In some situations, the transistor can be used as a switch, whereas in other situations, it can be used as an amplifier.

1. Working of Different types of Transistors
Austin February 24, 2022 no Comments
A transistor is a small device that can be used to serve two different
purposes. In some situations, the transistor can be used as a switch,
whereas in other situations, it can be used as an amplifier.
It can serve two-fold purposes in a way that it can be used to increase the
flow of electricity or voltages, and it can be used to inhibit or turn off the
flow of current.
Do you wish to learn about all the aspects of transistors in detail? Our
private physics tutor can guide you thoroughly to gain all the knowledge of
the subject.
Here we have discussed the different uses and applications of transistors
based on their modes of working.

2. What is a Transistor
A transistor is mainly considered as a semiconductor device because it can
be used as both; an electrical conductor and an inhibitor. The two main
purposes of a transistor are either working as an amplifier or a switch in
the circuit. There are basically three main terminals of a transistor that
makes it function properly. The three terminals include base, emitter, and
collector. Based on the usage and functioning of a transistor, there are two
ways a transistor can be used, including:
• Amplifier
• Switch
Let’s discuss how the functioning of a transistor differs while functioning
as an amplifier and a circuit.
Configuration of a Transistor
Here the working of a transistor is based on the configuration of the circuit.
Different configurations determine different workings of a transistor. There
can be three ways through which the transistor configuration can occur,
including:
• Common base
• Common emitter
• Common collector
Two terminals work for input and two terminals for output. In this situatio n,
the third terminal is usually taken as a common terminal for both input and
output; the configuration changes based on the common terminal that has
been taken.
Common base

3. If the transistor is working with the common base configuration, it means
that the base terminal has been used as a common terminal for input and
output. At the same time, the other two terminals work individually for the
input and output.
In this way, the base here provides two folded functions, i.e., for the input
and the output of voltages at the same time. This is why base in this
situation is known as a common base.
Common emitter
When the working of a transistor is in a way that the emitter is taken as a
common terminal for both input and output while the two other terminals
work separately for the input and output, then the configuration is named
as a common emitter.

4. Here the emitter is being used to provide the working of
both the input and output voltages at the same time. This
is why it is known as a common emitter.
Common collector
If the transistor is working with the collector terminal as a common
terminal for both; input and output, then while the two other terminals are
working individually for input and output, then the configuration is named
as a common collector.
In this situation, this collector is giving a purpose of both the input and the
output voltages at the same time. In this way, this terminal is known as the
common collector.

5. Transistors as amplifiers
When a transistor is working as an amplifier, it tends to increase the current
by changing a weak signal into a strong one. When we are using a transistor
in the form of an amplifier, then it is used for the conversion or
amplification of a weak signal from the input to a strong one at the output.
By using the transistor as an amplifier, the input signal with a rather low
value is converted into an output signal with a higher value.
The main purpose of using amplifier transistors is to enhance the signal
quality to increase the work effectiveness of the circuit. This property of a
transistor is quite helpful in the working of radio signals or distant
communication workings.
Active region
This region comes in between the saturation and cut -ff region. While
operating in the active region, the transistor is usually working as an

6. amplifier. In this region, the base-emitter junction is found to be in the
reverse bias, whereas the base-collector junction is found to be in the
forward bias. Just like the switching p roperty of the transistor happens in
the saturation or cut-off region, the amplification by the transistors usually
occurs when the transistor is present in the active region.
An example of amplified current
Let’s discuss through an example how a small amo unt of voltage can have
a large impact on the output voltage hence amplifying the output voltages.
If a small amount of voltage like 0.2V has been applied to the emitter, this
will eventually produce a current of 2 mAmp in the emitter. This will lead
to an increase in the voltages that are being collected as output voltages.
With this voltage and current, there would be a resistance of some extent
like 5 kΩ
Since V=IR
We can say that,
5 x 2= 10V
From this example, we can conclude that a small input voltage that was 0.2v
created a larger output voltage effect that was 10v.
In this way, the smaller voltage was amplified into a larger voltage amount
by the use of a transistor.

7. Transistors as switches
If a transistor is working as a switch, it means that a small amount of
current in the circuit will be turned into a larger amount of current, having
a major impact. The working of the transistor as a switch will determine
whether the current is flowing or not. In other words, it determines if the
circuit is open (OFF) or is it closed (ON).
When working as a switch, the transistor operates in two main regions,
including the cut-off region and the saturation region.
Cut-off region
A transistor as a switch is responsible for determining whether the circuit
is open or closed. When there is zero current present at the emitter, there
will be zero current at the collector; hence the switch will be considered as
an open switch or off mode. When the switch is open, the transistor is
working in the cut-off region.

8. Since the circuit is open when the transistor is in the cut -off region, there
is no current flowing through the circuit; hence the circuit will be
considered in OFF mode. In this way, the transistor in this region is used as
a switch.
Saturation region
On the other hand, when a certain amount of voltage, usually greater than
0.7V, is applied to the base terminal, then there is a flow of current at the
collector terminal. In this situation, the circuit is closed and in ON mode.
When the circuit is closed, the transistor is said to be working in the
saturation region. In the saturation region, there is the voltage, and current
is maximum at the base and the collector terminal.
Since the circuit is closed when the transistor is present in the saturation
region, there is maximum current flowing through the circuit because of
the voltages that are coming from the source. In this way, the circuit is said
to be turned ON with current flowing through it. In this way, the transistor
is used as a switch when present in this region.

9. Terminals of transistors
There are three main terminals of a transistor that work together for the
normal functioning of the transistors and transfer the charge from one part
of the circuit to the other. Let’s discuss the working of the three terminals
of a transistor.
Emitter
This is the first terminal of the transistor with a moderate size. Being
located at the beginning of the transistor, the emitter is mainly responsible
for transferring the charge to the collector through the base terminal. Since
it is responsible for supplying the charge, it is connected to the base via
the forward bias. The charges are transferred to the base terminal via the
junction present between the emitter and base.
Base

10. This is the second terminal on the resistor. There are two junctions at the
base terminal. One that connects the base with the emitter is known as the
input junction, and the other that connects the base to the collector is
known as the output junction. The input junction is in a forward bias,
whereas the output junction is in a reverse bias.
The base is located in the middle region of the transistor with a relatively
small size in comparison to the emitter and the collector region.
Collector
This is the final terminal of the transistor. It is located at the end of it, and
the size is relatively larger than both terminals, i.e., base and emitter. The
large size of this terminal is for collection purposes as the charges sent
from the emitter via base are supposed to be collected in this region.
There is a reverse-biased junction present here that connects the base with
the collector.
Types of transistors
Based on the configuration and material of the terminals, there are two
main types of transistors that are further subdivided into other categories.
The two main types of a transistor are:
• Bipolar junction transistor
• Field-effect transistor
The bipolar junction transistor (BJP) is further divided into two other
configurations that are PNP and NPN, whereas the field -effect transistor
(FET) further has two more configurations that are JFET and MOSFET.

11. Let’s discuss how these configurations and their uses distinguish the
different types of the transistor from each other.
Bipolar junction transistor (BJT)
The bipolar junction transistors are the ones that contain two junctions of
either P or N material. Here the N is supposed to indicate the negat ively
charged material present in the transistor, whereas the P would indicate
the positively charged material.
The BJP transistor has further two types of configurations that bring the
differences in the functioning of a transistor, including:
• NPN
• PNP
NPN
The NPN transistor is the one that has the configuration in a way that the
P material is situated in the middle of the N material. There are three
terminals in the NPN transistor, i.e., the emitter, base, and the collector,
where the base is connected to the emitter via the forward bias and to the
collector via the reverse bias.
The flow of electrons in the NPN configuration is from the emitter to the
collector since the two ends are both equipped with negative material.
Working
In the NPN transistor, when there is no voltage being applied, then the
transistor is said to be unbiased. When voltages are being applied, there is
a flow of charge that generates the current in the circuit. Since the left and
right sides of NPN transistors are negative material, i t is known that the
majority of charge carriers in these two regions are negatively charged

12. electrons, whereas the middle region (base) that is made of P material has
majority carriers as holds which are positively charged ions.
In this way, the flow of charges in this transistor is from emitter to base,
which is then transferred to the collector.
PNP
The PNP transistor, on the other han d, is the one with the opposite
configuration. The N material in this transistor is placed between the P
materials on both sides. In this transistor, the electrons are taken from the
base that is present in the middle. When the current comes in the base, i t
is then further sent to the collector, where it is amplified.
This transistor has two PN junctions. These are formed when the emitter is
connected to the base forming one PN junction, and the other is formed
when the base is connected to the collector fo rming another PN junction.

13. Working
If there is no voltage being applied to the given transistor, then it is said
to be an unbiased PNP transistor. After the application of voltage, there is
a flow of charges that generate the current. In the case of a PNP transistor,
the P region is constituted of holes as their majority charge carrier, and the
negatively charged electrons are considered as the minority charge
carriers. Whereas, in the base that is N material, the electrons are
considered as majority charge carriers, and the holes are consid ered as
minority charge carriers.
In this way, the major overall charge carriers in the PNP transistor are the
holes instead of the negatively charged electrons.
The preference of NPN over PNP
Usually, in electric circuits, the NPN transistors are preferred more over
the PNP transistors. Although the major difference between both

14. transistors is the configuration of the P and N material, this changes the
mode of operation for both transistors.
Here are some reasons why NPN transistors are preferred more over the
PNP ones:
Electrons movement
The majority carriers in the NPN transistors are the negatively charged
electrons, and it is known that electrons have a higher mobility rate and
travel faster in comparison to the holes present as majority carriers in the
PNP transistors. Being positively charged ions, they are less mobile than
the electrons.
The higher mobility of the electron allows more electricity conduction in
the given circuit, which makes NPN a more preferred transistor over the
PNP one.
Grounding the negative terminal
Grounding the negative terminal helps to keep the balance in the flow of
current and to minimize the unbalanced current flow. The NPN has a more
prominent negative field which helps to easily ground the negative
terminal, whereas the PNP transistor has a positive field being more
prominent. This inhibits the prominent negative exposure of the transistor
and inhibits the ease of grounding the negative terminal.
In this way, the NPN transistor is preferred more over the PNP so that the
negative terminal can be grounded in an easy way.
NPN transistor as switches
When the transistor is being used as a switch, the NPN transistor works
more efficiently in comparison to the PNP tran sistor. The power of
switching is more when the working configuration of the transistor in NPN
with more negative field instead of the PNP transistor with a positive field.

15. Similarly, the frequency rate of the NPN transistor is better than PNP, which
makes the amplification output rather better than the PNP configuration.
The switching speed and the amplification rate both are higher in the NPN
transistor. Hence both modes of a common transistor work better when the
configuration is done with P material bei ng centered with the N material.
Rather than having the opposite configuration, that is the PNP.
Costing benefits
The manufacturing and production cost of an NPN transistor is quite less
in comparison to the PNP transistor. To limit the cost of transistor
manufacturing, the NPN transistor is used more commonly than the PNP
transistor.
More reliable
Being smaller in size and with low production cost, the NPN transistor is
considered rather more efficient and reliable than the PNP transistor.
Field effect transistor
This is the type of unipolar transistor that does not have double junct ions
like the BJT transistor. Being unipolar, the FET transistor either has P
material or N material embedded in it and because of this, there are either
electrons or holes working as the charge carriers in this transistor. Both
charge carriers won’t be presented at the same time in FET.
FET is known to have a high input impedance which makes it perfect to be
used in an integrated circuit.
FET is considered as a voltage-controlled device as its functioning depends
on the output voltages that are provided to it by the input voltages. There
are three different terminals of the FET which are different from that of the
BJT. these three terminals are:

16. • Gate
• Source
• drain
Let’s discuss how the placement and roles of these terminals are important
for the normal functioning of FET.
Gate
This is the terminal that is mainly responsible for the flow of charge carriers
in the transistor. It is made by the diffusion of an N -type semiconductor
and a P-type semiconductor. It is placed close to the channel so that it can
ensure that the charge carriers are flowing in the right way from the source
to the drain.
Source
This is the terminal of the FET where the electrons or holes enter the circuit
channel to travel further towards the drain for the normal functioning of
the circuit. The entrance of charge carriers here is controlled by the gate.
Drain
This is the final terminal in the FET where the incoming charge carriers are
exited from. The electrons basically enter the channel from the gate and
exit from the drain; this flow of charge carriers from the source to the drain
is controlled and regulated by the third terminal that is the gate.
Based on the working of this transistor, there are two different types of FET
transistor as discussed below:
JFET

17. The junction field-effect transistor (JFET) is a type of FET that is usually
used as a switch in a circuit. JFET is a voltage -controlled device and works
with either the P-type material or the N-type material. Based on this
configuration, the JFET is further divided into n -type JFET and P-type JFET.
Since it is controlled by the voltages provided to it when there is no voltage
provided to it, the transistor is considered to be in an ON state, and when
there is a supply of voltages, the transistor allows a limited amount of
current to flow through the circuit.
If the P-type material is the main constituent in the JFET, then it is known
as the P-type JFET, and the main charge carriers in this transistor are the
positively charged ions that are the holes.
On the other hand, if the main constituent of the transistor is the N-type
material, then it is known as the N type JFET, and the negatively charged
electrons are the main charge carriers in it.
MOSFET is a type of Field effect transistor that is m ainly used for
amplification or switching purposes in a given circuit. This is also a voltage -

18. controlled device, as the small amount of voltage brings changes in the
current flow of the circuit.
The MOSFET further has two types of configurations that are t he depletion
type and the enhancement type.
If the MOSFET is the depletion type, then it means it is in the close switch
form, and in order to turn off the switch, there needs to be source gate
voltage provided.
On the other hand, if this transistor is an enhancement type, it is
considered to be in an open switch form, and to turn on the switch, there
should be source gate voltages.
Difference between JFET and MOSFET
There are a few differences between the J FET and MOSFET, as discussed
below:

19. • The JFET works only when present in the depletion mode, which
means it works in a close switch form, whereas MOSFET works in
both modes, whether in depletion mode or enhancement mode.
• The conductivity in the JFET is usua lly affected by the reverse
biasing in the channel, whereas in the MOSFET, the conductivity is
affected by the carries that are present in the channel.
• The resistance offered in the drain terminal of JFET is rather high
than the drain resistance in the MOS FET.
• When it comes to the manufacturing process, the process is rather
simple for the JFET but a bit complex for the MOSFET.
• The JFET is considered as the transistor with three terminals,
whereas the MOSFET is considered as the transistor having four
terminals.
The uses and applications of transistors
There are several uses of transistors in our daily lives. Some of them have
been discussed here as follows.
• Transistors can be used to serve two major functions either the
switching of the circuit or the amplification of current in a given
device.
• As a switch, transistors are used in digital and analog circuits as
well.
• When a greater amount of current is required, the bipolar junction
transistors can come to work by amplifying the current that passes
through the base of it.
• Transistors are used widely in the production of our mobile phones.
The transistors present in mobile phones are responsible for storing

20. and regulating current flow and also amplifying the signals that are
coming to the mobile phone.
• Integrated circuits are embedded in many of our daily life devices.
These small units are responsible for transmitting e lectrical signals
from one place to another via other units that mainly includes the
use of transistors. Transistors are the main functioning units of
these microchips that are known as integrated circuits.
• All the electric devices are providing sound func tions, including the
radio, cell phones, or the computer use transistors to amplify the
sounds from the device and hence enhancing the quality of the
sound.
Advantages and disadvantages of using a transistor
Just like everything else, there are several adv antages and some
disadvantages associated with the use of a transistor. Some of them have
been discussed below:
Advantages
Some of the advantages of using a transistor have been discussed below.
• The many advantages of a transistor are because of its very s mall
size and less manufacturing cost. A transistor is rather small in size
and provides the work efficiently by occupying very little space in a
device.
• It can act as an insulator and a conductor both. This is why it is
sometimes referred to as a semicond uctor. When there is a need for
a flow of current, the transistor acts as a conductor, whereas when
the current flow needs to be stopped or reduced, the transistor acts
as an insulator.

21. • When a transistor is being used as a bipolar junction transistor, it
allows the flow of positive and negative charge carriers at the same
time. There are negative and positive charges flowing across in the
form of electrons and holes, respectively.
• With a low production cost, small transistors can be used to
manufacture a rather large size of integrated circuit.
• Transistors are quite reliable because of their small size and less
sensitivity to mechanical exposure; because of this, there are lesser
chances of physical damage to the equipment.
• The use of silicon in the transist ors is essential in the devices that
work in the higher current environment. AC devices that require the
working of higher current levels can work right with BJ transistors
made up of silicon base.
• It does not require a large number of voltages for a large amount of
voltage output. In this way, the transistor offers large voltages with
the input of a small number of voltages.
• There is no noise related to the working of a transistor providing
noiseless operations. Being so small in size, there is no moving
element in the equipment of a transistor which helps to avoid the
wear and tear situation of a machine.
Disadvantages
With many advantages associated with the use of transistors, there are
some disadvantages as well. Some of the disadvantages have been
discussed below:
• Higher voltages and current can have a bad effect on the quality of
a transistor. It should be placed wh ere the voltages and current
levels are relatively higher.

22. • In the case of reversing voltages, transistors usually fail to work, so
they should be used with care when operating in an AC device so
that the reverse voltages would not destroy the working of th e
transistor.
• Transistors are rather sensitive to the application of temperature. If
there are higher levels of temperature being applied to the device,
it can eventually damage the transistor.
• The small size of a transistor can be a drawback of it as well . If
there is circuit damage or particularly in the transistor, the
repairing process is rather difficult due to its small size.
Conclusion
Transistors are very small devices that are installed in a circuit and can be
used to perform many functions, including the amplification of the current
or the voltage and the switching on and off of a given circuit. The transistor
has three main components that make the transistor work normally,
including the emitter, base, and collector, whereas in some types of
transistors, the three terminals include the gate, drain, and source. There
are two types of charge carriers flowing in a transistor that makes t he
current flow, including the negatively charged ion that are electrons and
the positively charged ions that are the holes. If you wish to study the
detailed aspects of the transistor and related elements, learn all about it
from our private home tutors.
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