TRANSISTOR

A transistor consists of two pn junctions formed by sandwitching either p-type or n-type semiconductor between a pair of opposite types. There are two types of transistor

(a) n-p-n transistor

(b) p-n-p transistor

An n-p-n transistor is composed of two n-type semiconductor separated by a thin section of p-type. However, a p-n-p transistor is formed by two p-separated thin section of n-type.

In each type of transistor, the following points may be noted:

(a) There are two pn junction. Therefore, a transistor may be regarded as a combination of two diodes connected back to back.

(b) There are three terminals, one taken from each type of semiconductor.

(c) The middle section is a very thin layer. This is the most important factor in the function of a transistor.

TRANSISTOR TERMINALS

There are three terminals in terminals

(a) Emitter (b) Base (c) Collector

SOME FACTS ABOUT THE TRANSISTOR

(a) The transistor has three regions; emitter, base and collector. The base is much thinner than the emitter while collector is wider than both. However, for the sake of convenience, it is customary to show emitter and collector to be of equal size.

(b) The emitter is heavily doped so that it can inject a large number of charge carriers (electrons or holes) into the base. The base is lightly doped and very thin, it passes most of the emitter injected charge carries to the collector. The collect is moderately doped.

(c) The transistor has two pn junction i.e. it is like two diodes. The junction between emitter and may be called emitter-base diode or emitter diode. The junction between the base and collector may be called collector-base diode or simply collector diode.

(d) The emitter diode is always forward biased whereas collector diode is always reverse biased.

(e) The resistance of emitter of emitter diode (forward biased) is very small as compared to collector diode (reverse biased). Therefore, forward bias applied to the emitter diode is generally very small whereas reverse bias on the collector diode is much higher.