitter are repelled towards base
by negative potential of VEE on emitter, resulting emitter current l E. The base being thin and
lightly doped has low density of holes, thus when electrons enter the base region, then only a
few holes get neutralised by electron hole combination, resulting in base current (IB). The
remaining electrons pass over to the collector, due to high positive potential of collector,
resulting in collector current (Ic) . As,’ e electron reaches to collector, it gets neutialised by the
flow of one electron from the negative terminal of the battery Vcc to collector through
connecting wire. Then, one electron flow from negative terminal of battery Vcc to positive
terminal of battery VEE and one electron flow from negative terminal of V EE to emitter.
When the electron coming from emitter combines with the holes in base, then deficiency of
hole in the base is compensated by the breaking of covalent bond there. The electron, so
released flows to the positive terminal of battery VEE, through connecting wire.
Thus, in n-p-n transistor, the current is carried inside as well as in external circuit by the
electrons
(ii) Common-emitter Transistor Characteristics
To study the characteristics of an n-p-n transistor in common-emitter mode, required circuit is
shown in the figure. Here, base-emitter circuit is forward biased with battery VBE and emitter-
collector circuit is reverse biased with battery Vcc .

From circuit diagram, we come across to know that it is made up of two sections, i.e. input
and output.
These two characteristics can be studied as shown below:
(a) Emitter or Input Characteristics
A graphical relation between the emitter voltage and the emitter current by keeping collector
voltage constant is called input characteristics of the transistor.
Adjust collector-emitter voltage at a suitable high value VCE (say = + 10 V). It is necessary so
as to make the base-collector junction reverse biased.
Now, with the help of rheostat gradually increases, the value of base-emitter voltage VBE in
small steps and note the corresponding values of base current lB.
(c) Feedback When a portion of the output power is returned back to the input in phase this is
termed as positive feedback.

Feedback network The phenomenon of mutual inductance is used to take a part of output in
coil L’ back into input coil L. When the switch K is closed, collector current begin to flow
through L’, which in turn increases the magnetic flux linked with L’ and hence with L This leads
to produce an induce emf in L, which increases the forward bias. This also increases the base
current and hence collector current along with the charging of capacitor takes place with
upper plate as positive. This phenomenon is repeated again and again till the collector current
reaches to its maximum value.

At maximum value of lc, current through L’ does not change and therefore flux remains
unchanged and emf in L’ and L reduces to zero. Now, the discharging of capacitor begins
through L. The positivity of upper plate decreases and forward bias decrease, which results in
the form of decrease in base current and hence, decrease in collector current. This
phenomenon repeats till collector current reduces to zero and emf in the coil L also reduces
to zero.
Thus, the time duration in which collector current grows from zero to maximum, the current in
coil L of tank circuit complete its half cycle. The duration in which collector current reduces
from maximum to zero, the current in L’ completes its next half cycle

Thus, the AC of desired frequency and amplitude can be obtained by taking appropriate
value of inductance,capacitance and strength of battery B
(iii)Circuit is as shown below:
Ans.While finding gain for CE configuration we should mind that it will depend upon the load
resistance, input resistance as well as output will be inverted.
Working In the circuit, emitter is forward biased and collector is reversed biased. This makes
input resistance (R in) very low and output resistance (R out) high. During the positive half cycle
of input AC decrease the forward bias.
Ans.Circuit is as shown below:
63.(i)Using the necessary circuit diagram, draw the transfer characteristics of a base-biased
transistor in CE configuration. With the help of these characteristics, explain briefly how the
transistor can be used as an amplifier?
(ii) Why are NAND gate called universal gates? Identify the logical operations carried out by
the circuit given as below:

Ans.(i)
Ans.(i) For n-p-n transistor in CE configuration circuit diagram
The transfer characteristic curve of base biased transistor in CE configuration as shown
below:
65.(i)Draw the circuit diagram used for studying the input and output characteristics of an
n-p-n transistor in the CE configuration. Show the typical shapes of these two
characteristics.
(ii) How are the
(a)input resistance
(b)current amplification factor of the transistor determined from these characteristics?
[Delhi 2010 C]
Ans .(i) Common-emitter Transistor Characteristics
To study the characteristics of an n-p-n transistor in common-emitter mode, required circuit is
shown in the figure. Here, base-emitter circuit is forward biased with battery VBE and emitter-
collector circuit is reverse biased with battery Vcc.

From circuit diagram, we come across to know that it is made up of two sections, i.e. input
and output.
These two characteristics can be studied as shown below:
(a) Emitter or Input Characteristics
A graphical relation between the emitter voltage and the emitter current by keeping collector
voltage constant is called input characteristics of the transistor.
Adjust collector-emitter voltage at a suitable high value VCE (say = + 10 V). It is necessary so
as to make the base-collector junction reverse biased.
Now, with the help of rheostat gradually increases, the value of base-emitter voltage VBE in
small steps and note the corresponding values of base current lB.
(ii) Feedback When a portion of the output power is returned back to the input in phase this is
termed as positive feedback.

Feedback network The phenomenon of mutual inductance is used to take a part of output in
coil L’ back into input coil L. When the switch K is closed, collector current begin to flow
through //, which in turn increases the magnetic flux linked with L’ and hence with L This leads
to produce an induce emf in L, which increases the forward bias. This also increases the base
current and hence collector current along with the charging of capacitor takes place with
upper plate as positive. This phenomenon is repeated again and again till the collector current
reaches to its maximum value.

At maximum value of lc, current through L’ does not change and therefore flux remains
 c
unchanged and emf in L’ and L reduces to zero. Now, the discharging of capacitor begins
through L. The positivity of upper plate decreases and forward bias decrease, which results in
the form of decrease in base current and hence, decrease in collector current. This
phenomenon repeats till collector current reduces to zero and emf in the coil L also reduces to
zero.
Thus, the time duration in which collector current grows from zero to maximum, the current in
coil L of tank circuit complete its half cycle. The duration in which collector current reduces
from maximum to zero, the current in L’ completes its next half cycle

Thus, the AC of desired frequency and amplitude can be obtained by taking appropriate
value of inductance,capacitance and strength of battery B.

(ii)

66.(i)Draw a circuit diagram to study the input and output characteristics of an n -p- n
transistor in its common- emitter configuration. Draw the typical input and output
characteristics.
(ii) Explain with the help of a circuit diagram, the working of an n-p-n transistor as a
common-emitter amplifier. [Delhi 2009 C]
Ans.(i) Common-emitter Transistor Characteristics
To study the characteristics of an n-p-n transistor in common-emitter mode, required circuit is
shown in the figure. Here, base-emitter circuit is forward biased with battery VBE and emitter-
collector circuit is reverse biased with battery Vcc.
From circuit diagram, we come across to know that it is made up of two sections, i.e. input
and output.
These two characteristics can be studied as shown below:
(a) Emitter or Input Characteristics
A graphical relation between the emitter voltage and the emitter current by keeping collector
voltage constant is called input characteristics of the transistor.
Adjust collector-emitter voltage at a suitable high value VCE (say = + 10 V). It is necessary so
as to make the base-collector junction reverse biased.
Now, with the help of rheostat gradually increases, the value of base-emitter voltage VBE in
small steps and note the corresponding values of base current lB.
(ii) Feedback When a portion of the output power is returned back to the input in phase this is
termed as positive feedback.

Feedback network The phenomenon of mutual inductance is used to take a part of output in
coil L’ back into input coil L. When the switch K is closed, collector current begin to flow
through //, which in turn increases the magnetic flux linked with L’ and hence with L This leads
to produce an induce emf in L, which increases the forward bias. This also increases the base
current and hence collector current along with the charging of capacitor takes place with
upper plate as positive. This phenomenon is repeated again and again till the collector current
reaches to its maximum value.

At maximum value of lc, current through L’ does not change and therefore flux remains
unchanged and emf in L’ and L reduces to zero. Now, the discharging of capacitor begins
through L. The positivity of upper plate decreases and forward bias decrease, which results in
the form of decrease in base current and hence, decrease in collector current. This
phenomenon repeats till collector current reduces to zero and emf in the coil L also reduces to
zero.
Thus, the time duration in which collector current grows from zero to maximum, the current in
coil L of tank circuit complete its half cycle. The duration in which collector current reduces
from maximum to zero, the current in L’ completes its next half cycle
Thus, the AC of desired frequency and amplitude can be obtained by taking appropriate
value of inductance,capacitance and strength of battery B.

(ii)
67.Draw a circuit diagram of an n-p-n transistor with its emitter base junction forward biased
and base- collector junction reverse biased. Describe briefly its working.
Explain, how a transistor in active state exhibits a low resistance at its emitter-base junction
and high resistance at its base-collector junction? [Foreign 2009]
Ans. n-p-n transistor in CB configuration
Since, the base is common in input and output circuits, therefore transistor is connected in CB
configuration.

Working When input voltage, VBE is sufficient to make flow of emitter current, collector current
flows in output circuit. In this condition, the circuit is said to be in active state.
The small change in VEB, produces sufficient change in emitter current and hence, in collector
current. The input circuit offers very small resistance as ample change in emitter current
occurs corresponding to small change in input voltage.
This lead to produce large change in output voltage inspite of smaller change in collector
current (l E < l c). This shows that output circuit offer high resistance

68.Draw a labelled circuit diagram of a base-biased transistor in common-emitter

configuration. Plot the transfer characteristics of this base biased transistor indicating the
different regions of its operation.[Delhi 2009 c]
Ans.
Ans.(i)
70.(i) The same input -a! is applied to both the (input) terminals of a given logic gate. If the
output is(a)same as the (common) input signal.
(b)inverted with respect to the (common) input signal.
Identify the logic gates involved in each case.
(ii) Write the truth tables for each of the combinations shown below. Also identify the logic
operations performed by them. [All India 2008 C]

Ans.