edcThe valence band is simply the outermost electron orbital of an atom of any specific material that electrons actually occupy The conduction band is the band of electron orbitals that electrons can jump up into from the valence band when excited. When the electrons are in these orbitals, they have enough energy to move freely in the material The energy difference between the highest occupied energy state of the valence band and the lowest unoccupied state of the conduction band is called the band gap

1. UNIT – III
Transistor Characteristics: BJT: BJT - Construction,
Operation, Transistor Current Components, Transistor
as an Amplifier, Transistor Characteristics - CB, CE
and CC.
FET: Types, JFET- Construction, Working,
Characteristics, MOSFET - types, Construction,
Working, Characteristics, Comparison between JFET
and MOSFET.
SYLLABUS
ELECTRONIC DEVICES AND
CIRCUITS (19EC0402)

2. BJT-BIPOLAR JUNCTION
TRANSISTOR
Why Do We Need Transistors?
Suppose that you have a FM receiver which grabs the signal
you want.
The received signal will obviously be weak due to the
disturbances it would face during its journey.
Now if this signal is read as it is, you cannot get a fair output.
Hence we need to amplify the signal.
Amplification means increasing the signal strength.
This is just an instance. Amplification is needed wherever the
signal strength has to be increased.
This is done by a transistor. A transistor also acts as a switch to
choose between available options.
It also regulates the incoming current and voltage of the

3. BJT-BIPOLAR JUNCTION
TRANSISTOR
• BJT is the short form of Bipolar Junction Transistor, it is
a solid-state current-controlled device which can be used to
electronically switch a circuit, you can think of it as your
normal Fan or Light switch, but instead of you turning it on
manually it can be controlled electronically.

4. Construction of Bipolar Junction
Transistor
• The BJT is formed by three layers of semiconductor
materials, if it is a PNP transistor, it will have two P-type
regions and one N-type region, likewise, if it is an NPN
transistor, it will have two N-type regions and one P-type
region.
• The two outer layers are where the collector and emitter
terminals are fixed and the base terminal is fixed at the
center layer.

5. Construction of Bipolar Junction
Transistor

6. Construction of Bipolar Junction
Transistor
• The construction can simply be explained with a two diode
analogy for transistor as shown in the above image,it can
consider reading his article.
• Consider the two diodes connected with each other using
the cathode, then the meeting point can be extended to form
the base terminal and the two anodes end acts as the
collector and emitter of a PNP transistor.
• Similarly, if you connect the anode ends of the Diode then
the meeting point of the anodes can be extended to for the
base terminal and the two cathode ends act as the collector
and emitter of the NPN transistor.

7. Working of Transistor (BJT)
• Practically the working of a transistor is very simple,
it can be used as a switch or as an amplifier.
• But for basic understanding lets start with
how transistor as a switch works in a circuit.
• A typical working of PNP transistor BC558 is shown
below, Apart from this the BC547, 2N2222, BC557
are few among the popular transistors that are most
commonly used.

8. Working of Transistor (BJT)

9. Transistor Current Components
Based on the Kirchoff’s Current Law, we can frame the current
equation as
IE = IB + IC
Where, IE, IB, and IC are the emitter, base, and collector current
respectively. Here the base current will be very small when
compared with emitter and collector current, therefore, IE ~ IC
Similarly, when you consider the PNP Transistor, they operate in
the same way as the NPN transistor, but in NPN transistors the
majority charge carriers are holes (Positively charged particle)
but in the NPN transistor the charge carriers are the electrons
(negatively charged particle).

10. Characteristics of BJT
• BJT can be connected in three different configurations by
keeping one terminal common and using the other two
terminals for the input and output.
• These three types of configurations respond differently to
the input signal applied to the circuit because of the static
characteristics of the BJT.
• The three different configurations of BJT are listed below.
• Common Base (CB) configuration
• Common Emitter (CE) configuration
• Common Collector (CC) Configuration

11. Common Base (CB) Configuration

12. Input characteristics
• The input Characteristic curve for the Common Base
configurations is drawn between the emitter current IE and
the voltage between the base and emitter VEB.
• During the Common base configuration, the Transistor gets
forward biased hence it will show characteristics similar to
that of the forward characteristics of a p-n diode where
the IE increases for fixed VEB when VCB increases.

13. Output Characteristics
• The output characteristics of the Common Base
configuration are given between the collector current IC and
the voltage between the collector and base VCB, here the
emitter Current IE is the measuring parameter.
• Based on the operation, there are three different regions in
the curve, at first, the active region, here the BJT will be
operating normally and the emitter junction is reverse
biased.
• Next comes the saturation region where both the emitter
and collector junctions are forward biased.
• Finally, the cutoff region where both emitter and the
collector junctions are reverse biased.

14. Output Characteristics

15. Common Emitter (CE) Configuration
• The Common Emitter Configuration is also called the
grounded emitter configuration where the emitter acts as the
common terminal between the input applied between the
base and emitter and the output obtained between the
collector and the emitter.
• This configuration produces the highest current and
power gain when compared with the other two types of
configurations, this is because of the fact that the input
impedance is low as it is connected to a forward-biased PN
junction whereas the output impedance is high as it is
obtained for the reverse-biased PN junction.

16. Common Emitter (CE) Configuration

17. Input Characteristics
• The input characteristics of the Common Emitter
configuration are drawn between the base current IB and
the voltage between the base and emitter VBE.
• Here the Voltage between the Collector and the emitter is
the most common parameter.

18. Output Characteristics
• The output characteristics are drawn between the Collector
Current IC and the voltage between the collector and the
Emitter VCE.
• The CE configuration also has the three different regions, in
the active region the collector junction is reverse biased
and the emitter junction is forward biased, in the cut-off
region, the emitter junction is slightly reverse biased and
the collector current is not completely cut off, and finally, in
the saturation region, both the collector and the emitter
junctions are forward biased.

19. Output Characteristics

20. Common Collector (CC) Configuration
• The Common Collector Configuration is also called the
grounded Collector configuration where the collector
terminal is kept as the common terminal between the input
signal applied across the base and the emitter.
• The output signal obtained across the collector and the
emitter. This configuration is commonly called as
the Voltage follower or the emitter follower circuit.
• This configuration will be useful for impedance matching
applications as it has very high input impedance, in the
region of hundreds of thousands of ohms while having
relatively low output impedance.

21. Common Collector (CC) Configuration

22. Field Effect Transistor
Types of Field Effect Transistors
– Field Effect transistor’s are of mainly of two types based on
construction features of the device
– Junction Field Effect Transistor
– Metal oxide semiconductor Field Effect Transistor.

23. JFET
• JFET’s are again subdivided into two types based on the
type of channel namely
• p channel JFET
• n- channel JFET.

24. Construction of JFET
• N-channel JFET consists of a p+ type semiconductors
grown by doping acceptor impurities on either side of N-
type semiconductor as shown in the figure.
• Current is allowed to flow through the length of the N-
channel between the two p+ semiconductors. FET is a
three terminal device with the terminals being source,
gate and drain.

25. Construction of JFET
Source
• It is there terminal where the majority carriers enter in to JFET
bar. As for the FET bar concern this terminal is the source of
majority carriers so it is called as source terminal and the current
flow in this terminal is Is.
Drain
• This terminal is the end terminal which collects the majority
carriers sourced by the source. I.e. it is draining the majority
carriers from FET bar and giving to the output terminal so it is
called as drain terminal and the current in drain is Id.
Gate
• This important control element in FET as it acts like gate to the
majority carriers flow. I.e. by operating the gate terminal voltage
opening or closing of majority carries can be obtained, so this
terminal called Gate terminal and the current in gate is Ig.

26. Construction of JFET

27. Working of JFET
• When there is no voltage across gate and source, the
channel becomes a smooth path which is wide open for
electrons to flow.
• But the reverse thing happens when a voltage is applied
between gate and source in reverse polarity, that makes the
P-N junction reversed biased and makes the channel
narrower by increasing the depletion layer and could put the
JFET in cut-off or pinch off region.
• In the below image we can see the saturation mode and
pinch off mode and we will be able to understand
the depletion layer became wider and the current flow
becomes less.

28. Working of JFET

29. JFET Characteristics Curve
•In the above image, a JFET is biased through a variable DC
supply, which will control the VGS of a JFET.
•We also applied a voltage across the Drain and Source.
•Using the variable VGS, we can plot the I-V curve of a JFET.

30. JFET Characteristics Curve
In the above I-V image, we can see three graphs, for three different values of
VGS voltages, 0V, -2V and -4V.
There are three different regions Ohmic, Saturation, and Breakdown region.
During the Ohmic region, the JFET acts like a voltage controlled resistor,
where the current flow is controlled by voltage applied to it. After that, the JFET
gets into the saturation region where the curve is almost straight

31. MOSFET
• A metal–oxide–semiconductor field-effect
transistor (MOSFET, MOS-FET, or MOS FET) is
a field-effect transistor (FET with an insulated
gate) where the voltage determines the
conductivity of the device.
• It is used for switching or amplifying signals.
The ability to change conductivity with the
amount of applied voltage can be used for
amplifying or switching electronic signals.
MOSFETs are now even more common than BJT
(bipolar junction transistors) in digital and analog
circuits.

32. MOSFET
• MOSFETs are particularly useful in amplifiers due to their input
impedance being nearly infinite which allows the amplifier to
capture almost all the incoming signal. The main advantage is
that it requires almost no input current to control the load current,
when compared with bipolar transistors. MOSFETs are available
in two basic forms:
• Depletion Type: The transistor requires the Gate-Source voltage
(VGS) to switch the device “OFF”. The depletion-mode MOSFET
is equivalent to a “Normally Closed” switch.
• Enhancement Type: The transistor requires a Gate-Source
voltage(VGS) to switch the device “ON”. The enhancement-mode
MOSFET is equivalent to a “Normally Open” switch.

33. Classification of MOSFETs

34. Construction of N- Channel MOSFET
• Let us consider an N-channel MOSFET to understand its
working. A lightly doped P-type substrate is taken into
which two heavily doped N-type regions are diffused, which
act as source and drain. Between these two N+ regions,
there occurs diffusion to form an Nchannel, connecting
drain and source.

35. Working of N - Channel depletion
mode MOSFET
• For now, we have an idea that there is no PN junction
present between gate and channel in this, unlike a FET.
• We can also observe that, the diffused channel N ,
the insulating dielectric SiO2 and the aluminum metal layer
of the gate together form a parallel plate capacitor.

36. Working of N-Channel MOSFET
• The same MOSFET can be worked in enhancement mode,
if we can change the polarities of the voltage VGG.
• So, let us consider the MOSFET with gate source
voltage VGG being positive as shown in the following
figure.

37. Output Characteristics
Drain Characteristics
• The drain characteristics of a MOSFET are drawn
between the drain current ID and the drain source
voltage VDS. The characteristic curve is as shown below
for different values of inputs.

38. Output Characteristics
Transfer Characteristics
• Transfer characteristics define the change in the value
of VDS with the change in ID and VGS in both depletion
and enhancement modes. The below transfer
characteristic curve is drawn for drain current versus gate
to source voltage.

39. Comparison between JFET and
MOSFET
PARAMETERS JFET MOSFET
Mode of operation It operates only in depletion mode. It can be operated in either depletion or
enhancement mode.
Input impedance JFET have much smaller input
impedance mainly of the order of
10
8
Ω.
MOSFETs have much higher input
impedance of about 10
10
to 10
15
Ω due to
small leakage current.
Characteristic curve As JFET has higher drain resistance,
the characteristic curve is more flatter.
The characteristic curve is less flat than
those of JFET.
Drain resistance JFET has drain resistance of the order
of 10
5
to 10
6
Ω
Drain resistance in case of MOSFETs is
of the order of 1 to 50 K Ω.
Fabrication Fabrication process of JFET is more
difficult than MOSFET.
MOSFET can be easily fabricated thus it
is more widely used.
Cost Manufacturing of JFET is cheaper as
compared to MOSFET.
MOSFETs are slightly expensive as
compared to JFET.
Susceptibility to
damage
It does not require special handling. These are more susceptible to overload
voltage and requires special handling

40. References
• https://bu.edu.eg/portal/uploads/Engineering,%20Sho
ubra/Electrical%20Engineering/823/crs-
14136/Files/lecture8_Zener_Diodes.pdf
• http://ggn.dronacharya.info/APSDept/Downloads/Qu
estionBank/Basics-Electronics/Section-B/SectionB-
4_Voltage_regulator.pdf
• http://ggn.dronacharya.info/APSDept/Downloads/Qu
estionBank/Basics-Electronics/Section-B/SectionB-
4_Voltage_regulator.pdf
• https://www.electronics-
tutorials.ws/amplifier/transistor-biasing.html