Transistor

From ArticleWorld

A transistor is a device designed to transform resistance and hence voltage and current. The word transistor is derived from the words transfer and resistor. A transistor consists of two p-n junctions and three layers of semiconductors with different types of dopants i.e. p-type and n-type. It is also termed as bipolar transistor as the current flowing through the transistor is because of electrons as well as holes.

Layers in a transistor

The three distinct layers of the transistor are the emitter, base and collector. If the base is of n-type, then the emitter and collector will be of p-type.

1.The emitter: This region is on one extremity of the transistor. It has large conductivity. It is always forward biased with respect to the base of the transistor. The emitter, thus supplies a large number of majority carriers. This region is heavily doped.

2.The collector: This region is on the other extremity of the transistor. It collects charged carriers emitted by the emitter. It is always reverse biased with respect to the base of the transistor. It has low conductivity and is moderately doped. The collector region is made physically larger than the emitter region in most transistors since it is required to dissipate more heat.

3.Base: It is a thin region between the emitter and the collector. It is lightly doped.

Transistors - Different arrangements

In a transistor there are three terminals. One terminal is made common between input and output circuit. The input is fed between the common terminal and one of the other two terminals. The output is obtained between the common terminal and the remaining terminal. Thus transistor circuits can be arranged in three ways as follows:

1.Common base circuit: In this arrangement input is fed in between the emitter and base, and output is taken from the collector and base. The emitter- base junction is kept forward biased and collector - base junction is kept reverse biased.

2.Common emitter circuit: In this circuit the input is fed between the base and the emitter and output is taken by a load in between the collector and the emitter. It is widely used in practical circuits.

3.Common collector circuit: In this circuit, the input is given between the base and collector and the output is taken across the emitter and collector. This circuit has high input resistance and low output resistance. As the output resistance is lower than the input resistance, there is no voltage gain but there is high current gain, hence used for impedance matching.

Applications

In 1948, American physicists Dr. J. Bardeen and Dr. W. Brattain first invented a single crystal of germanium or silicon, having two p-n junctions.

Transistors today find place in nearly all modern electronic devices.Transistors have replaced vacuum tubes because of their advantages. They are small in size and require very small power. They do not need power for heating the cathode as in case of vacuum tubes. Thus they are used in light, handy electronic equipments, like controlling air-traffic and guided missiles.

Other devices which include transistors as their main component are small hearing aids, compact portable radio and T.V. receivers. They are used in miniature electronic equipments used in fields as varied as medicine and entertainment. A new application is the use of transistors in ignition systems of automobiles.