What is a Transistor?

A semiconductor device that has the ability to produce amplification is known as a transistor. Current is transferred from a low resistance circuit to a high resistance circuit to achieve the amplifying effect.

It’s made up of two PN junctions that link either a p-type or n-type semiconductor between two kinds.

Transistor

Terminals of Transistor

A transistor has three sections of doped semiconductors. They are emitter, base, and collector.

Emitter

The first section of the transistor is called the emitter. The function of an emitter is to supply majority charge carriers to the base. The emitter is always forward biased in relation to the base in order to supply the base with the bulk of charge carriers. The emitter is heavily doped.

Collector

The third section of the transistor is called the collector. The function of the collector is to collect the majority of charge carriers. To keep charge carriers away from the junction, the collector is always reverse biased in relation to the base. The collector is moderately doped.

Base

The base refers to the transistor’s center part. The base is thinner than the emitter and collector so that it may pass most of the charge carriers to the collector. The base is very lightly doped compared to the emitter and collector.

Types of Transistor

There are two types of transistors:

1. NPN transistor

An NPN transistor is a type of transistor in which a thin layer of p-type semiconductor is sandwiched between two thick layers of n-type semiconductor. The structural diagram of an NPN transistor is shown below.

NPN Transistor

2.PNP transistor

A PNP transistor is a type of transistor in which a thin layer of n-type semiconductor is sandwiched between two thick layers of p-type semiconductor. The structural diagram of a PNP transistor is shown below.

PNP transistor

Operation of transistor

1. Transistor as an amplifier.

For operating a transistor as an amplifier, we should operate the transistor in active mode. The circuit below shows the basic common emitter configuration of a transistor. While operating the transistor in a common emitter configuration, we can enable the transistor to work as an amplifier.

CE configuration of Transistor

The figure below shows the input and output characteristics of the transistor working in the CE (Common Emitter) configuration. In the following figure, VCE1 > VCE2.

In the following output characteristics curve, the transistor can work in the active region to operate it as an amplifier.

2.Transistor as a Switch

A transistor can work in one of the following modes of operations:

  1. Active mode
  2. Cutoff mode
  3. Saturation mode

For operating a transistor as a switch, the area of operation for the transistor are saturation mode and cutoff mode. The transistor driven in cutoff mode acts as an ‘OFF’ device and operating in saturation mode acts as an ‘ON’ device. The two regions of operation are shown below in which the shaded part represents cutoff and saturation mode.

Transistor as a switch

Operation in cutoff mode

For operating a transistor in cutoff mode, the base current should be zero. The base-emitter voltage VBE  is less than 0.7V which shows that this base-emitter junction is reverse biased. Hence, the base-collector junction is also reverse biased.

Since all the currents in the transistor are equal to zero, the transistor is in ‘OFF’ condition and acts as an open switch which is shown in the circuit diagram below.

Operation in saturation mode

For operating a transistor in saturation mode, the transistor is biased so that a sufficient amount of base current IB is applied which results in an insufficient amount of collector current IC. The voltage between the collector and the emitter drops as a result of this. As a result, the depletion layer becomes small, and maximum current flows through the transistor.

Here,

VCE=0 & Vout=0

Hence, the base-emitter junction is forward biased i.e. VBE>0.7V. The base-collector junction is also forward biased and the transistor acts as a closed switch which is shown in the circuit diagram below.



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