Introduction to Bipolar & Unipolar Transistors

Bipolar and unipolar transistors can be categorized according to their basic functionality. In bipolar transistor the electric current flows due to BOTH electrons and holes through it. However, in unipolar transistor the current flows EITHER due to electrons or holes through it.

A transistor is a semiconductor device having three terminals: they are collector, base and emitter. In this chapter, we shall study its basic structure, characteristics, different configurations and important applications. In addition, we shall also study brief details of some important unipolar transistors like FET, MOSFET and UJT.

What is a transistor?

Transistor is a semiconductor device having three layers, three terminals and two junctions. Since we have only two types of semiconductors i.e. the p-type and n-type, there are two types of transistors: they are NPN transistor and PNP transistor.

It is named as transistor which is an acronym of two terms: “transfer-of-resistor.” It means that the internal resistance of transistor transfers from one value to another values depending on the biasing voltage applied to the transistor. Thus it is called TRANSfer resISTOR: i.e. TRANSISTOR.

Bipolar Transistor

Definition: The bipolar transistor is a semiconductor device in which electric current flows due to electrons and holes BOTH, simultaneously. Thus both types of charges take part in the conduction of current through it. Hence it is called bipolar transistor. There are two types of bipolar transistors, NPN and PNP transistor.

Unipolar Transistor

Definition: The unipolar transistor is a type of transistor in which electric current flows due to only one type of charges i.e. either holes or electrons.

The unipolar transistor is categorized into two main types:

• Junction Field Effect Transistor (JFET)
  • N-channel JFET
  • P-channel JFET
• Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
  • N-channel MOSFET
  • P-channel MOSFET