Transistors

A transistor consists of two pn junction formed by sandwiching either p-type or n-type semiconductor between a pair of opposite types.

accordingly : there are two types of transistors namely 

(i) n-p-n transistor 

(ii)p-n-p transistor

Transistor is a semiconductor device,containing three layers and two junctions.

In n-p-n transistor,a thin layer of p-type is sandwiched between two N-type layers.In p-n-p transistor a thin layer of N-type sandwiched between two P-type layers.The symbol of transistor is shown in below 

There three portions of the transistor are emitter(E),collector(c) and base(B).An arrow marked in the emitter terminals,specifies the direction of current.Emitter layer is heavily doped,so that it can inject a large number of carriers into the base.Base is lightly doped and very thin.It passes most of the injected charge carriers from the emitter into the collector.collector moderately doped.It collects majority carriers from the emitter.

Naming the Transistor terminals:

A transistor has three sections of doped semiconductors.The sections on one side is the emitter and the section of the opposite side is the collector.The middle section is called Base and two junctions between the emitter and collector.

(i)Emitter :The section on one side that supplies charge carriers is called the emitter.The Emitter is always forward biased with respect to base so that it can supply a large number of majority carriers.The emitter (p-type)of p-n-p transistor is forward biased and supplies the hole charges to its junction with the base.similarly the emitter (n-type) of n-p-n transistor has a forward bias and supplies free electronics to its junction with the base.

(ii)Collector :The section on the other side that collects the charges is called collector.The collector is always reverse biased.Its function is to remove the charges from its junction with the base.The collector (p-type) of p-n-p transistor has a reverse bias and receives hole charges that flows in the output circuit.The collector (n-type)of n-p-n transistor has reverse bias and receives the electrons.

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

Transistor connections :

A Transistor can be connected in a circuit in the following three ways 

(i)  common base connection

(ii) common emitter connection

(iii)common collector connection

Important points about transistors :

1. A transistor has two pn junction.There are two depletion layer in the transistors.
2. A transistor is current operated device.
3. The base of the transistor is lightly doped.
4. The emitter of transistor is heavily doped.
5. The input impedance of the transistor is very low.
6. The most commonly used transistor arrangement is Common emitter arrangement.
7. The temperature of transistor goes up the base emitter resistance decreases.
8. The arrow of the transistor indicates the directions of hole current in the emitter.
9. The most commonly used semiconductors in the manufacture of a transistor is silicon.
10. The collector base junction in a transistor has reverse bias at all times.