Transistor

What is a Transistor?

A transistor is a type of a semiconductor device
that can be used to both conduct and insulate electric current or
voltage. A transistor basically acts as a switch and an amplifier. In
simple words, we can say that a transistor is a miniature device that is
used to control or regulate the flow of electronic signals
Transistors are one of the key components in most of the electronic
devices that are present today. Developed in the year 1947 by three
American physicists John Bardeen, Walter Brattain and William
Shockley, the transistor is considered as one of the most important
inventions in the history of science.

Parts of a Transistor

A typical transistor is composed of three layers of semiconductor
materials or more specifically terminals which helps to make a
connection to an external circuit and carry the current. A voltage or
current that is applied to any one pair of the terminals of a transistor
controls the current through the other pair of terminals. There are three
terminals for a transistor. They are:

Base: This is used to activate the transistor.
Collector: It is the positive lead of the transistor.
Emitter: It is the negative lead of the transistor.
Parts of a Transistor

Well, the very basic working principle of a transistor is based on
controlling the flow of current through one channel by varying intensity
of a very smaller current that is flowing through a second channel.

Also Read:

Transistor as a Switch
Transistor as Amplifier

Types of Transistors

Based on how they are used in a circuit there are mainly two types of
transistors.
NPN and PNP

P-N-P Transistor:

It is a type of BJT where one n-type material is
introduced or placed between two p-type materials. In such a
configuration, the device will control the flow of current. PNP transistor
consists of 2 crystal diodes which are connected in series. The right
side and left side of the diodes are known as the collector-base diode
and emitter-base diode, respectively.

N-P-N Transistor:

In this transistor, we will find one p-type material that
is present between two n-type materials. N-P-N transistor is basically
used to amplify weak signals to strong signals. In NPN transistor, the
electrons move from the emitter to collector region resulting in the
formation of current in the transistor. This transistor is widely used in the
circuit.

P-N-P and N-P-N Transistor:

There are three types of configuration as a common base (CB),
common collector (CC) and common emitter (CE).

In Common Base (CB) configuration the base terminal of the transistor
is common between input and output terminals.

Common Base (CB) configuration

In Common Collector (CC) configuration the collector terminals are
common between the input and output terminals.

Common Collector (CC) configuration

In Common Emitter (CE) configuration the emitter terminal is common
between the input and the output terminals.
Common Emitter (CE) Configuration
How do Transistors work?
Let us look at the working of transistors. We know that BJT consists of
three terminals (Emitter, Base and Collector). It is a current-driven
device where two P-N junctions exist within a BJT.

One P-N junction exists between emitter and base region and the
second junction exists between the collector and base region. A very
small amount of current flow through emitter to the base can control a
reasonably large amount of current flow through the device from emitter
to collector.

In usual operation of BJT, the base-emitter junction is forward biased
and the base-collector junction is reverse biased. When a current flows
through the base-emitter junction, a current will flow in the collector
circuit.

In order to explain the working of the transistor, let us take an example
of an NPN transistor. The same principles are used for PNP transistor
except that the current carriers are holes and the voltages are reversed.

Operation of NPN Transistor:

The emitter of NPN device is made by n-type material, hence the
majority carriers are electrons. When the base-emitter junction is
forward biased the electrons will move from the n-type region towards

the p-type region and the minority carriers holes moves towards the n-
type region.

When they meet each other they will combine enabling a current to flow
across the junction. When the junction is reverse biased the holes and

electrons move away from the junction, and now the depletion region
forms between the two areas and no current will flows through it.

Operation of NPN Transistor:

When a current flows between base and emitter the electrons will leave
the emitter and flow into the base as shown above. Normally the
electrons will combine when they reach the depletion region.

But the doping level in this region is very low and the base is also very
thin. This means that most of the electrons are able to travel across the
region without recombining with holes. As a result, the electrons will drift
towards the collector.

In this way, they are able to flow across what is effectively reverse-
biased junction and the current flows in the collector circuit.