When a 3rd doped element each added to crystal diode in such away that 2 p-n junctions are formed, the resulting, device is known as transistor. The transistor an entirely new type of electronics device is capable of achieving amplification of weak signals in a fashion comparable to that realized by vaccum tubes.

Transistor consists of 2 p-n junction formed by sandwitching either p-type or n-type semiconductors between a pair of opposite type. Accordingly there are 2 types.

1. n-p-n transistor
2. p-n-p transistor

An n-p-n transistor is composed of 2 n-type semiconductors separated by a thin section of p-type .A p-n-p transistor is formed by 2 p-type sections separated by a thin of n-type.

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

1. There are two p-n junctions there or transistor may be regarded as a combination ....of two diodes, connected back to back.
2. There are three terminals, taken from each type of semiconductors
3. The middle section is very thin layer. this is the important function of a transistor

A transistor has 3 sections of' doped semi- conductors. The section on one side is the emitter and the section on the opposite side is the collector. The middle section is called the opposite side is the collector. The middle section is called base and forms two junctions between the emitter and collector.

(1). Emitter: - the section on one side that supplies charging carriers is called the emitter. The emitter is always forward biased with respect to base so that it can supply large number of majority carriers.

(2). Collector: - the section on the other side that collects charges is called the collector. The collector is always reverse biased. Its function is to remove charges from its junction with the base.

(3). Base: - the middle section which forms two PN junctions between the emitter and collector is called the base. The emitter junction is forward biased, allowing low resistance, for the emitter circuit. The collector junction is reverse biased and provides high resistance in the collector circuit.