Transistor Operation

1.) Transistor construction
NPN transistor.

The transistor consists of two PN junctions to form three regions. And, these three regions of the transistors are called:

- Base, very thin and lightly doped, thus less mobile charge carriers.

- Emitter, highly doped, thus more mobile charge carriers.

- Collectors, largest region, and is less doped than the emitter.

PNP Transistor.

2.) Transistor Biasing

The setting up of dc voltage properly onto a transistor is called biasing.

For proper and correct biasing, please refer to the picture below,

- The emitter-base (or base-emitter) must be forward-biased.

- The collector-base must be reversed-biased.

Free electrons in the emitter region move to the base because the emitter-base is forward-biased. Because the base is deliberately made very thin and is very lightly doped. Most of these free electrons do not combine.

These free electrons diffused into the collector-base depletion where they are swept across the junction into the collector and are attracted by the higher positive potential (voltage) of collector supply.

Almost all these free electrons that enter the base, will pass the collector.

3.) Bipolar Transistor

For transistor, the charge carriers are:
- Electrons
- Holes

Since the conduction of a transistor is by electrons, as well as holes. a transistor is often called the bipolar transistor (BJT).

The two junctions of a bipolar transistor are known as:
- BE junction
- BC junction