What is a transistor?

A transistor is a semiconductor device that is used to switch digital signals or amplify analog signals or generate electrical power. Transistors are one of the basic building blocks of modern electronics. It is made out of semiconductor material and has at least three terminals for connection to an external circuit.

A transistor is composed by placing an oppositely doped semiconductor material between two similarly doped semiconductors. Thus By placing n-type material between two p-type materials forms the PNP transistor while by placing a p-type material between two n-type semiconductor forms NPN transistor.

There are typically three electrical leads in a transistor, called the emitter, the collector, and the base. By applying a lower input current at the base of the transistor we can control higher output currents in between emitter and collector of the transistor.

What does it do?

When we apply a low current signal at the base-emitter junction of the transistor, electrons leave the emitter and flow into the base region. However the doping in the base region is kept very low and therefore comparatively few holes available for recombination.

As a result most of the electrons coming from the emitter region are able to easily overcome the base region and enter into the collector region, attracted by the positive potential of that region.

Only a small amount of the electrons from the emitter combine with holes in the base region which increases the current in the base-emitter circuit. This means that the collector current gets much higher.

The ratio between the collector current and the base current is given the Greek symbol Β. For most small signal transistors this may be in the region 50 to 500. In some cases it can be even higher. This means that the collector current is typically between 50 and 500 times that flowing in the base. For a high power transistor the value of Β is much less, 20 is a fairly typical value.

How does it work?

Depending on the biasing conditions, transistors have three different modes of operation.

Active Mode

The emitter-base junction is forward biased and the collector-base junction is reverse biased in the active mode. The current flows between emitter and collector and the amount of current flow is proportional to the base current.

Cutoff Mode

In this mode, both collector-base junction and emitter-base junction are reverse biased. Therefore almost no current flows except for some very small amount of leakage currents. Transistor in this mode is normally switched OFF and it is essentially an open circuit.

Saturation Mode

In this mode of operation, both the emitter-base and collector-base junctions are forward biased. Current flows freely from collector to emitter with almost zero resistance. In this mode, the transistor is fully switched ON and essentially it’s a close circuit.

Where to use it?

In the active mode the transistor generally acts as a current amplifier. Thus we can amplify weak signals to a much stronger signal. This principle is used in old AM radio. Cutoff Region & Saturation Region are primarily used as a switching in the digital logic circuits where cutoff mode is used for normally open circuits and saturation mode is used for normally closed circuit.

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