Bipolar Junction Transistor(BJT)

  Bipolar Junction Transistor

Introduction

Bipolar junction transistor definition

A bipolar junction transistor or BJT is a three terminal electronic device that amplifies the flow of current. It is a current controlled device. In bipolar junction transistor, electric current is conducted by both free electrons and holes.

Unlike a normal pn junction diode, the transistor has two p-n junctions.

Types of Bipolar Junction Transistors (BJTs)

The bipolar junction transistors are formed by sandwiching either n-type or p-type semiconductor layer between pairs of opposite polarity semiconductor layers.

Bipolar junction transistors are classified into two types based on their construction: They are

• NPN transistor
• PNP transistor

NPN transistor

When a single p-type semiconductor layer is sandwiched between two n-type semiconductor layers, the transistor is said to be an npn transistor

PNP transistor

When a single n-type semiconductor layer is sandwiched between two p-type semiconductor layers, the transistor is said to be a pnp transistor.

Both PNP and NPN transistors consist of three terminals: they are emitter, base, and collector. 

Terminals of BJT

Emitter:

As the name suggests, the emitter section supplies the charge carriers. The emitter section is heavily doped so that it can inject a large number of charge carriers into the base. The size of the emitter is always greater than the base.

Base:

The middle layer is called base. The base of the transistor is very thin as compared to emitter and collector. It is very lightly doped.

Collector:

The function of the collector is to collect charge carriers. It is moderately doped. That is the doping level of the collector section is in between emitter and base. The size of the collector is always greater than emitter and base. The collector area in the transistor is considerably larger than the emitter area. This is because the collector region has to handle more power than the emitter does and more surface area is required for heat dissipation.

In transistor, the amplification is achieved by passing input current from a region of low resistance to a region of high resistance.

Applications of bipolar junction transistor

The various applications of bipolar junction transistors include:

• Televisions
• Mobile phones
• Computers
• Radio transmitters
• Audio amplifiers