Transistor: Transistor is a three layered semiconducting device. These three layers have three terminals - emitter, base and collector. The place where the two layers touch each other is called a junction. The junction where the emitter and base touch each other is called emitter base junction. The junction where the collector and base touch each other is called collector base junction.

Transistor will act as a switch or can be used for amplification. It is controlled by an electrical signal. Transistor can also be used for enhancing the applied signal strength. To understand the functioning of a transistor, we can relate it to the water supply system in our home. A storage tank which is kept at the roof of the building is similar to an emitter in a transistor which acts as the source of charge carrier (i.e. electrons and holes in semiconductor). The tap at the ground is similar to the base of the transistor. this tap controls the flow of water, likewise, the base controls the flow of the charge carrier. The bucket on the ground collects the water, coming from the storage tank. Similarly, the collector of the transistor collects the charge carriers coming from the emitter.

Transistor Terminals

The transistor has three terminals namely, emitter, collector and base. The terminals of the diode are explained below in details.

Emitter – The section that supplies the large section of majority charge carrier is called emitter. The emitter is always connected in forward biased with respect to the base so that it supplies the majority charge carrier to the base. The emitter-base junction injects a large amount of majority charge carrier into the base because it is heavily doped and moderate in size.

Collector – The section which collects the major portion of the majority charge carrier supplied by the emitter is called a collector. The collector-base junction is always in reverse bias. Its main function is to remove the majority charges from its junction with the base. The collector section of the transistor is moderately doped, but larger in size so that it can collect most of the charge carrier supplied by the emitter.

Base – The middle section of the transistor is known as the base. The base forms two circuits, the input circuit with the emitter and the output circuit with the collector. The emitter-base circuit is in forward biased and offered the low resistance to the circuit. The collector-base junction is in reverse bias and offers the higher resistance to the circuit. The base of the transistor is lightly doped and very thin due to which it offers the majority charge carrier to the base.

Working of Transistor


Usually, silicon is used for making the transistor because of their high voltage rating, greater current and less temperature sensitivity. The emitter-base section kept in forward biased constitutes the base current which flows through the base region. The magnitude of the base current is very small. The base current causes the electrons to move into the collector region or create a hole in the base region.

The base of the transistor is very thin and lightly doped because of which it has less number of electrons as compared to the emitter. The few electrons of the emitter are combined with the hole of the base region and the remaining electrons are moved towards the collector region and constitute the collector current. Thus we can say that the large collector current is obtained by varying the base region.