Sri Ramakrishna College of Arts & Science, Coimbatore, Department of Physics

1. Electronic
Devices and
Electrical
Network Skills
21PHE01

2. Course Outcome
CO1 – Explain the
Characteristics of Special
Diodes and Transistors
CO2 – Describe the
working Principle of JFET
and UJT
CO3 – Design Basic
electronic circuits such as
voltage amplifiers and
oscillators implementing
the principles of negative
and positive feedbacks
CO4 – Distinguish
between AC and DC
signals and explain the
response of circuit
elements to these signals
CO5 – Examine
Thevenin’s and Norton’s
theorems to a network,
convert star to delta and
vice versa

3. Unit I – Diodes and Transistors
DIODES:
Characteristics of PN Junction Diode
Characteristics of Zener Diode
Characteristics of tunnel Diode
Characteristics of Varactor Diode
Characteristics of Schottky Diode
Characteristics of Light Emitting Diode
TRANSISTORS:
Working – Types of Transistor
Characteristics of CE, CB & CC
Configuration

4. Diode
• Diode, an electrical component that allows the flow of current in only one direction.
• A diode is a specialized electronic component with two electrodes called the anode
and the cathode.
• Most diodes are made with semiconductor materials such as silicon, germanium, or
selenium.
• Some diodes are comprised of metal electrodes in a chamber evacuated or filled
with a pure elemental gas at low pressure.

5. Types of Diode
• Light Emitting Diode
• Laser diode
• Avalanche diode
• Zener diode
• Schottky diode
• Photodiode
• PN junction diode
• Varactor Diode
• Tunnel Diode
• Shockley Diode

6. Applications of Diode
• Rectifying a voltage: turning AC into DC voltages
• Drawing signals from a supply
• Controlling the size of a signal
• Mixing (multiplexing) signals
Reference: Learn More about Diodes

7. PN Junction Diode
• Junction generally means the area or point that
bounds two different parts, similarly in diodes junction
is a boundary of two semiconductor materials i.e. the
p-type and the n-type, semiconductor

8. Forward and Reverse Bias

9. Forward and Reverse Bias
• Increase Voltage in steps of 0.1V
• Junction Voltage 0.7V for Si & 0.3V for Ge
• Not to Increase Voltage beyond Safe limit – Burnout
• Knee Voltage / Cut-in-Voltage / threshold Voltage - Voltage at which Diode starts
to conduct current (0.6V for Si and 0.2V for Ge)
• Breakdown Votage VBR
• Reverse Saturation Current Io (1 nA for Si and 1µ A for Ge)

10. Forward and Reverse Bias

11. V-I Characteristics

12. Time to Think and Answer
• Question 1: When silicon is doped with indium it leads to which type of
semiconductor?
• Question 2: A transistor has a current gain of 30 Ampere. If the collector resistance
is 6 kΩ, the input resistance is 1 kΩ, calculate its voltage gain.
• Question 3: Write characteristics of holes.
• Question 4: Name the kind of biasing which leads the following result:
• a) Increase in resistance,
• b) Decrease in resistance and
• c) Increase in width of the depletion region.
• Question 5: What is the ratio of electrons and holes in the intrinsic semiconductor?
• Question 6: Define the term breakdown voltage of p-n junction.

13. Question & Answers:
• When silicon doped with indium it leads to which type of semiconductor?
Answer:
• As we know, Valency of Indium is 3 therefore it is Trivalent in nature, when it is doped in
Silicon it has majority of holes, that’s why it is of p-type semiconductor.
• A transistor has a current gain of 30 Ampere. If the collector resistance is 6 kΩ, the
input resistance is 1 kΩ, calculate its voltage gain.
Answer:
Given, Rin =1 kΩ and Rout = 6k Ω ; ∴ Rgain = Rout/Rin = 6/1 = 6
Voltage gain = current gain × Resistance gain = 30 × 6 =180

14. Question & Answers:
• Write characteristics of holes.
Answer:
• Following are the characteristics of holes:
• A hole is equivalent to a positive electric charge.
• The mobility of a hole as compared to that of an electron is less.

15. Question & Answers:
• Name the kind of biasing which leads the following result:
• a) Increase in resistance,
• b) Decrease in resistance and
• c) Increase in width of the depletion region.
Answer:
• a) In reverse bias resistance increases.
• b) In forward bias resistance decrease.
• c) In reverse bias there is increase in the width of depletion region take place.

16. Question & Answers:
• What is the ratio of electrons and holes in the intrinsic semiconductor?
Answer:
• Number of electrons = ne
• Number of holes = nh
• In intrinsic semiconductor, ne = nh
• ne/nh = 1
• Define the term breakdown voltage of p-n junction.
Answer:
• In reverse bias condition, when the applied voltage increases gradually at a certain point
there is increase in reverse current noticed, this is junction breakdown, corresponding
applied voltage is known as breakdown voltage of p-n junction diode.

17. Zener Diode
17
DHIVYA R
Assistant Professor
Department of Physics
Sri Ramakrishna College of Arts and Science
Coimbatore - 641 006
Tamil Nadu, India

18. Contents
Zener Diode
Avalanche breakdown
Zener breakdown
Avalanche vs Zener breakdown
VI characteristics of Zener diode
Advantages and Applications of Zener
Diode

19. Zener
Diode
• A Zener diode is a special type of
device designed to operate in the
Zener breakdown region
• Zener diodes acts like normal p-n
junction diodes under forward
biased condition.
• Zener diode is heavily doped than
the normal p-n junction diode.
Hence, it has very thin depletion
region.
• Zener diodes allow more electric
current than the normal p-n
junction diodes.
• . Zener diode is always connected
in reverse direction because it is
specifically designed to work in
reverse direction.

20. Avalanche breakdown
• Avalanche breakdown occurs due to the
rapid collision of electrons with other
atoms.
• Zener breakdown occurs because of the
high electric field.
• The avalanche breakdown occurs in both
normal diodes and Zener diodes at high
reverse voltage
• However, avalanche diodes may not be
destroyed because they are carefully
designed to operate in avalanche
breakdown region.

21. Zener breakdown
• Breakdown occurs in heavily doped p-n junction
diodes because of their narrow depletion region.
• When reverse biased voltage applied to the diode
is increased, the narrow depletion region
generates strong electric field.
• When reverse biased voltage applied to the diode
reaches close to Zener voltage, the electric field in
the depletion region is strong enough to pull
electrons from their valence band.
• The valence electrons which gains sufficient
energy from the strong electric field of depletion
region will breaks bonding with the parent atom.
• The valance electrons which break bonding with
parent atom will become free electrons.
• This free electrons carry electric current from one
place to another place. At Zener breakdown
region, a small increase in voltage will rapidly
increases the electric current.

22. Avalanche
vs Zener
breakdown
Zener breakdown occurs at low reverse
voltage whereas avalanche breakdown occurs
at high reverse voltage.
Zener breakdown occurs in Zener diodes
because they have very thin depletion
region.
Breakdown region is the normal operating
region for a Zener diode.
Zener breakdown occurs in Zener diodes with
Zener voltage (Vz) less than 6V.
Avalanche breakdown occurs due to the rapid
collision of electrons with another atoms &
Zener breakdown occurs because of the high
electric field.

23. VI characteristics
of Zener diode
• At this point, a small increase in reverse voltage will rapidly increases the electric
current.
• Because of this sudden rise in electric current, breakdown occurs called Zener
breakdown.
• However, Zener diode exhibits a controlled breakdown that does damage the device.
• The Zener breakdown voltage of the Zener diode is depends on the amount of
doping applied.
• If the diode is heavily doped, Zener breakdown occurs at low reverse voltages.
• On the other hand, if the diode is lightly doped, the Zener breakdown occurs at high
reverse voltages. Zener diodes are available with Zener voltages in the range of 1.8V
to 400V.
• When reverse biased voltage is applied to
a Zener diode, it allows only a small
amount of leakage current until the
voltage is less than Zener voltage.
• When reverse biased voltage applied to
the Zener diode reaches Zener voltage, it
starts allowing large amount of electric
current.

24. Applications
Advantages
• Power dissipation capacity
is very high
• High accuracy
• Small size
• Low cost
It is normally used as voltage
reference
Zener diodes are used in voltage
stabilizers or shunt regulators.
Zener diodes are used in
switching operations
Zener diodes are used in clipping
and clamping circuits.
Zener diodes are used in various
protection circuits

25. Tunnel Diode
Unit I

26. Tunnel Diode
• A Tunnel Diode is a heavily doped p-n junction diode (Normal Doping - 1 part in 108; Tunnel Diode - 1 part in 103)
• The tunnel diode shows negative resistance (i.e., When voltage value increases, current flow decreases)
• Tunnel diode works based on Tunnel Effect.
• The current induces because of the tunnelling.

27. Tunnelling
•The tunnelling is the
phenomenon of
conduction in
the semiconductor mat
erial in which the
charge carrier punches
the barrier instead of
climbing through it.

28. Construction
• The device is constructed by using the two
terminals namely anode and cathode.
• Gallium arsenide, germanium and gallium
antimonide are used for manufacturing the
tunnel diode.
• The ratio of the peak value of the forward current
to the value of the valley current is maximum in
case of germanium and less in silicon. Hence
silicon is not used for fabricating the tunnel
diode. The doping density of the tunnel diode is
1000 times higher than that of the ordinary
diode.

29. V-I
Characteristics

30. V-I
Characteristics
• Forward Biasing
• Immediate conduction occurs in the diode because of
their heavy doping
• Reached Maximum Currect Ip at an Voltage Vp
• Applied Voltage is increased above Vp, the Ip Value
decreases until it reaches a minimum value – Valley
Current Iv.
• In the region A – B in the graph the diode produces
power instead of absorbing.

31. V – I Characteristics

32. Advantages &
Disadvantages
The tunnel diode is low cost.
It produces low noise, and their fabrication is also very
simple.
The diode gives a fast response, and it is moderate in
operation.
The tunnel diode works on low power.
The disadvantage of the tunnel diode is that output voltage
of the diode swings.
It is a two-terminal device, but their input and output
circuits are not isolated from each other.

33. Applications
The tunnel diode can be used as an
amplifier and as an oscillator for detecting
small high-frequency or as a switch.
It is a high-frequency component because
it gives the very fast responses to the
inputs.
The tunnel diode is not widely used
because it is a low current device.

34. Time to Think
Why silicon cannot be used in tunnel diode?
What is the tunnel diode needed for?
In which region does tunnel diode operates very
fast?
What are the characteristics of tunnel diode?
Who invented The tunnel diode?
The tunneling phenomena is otherwise known
as?

35. Time to Think
Why silicon is not used in tunnel diode?
• Answer
• The ratio of the peak value of the forward current
to the value of the valley current is maximum in
case of germanium and less in silicon. Hence
silicon is not used for fabricating the tunnel diode.
What is the tunnel diode needed for?
• Answer
• Tunnel diode can be used as a switch, amplifier,
and oscillator. Since it shows a fast response, it is
used as high frequency component. Tunnel diode
acts as logic memory storage device. They are used
in oscillator circuits, and in FM receivers.

36. Time to Think
In which region tunnel diode operates very fast?
• Answer
• Tunnel diode is a type of semiconductor which works on
tunneling effect of electrons in microwave region. So,
tunnel diode has a very fast operation in microwave
region.
What are the characteristics of tunnel diode?
• Answer
• The I-V characteristics of a tunnel diode exhibit
• current-controlled negative resistance.
• voltage-controlled negative resistance.
• temperature-controlled positive resistance.
• current-controlled positive resistance.

37. Time to Think
The tunnel diode was
invented by
• Answer
• Leo Esaki
The tunneling phenomena is
also known as
• Answer
• Auto electronic phenomena

38. VARACTOR
DIODE

39. Varactor Diode
Varactor diode is a p-n
junction
diode whose capacitance is
varied by varying the
reverse voltage.
Varactor – Variable
Capacitor
Operates only in reverse
bias
Varactor diode – Variable
Capacitor under reverse bias
Varactor diode is also
sometimes referred to as
varicap diode, tuning
diode, variable reactance
diode, or variable
capacitance diode.
The varactor diode is
manufactured in such as way
that it shows better transition
capacitance property than
the ordinary diodes

40. Capacitance

41. Capacitor and capacitance
•
•
•

42. Construction
•
•
•
•
•
•

43. Working When a p-type semiconductor is in contact with the n-
type semiconductor, a p-n junction is formed between
them. This p-n junction separates the p-type and n-type
semiconductor.
At the p-n junction, a depletion region is created. A
depletion region is a region where mobile charge
carriers (free electrons and holes) are absent.
The depletion region is made up of positive and
negative ions (charged atoms). These positive and
negative ions does not move from one place to another
place.
The depletion region blocks free electrons from n-side
and holes from p-side. Thus, depletion region blocks
electric current across the p-n junction.

44. Working A heavily doped varactor diode - thin depletion layer
whereas
A lightly doped varactor diode - wide depletion layer.
An insulator or a dielectric - does not allow electric current
through it.
The depletion region - does not allow electric current
through it.
Depletion region acts like a dielectric of a capacitor.
The p-type and n-type semiconductor acts like the electrodes or
conductive plates of the capacitor for a Varactor Diode
In an unbiased varactor diode, the depletion width is
very small.
The capacitance (charge storage) is very large.

45. Working
•
•
•
•
•

46. Working
•
•
•
•
•
•

47. Working
•
•

48. Advantages and
Applications
The capacitance of
a varactor diode is
measured in
picofarads (pF).
Varactor diode is
used in frequency
multipliers.
Varactor diode is
used in parametric
amplifiers.
Varactor diode is
used in voltage-
controlled
oscillators.

49. Time to Think
What is the use of a
varactor diode, In a
Colour Television
Receiver?
This circuit is an
equivalent for _____ ?
Which Value do we vary
when we tune a radio to
any desired station?
A varactor diode cannot
be used in which of the
following applications?
• Parametric amplifier;
• Frequency tuner;
• Voltage controlled
oscillator;
• Phase shifter
What does the following
represent?
𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒 𝑎𝑡 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑒𝑑
𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒 𝑎𝑡 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑒𝑑
𝑚𝑖𝑥𝑖𝑚𝑢𝑚 𝑣𝑜𝑙𝑡𝑎𝑔𝑒

50. Think and Answer
•
•
•

51. Think and Answer
•
•
•
•
•
•
•

52. Think and Answer
•
•
•
•
•
•

53. Schottky Diode
Unit I

54. Schottky
Diode
• Schottky diode is a metal-semiconductor
junction diode.
• It has less forward voltage drop than the P-N
junction diode.
• It can be used in high-speed switching
applications.
• It is named after German physicist Walter H.
Schottky.
• It is also known as Schottky barrier diode,
surface barrier diode, majority carrier
device, hot-electron diode, or hot carrier
diode.

55. Construction
• M-S junction – Metal Semiconductor Junction -
aluminum or platinum metal is joined with N-
type semiconductor
• M-S Junction - creates a barrier or depletion
layer known as a Schottky barrier
• M-S Junction - can be either non-rectifying or
rectifying
• Non-rectifying M-S Junction - ohmic contact.
Rectifying M-S Junction - non-ohmic contact.

56. Schottky Barrier Height
• The most important characteristics of a Schottky
barrier is the Schottky barrier height
• The value of this barrier height depends on the
combination of semiconductor and metal
• The Schottky barrier height of ohmic contact (non-
rectifying barrier) is very low
• The Schottky barrier height of non-ohmic contact
(rectifying barrier) is high
• In non-rectifying Schottky barrier, the barrier
height is not high enough to form a depletion
region
• Depletion region is negligible or absent in the
ohmic contact diode

57. Rectifying and
Non rectifying
The rectifying
Schottky barrier
- metal is in
contact with the
lightly doped
semiconductor.
The non-
rectifying barrier
- metal is in
contact with the
heavily doped
semiconductor.
The ohmic
contact has a
linear current-
voltage (I-V)
curve.
The non-ohmic
contact has a
non-linear
current-voltage
(I-V) curve.

58. Unbiased
Schottky diode

59. Unbiased Schottky
diode
• Positive ions are created the n-side
junction
• Negative ions are created at the
metal junction
• Hence a depletion region is created
• The metal has a sea of free electrons
but the width over which these
electrons move into the metal is
negligibly thin as compared to the
width inside the n-type
semiconductor
• A Built-in-potential or built-in-
voltage is primarily present inside
the n-type semiconductor – Barrier
to electrons from SC.

60. Forward biased
Schottky diode
• Under FB - a large number of free electrons are
generated in the n-type semiconductor and metal.
• The free electrons in n-type semiconductor and
metal cannot cross the junction unless the applied
voltage is greater than 0.2 volts.
• Above 0.2 V, the free electrons gain enough energy
and overcomes the built-in-voltage of the
depletion region and hence an electrc current
starts to flow.
• On increasing the applied voltage, the depletin
region becomes thin and finally disappears.

61. Reverse bias Schottky
diode
• Reverse bias – Width of depletion region
increases – electric current stops flowing –
only a small leakage current is detected.
• If the reverse bias voltage is continuously
increased, the electric current gradually
increases due to the weak barrier.
• RBV – Largely increased, sudden raise in
current causing a breakdown to depletion
region will be detected, which may damage
the diode.

62. V-I
Characteristics

63. Difference between Schottky diode and P-N junction
diode
SL
No
Schottky Diode P-N Junction Diode
1
In Schottky diode, the free electrons carry most of the
electric current and the holes carry negligible electric
current hence they are known as Unipolar Devices.
In P-N junction diode, both free electrons
and holes carry electric current. So P-N junction diode is
a bipolar device.
2
The reverse breakdown voltage of a Schottky diode is
comparatively very small
The reverse breakdown voltage of a P-N junction diode
is comparatively large
3
In Schottky diode, the depletion region is absent or
negligible
In P-N junction diode the depletion region is present
4 The turn-on voltage for a Schottky diode is very low The turn-on voltage for a P-N junction diode is high
5
In Schottky diode, electrons are the majority carriers in
both metal and semiconductor.
In P-N junction diode, electrons are the majority carriers
in n-region and holes are the majority carriers in p-
region.

64. Advantages and
Disadvantages
 Low junction capacitance
 Fast reverse recovery time
 High current density
 Low forward voltage drop or low turn on voltage
x Large reverse saturation current

65. Applications
Schottky diodes are
used as general-
purpose rectifiers.
Schottky diodes are
used in radio frequency
(RF) applications.
Schottky diodes are
widely used in power
supplies.
Schottky diodes are
used to detect signals.
Schottky diodes are
used in logic circuits.

66. Time to Think
1. Which metal cannot be used in Schottky diode?
2. What are all the metal used in Schottky diode?
3. Schottky diode is used on high frequencies
(More than 300 MHz) as Rectifier. True / False?
4. What are the other terms used to denote a
Schottky diode?
5. Schottky diode is unipolar – Justify.

67. Think and Answer
Which metal is not used
in Schottky diode?
Titanium silicide and other refractory
silicides, which are able to withstand the
temperatures needed for source/drain
annealing in CMOS processes, usually
have too low a forward voltage to be
useful, so processes using these silicides
therefore usually do not offer Schottky
diodes
What metal is used in
Schottky diode?
Typical metals used are molybdenum,
platinum, chromium or tungsten.
Schottky diode is used
on high frequencies
(More than 300 MHz) as
Rectifier.
True / False

68. Think and Answer
Schottky diode is also known
as
Schottky barrier diode
Hot carrier diode
Why is Schottky diode
unipolar?
The Schottky barrier diode has electrons
as majority carriers on both sides of the
junction. So, it is a unipolar device.

69. Light Emitting Diode
Unit I

70. Light Emitting Diode
• Light emitting diodes emit either visible light or
invisible infrared light when forward biased.
• The LEDs which emit invisible infrared light are
used for remote controls.
• A light Emitting Diode (LED) is an optical
semiconductor device that emits light when
voltage is applied.
• LED is an optical semiconductor device that
converts electrical energy into light energy.

71. LED - Construction
• Three layers: p-type
semiconductor, n-type
semiconductor and depletion
layer.
• Has a Depletion/Active region
in-between.

72. Working
• Light Emitting Diode (LED) works only in forward bias
condition
• When LED is forward biased, the free electrons from n-
side and the holes from p-side are pushed towards the
junction.
• When free electrons from n-region reach the junction
or depletion region, recombination occurs.
• Similarly, holes from p-side recombine with electrons in
the depletion region.
• Because of the recombination in the depletion region,
the width of depletion region decreases, and more
charge carriers will cross the p-n junction.

73. Working
• Recombination takes place in depletion region as well as in
p-type and n-type semiconductor.
• The free electrons in the conduction band releases energy
in the form of light before they recombine with holes in the
valence band.
• In silicon and germanium diodes, most of the energy is
released in the form of heat and emitted light is too small.
• However, in materials like gallium arsenide and gallium
phosphide the emitted photons have sufficient energy to
produce intense visible light.

74. Biasing of LED
• The safe forward voltage ratings of most LEDs is
from 1V to 3 V and forward current ratings is from
200 mA to 100 mA.
• Above 3 V, LED may get damages. To avoid this a
resistor in series- current limiting resistor (Rs)is
placed with the LED.
• This resistor restricts extra current which may
destroy the LED. Thus, current limiting resistor
protects LED from damage.
• The current flowing through the LED is
mathematically written as
𝐼𝑓 =
𝑉𝑠 − 𝑉𝑑
𝑅𝑠

75. Output characteristics
of LED
• The amount of output light emitted by
the LED is directly proportional to the
amount of forward current flowing
through the LED
• More the forward current, the greater is
the emitted output light.

76. Visible LEDs and invisible LEDs
Visible LED is a type of LED
that emits visible light. These
LEDs are mainly used for
display or illumination where
LEDs are used individually
without photosensors.
Invisible LED is a type of LED
that emits invisible light
(infrared light). These LEDs
are mainly used with
photosensors such as
photodiodes.

77. Disadvantages
and
Applications
of LED
• LEDs need more power to operate than normal p-n
junction diodes.
• Luminous efficiency of LEDs is low.
THE FOLLOWING ARE SOME OF THE APPLICATIONS OF LED
• Burglar alarms systems, Calculators, Picture phones, Traffic
signals, Digital computers, Multimeters, Microprocessors,
Digital watches, Automotive heat lamps, Camera flashes,
Aviation lighting.

78. What determines the
color of an LED?
The material used for constructing LED determines its
color. In other words, the wavelength or color of the
emitted light depends on the forbidden gap or energy gap
of the material.
• Gallium arsenide LEDs emit red and infrared light.
• Gallium nitride LEDs emit bright blue light.
• Yttrium aluminium garnet LEDs emit white light.
• Gallium phosphide LEDs emit red, yellow and green
light.
• Aluminium gallium nitride LEDs emit ultraviolet
light.
• Aluminum gallium phosphide LEDs emit green light.
Applications of LED

79. Time to Think
1. Why LED is heavily doped?
2. Why does LED emit the light in forward bias only?
3. Which Semiconducting materials are not suitable for
making an LED?
4. On what factors Colour of LED depends?
5. Which materials can be used to produce infrared LED?
6. How do LED lights save energy?

80. Think and
Answer
• Why LED is heavily doped?
• The amount of light emitted will depend on the
rate of doping, the diode allows the flow of
current only when it is forward biased. Hence,
the LED must be forward biased and heavily
doped.
• Why does LED emit the light in forward bias only?
• LED is a PN diode but emits light instead of heat
cause the semiconductor material used in LED is
a direct bandgap semiconductor.
• When the diode is forward biased, an electric
current starts to flow.
• Which Semiconducting materials are not suitable
for making an LED?
• Silicon and germanium are not suitable because
those junctions produce heat and no
appreciable IR or visible light.

81. Think and
Answer
• On what factors Colour of LED depends?
• The colour of the light emitted by the LED
depends on the wavelength of the light which in
turn depends on the semiconductor material
used in the diode while manufacturing LED. No
other factor is responsible for the colour of light.
• Which materials can be used to produce
infrared LED?
• Infrared LEDs are usually made of crystals of
such materials as indium gallium arsenide.
• How do LED lights save energy?
• LEDs use much less energy than incandescent
bulbs because diode light is much more efficient,
power-wise, than filament light. LED bulbs use
more than 75% less energy than incandescent
lighting.

82. Transistors
Unit I

83. Transistor
A transistor is a semiconductor device
used to amplify or switch electronic
signals.
When it works as an amplifier, it takes in a tiny electric current at one
end (an input current) and produces a much bigger electric current (an
output current) at the other.
A tiny electric current flowing through one part of a transistor can make
a much bigger current flow through another part of it. In other words,
the small current switches on the larger one.
W. Shockley, J. Barden, and W. Brattain invented the transistor in 1947.
The term ‘transistor’ is derived from the words ‘transfer’ and ‘resistor.’

84. Types
• Transistors are broadly
divided into two types:
• Bipolar transistors
(bipolar junction
transistors: BJTs)
• Field-effect transistors
(FETs)

85. BJT & FET

86. BJT and FET

87. Bipolar Junction
Transistor
• A bipolar junction transistor is a three-terminal semiconductor device
that consists of two p-n junctions which can amplify or magnify a signal.
• It is a current controlled device.
• The three terminals of the BJT are the base, the collector, and the
emitter.
• A signal of a small amplitude applied to the base is available in the
amplified form at the collector of the transistor.
• This is the amplification provided by the BJT.

88. Emitter Base and Collector
• Emitter :
• Supplies charges to other two regions.
• Heavily doped.
• Base :
• Middle region – thin – forming two PN Junctions
• Lightly doped
• Collector :
• Collects charges
• Larger than emitter and base
• Intermediate doping

89. Types of Bipolar
Junction Transistor
• There are two types of bipolar junction transistors:
• PNP bipolar junction transistor
• NPN bipolar junction transistor

90. Bipolar Junction
Transistor
• BJT is a semiconductor device that is constructed with 3 doped semiconductor Regions i.e. Base, Collector &
Emitter separated by 2 p-n Junctions.
• Bipolar transistors are manufactured in two types, PNP and NPN.
• There are three operating regions of a bipolar junction transistor:
• Active region: EB junction is forward biased and CB junction is reverse biased. The region in which the
transistors operate as an amplifier.
• Saturation region: EB and CB junction are both forward biased. The region in which the transistor is
fully on and operates as a closed switch such that collector current is equal to the saturation current.
• Cut-off region: EB and CB junction are both reverse biased. The region in which the transistor is fully
off, and collector current is equal to zero. This mode of operation is used when transistor is used as an
open switch
• Reverse active: EB junction is reverse biased and CB junction is forward biased. This mode of
application has limited applications

91. Bipolar Junction
Transistor biasing

92. Operation of NPN
transistor
• Emitter Current – IE (Base + Collector)
• Base Current – IB – Recombination
does not occur since base is made
thin
• Collector current –Ic – Injected
current (injected electrons + Thermal
agitated electrons)

93. Operation of PNP
transistor
• Current flow due to movement of
holes.
• Collector Current is Injected Current
• Direction of current flow is opposite
to that of NPN
• 𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶
• 𝐼𝐵 𝑖𝑠 𝑛𝑒𝑔𝑖𝑔𝑖𝑏𝑙𝑒
• 𝐼𝑐 ≈ 𝐼𝐸
PROBLEM:
In a common base connection, a certain transistor has an emitter current of 10 mA and a collector current of
9.8 mA. Calculate the value of Base current.

94. Configuration
of Bipolar
Junction
Transistors
• Since Bipolar Junction Transistor is a three-
terminal device, there are three ways to
connect it within an electric circuit while one
terminal is the same for both output and
input.
• Every method of connection responds
differently to the input signals within a
circuit.
• Common Emitter Configuration – has both
voltage and current gain
• Common Collector Configuration – has no
voltage gain but has a current gain
• Common base configuration – has no current
gain but has a voltage gain

95. Common Base

96. Common Emitter

97. Common
Collector

98. Characteristics in CB, CE & CC configuration

99. Time to Think
Who invented BJT?
What are the operating regions of BJT?
What are the applications of BJT?
What happens if the transistor is not
biased properly?
Why there is a maximum limit for the
collector supply voltage for a transistor?

100. Think and Answer
Who invented BJT?
• BJT was invented by W.H
Brattin, Bardeen, and William
Shockley.
What are the operating regions
of BJT?
• The operating regions of BJT
are:
• Forward active or active region
• Reverse active or inverted
region
• Saturation
• Cut-off

101. Think and Answer
• Following are the applications of Bipolar Junction Transistor:
• It is used as an amplifier
• It is used as an oscillator
• It is used as a demodulator
What are the applications of BJT?
• Following is the list of consequences if the transistor is not biased properly:
• The work efficiency of the transistor reduces
• There will be a distortion in the output signal
• The operating point may shift
• Transistor parameters will change
What happens if the transistor is not biased properly?
• There is a maximum limit for the collector supply voltage for a transistor because when the
collector current is increased rapidly there are chances of transistor getting damaged. To
avoid this, the voltage in the collector should have a maximum limit.
Why there is a maximum limit for the collector supply voltage for a transistor?

102. Transistor Characteristics
Unit I

103. Characteristics • Input Characteristics:
• These curves give the relationship
between the input current and input
voltage for a given output voltage
• Output Characteristics:
• These curves give the relationship
between the output current and the
output voltage for a given input current.

104. Common base
I/P
characteristics

105. Common base
O/P
characteristics

106. Current Gain In CB
Configuration
• DC Current Gain
• It is defined as the ratio of collector
current (Ic) to the emitter current (IE)
𝛼 =
𝐼𝐶
𝐼𝐸
• In a transistor, Ic is less than IE
• Hence 𝛼 is always less than unity
𝐼𝐶 = 𝛼. 𝐼𝐸
• WKT 𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶
𝐼𝐵 = 𝐼𝐸 − 𝐼𝐶
𝐼𝐵 = 𝐼𝐸 − 𝛼. 𝐼𝐸 = 1 − 𝛼 𝐼𝐸

107. Common emitter I/P characteristics

108. Common
emitter I/P
characteristics

109. Common
emitter O/P
characteristics

110. Common Emitter Current Gain
• It is defined as the ratio of collector current (Ic) to the emitter current
(IE)
𝛽 =
𝐼𝐶
𝐼𝐵
• The current gain in CE mode is much larger than that in CB mode.
• 𝛽 is much greater than unity ( 20 to 250)
• 𝛽 is also known as large signal common emitter current gain.

111. Relation between α and 𝛽
𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶
Dividing both the sides by 𝐼𝐶
(𝐼𝐸/𝐼𝐶) = (𝐼𝐵/𝐼𝐶) + 1
1/α =
1
𝛽
+ 1 =
1 + 𝛽
𝛽
𝛼 =
𝛽
𝛽 + 1
Similarly, 𝛽 =
𝛼
1−𝛼

112. Common collector I/P characteristics

113. Common
collector O/P
characteristics

114. Related Problems
1. Determine the value of IC and IB for the transistor show below
2. If the emitter current of a transistor is 8 mA and IB= 1/100 of IE Determine the
level of IC and IB
3. A transistor has IB = 105 mA and IC = 2.05 mA. Find
a) 𝛽 of the transistor
b) 𝛢 of the transistor
c) Emitter current IE
d) Now, if IB changes by +27 mA and IC changes by +0.65mA, find the new value of 𝛽