BIPOLAR JUNCTION TRANSISTOR (BJT): Unipolar Junction Transistor, Bipolar Junction Transistor, n-p-n Transistor and p-n-p Transistor

BIPOLAR JUNCTION TRANSISTOR (BJT)

The transistor was invented in 1947 by John Bardeen, Walter Brattain and William Shockley at

Bell Laboratory in America. A transistor is a semiconductor device, commonly used as an Amplifier or an electrically Controlled Switch.

There are two types of transistors:

1) Unipolar Junction Transistor

2) Bipolar Junction Transistor

In Unipolar transistor, the current conduction is only due to one type of carriers i.e., majority charge carriers. The current conduction in bipolar transistor is because of both the types of charge carriers i.e., holes and electrons. Hence it is called as Bipolar Junction Transistor and it is referred to as BJT.

BJT is a semiconductor device in which one type of semiconductor material is sand witched between two opposite types of semiconductor i.e., an n-type semiconductor is sandwiched between two p-type semiconductors or a p-type semiconductor is sandwiched between two n-type semiconductor.

Hence the BJTs are of two types.They are:

1) n-p-n Transistor

2) p-n-p Transistor

The two types of BJTs are shown in the figure below.

The arrow head represents the conventional current direction from p to n. Transistor has three terminals.

1) Emitter 

2) Base 

3) Collector

Transistor has two p-n junctions. They are:

1) Emitter-Base Junction

2) Collector-Base Junction

Emitter: Emitter is heavily doped because it is to emit the charge carriers.

Base: The charge carriers emitted by the emitter should reach collector passing through the base.

Hence base should be very thin and to avoid recombination, and to provide more collector current base is lightly doped.

Collector: Collector has to collect the most of charge carriers emitted by the emitter. Hence the area of cross section of collector is more compared to emitter and it is moderately doped.

Transistor can be operated in three regions.

1) Active region.

2) Saturation region.

3) Cut-Off region.

Active Region: For the transistor to operate in active region base to emitter junction is forward biased and collector to base junction is reverse biased.

Saturation Region: Transistor to be operated in saturation region if both the junctions i.e., collector to base junction and base to emitter junction are forward biased.

Cut-Off Region: For the transistor to operate in cut-off region both the junctions i.e., base to emitter junction and collector to base junction are reverse biased.

Transistor can be used as

1) Amplifier 

2) Switch

For the transistor to act as an amplifier, it should be operated in active region. 

For the transistor to act as a switch, it should be operated in saturation region for ON state, and cut-off region for OFF state.

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