Bipolar Junction Transistor (BJT) Operation

A bipolar junction transistor (BJT) is a three-layer semiconductor device consisting of two pn junctions. The three regions of a BJT are the emitter, base, and collector. The operation of a BJT is based on the movement of charge carriers (electrons and holes) across the different regions.

Three regions of operation:

1. Active Region:
In the active region, both junctions of the BJT are forward biased. The base-emitter junction is forward biased, allowing the flow of electrons from the emitter to the base. The base-collector junction is also forward biased, allowing the flow of majority charge carriers (either electrons or holes) from the base to the collector. The BJT operates as an amplifier in this region, where a small input current at the base controls a larger output current at the collector.

2. Saturation Region:
In the saturation region, both junctions of the BJT are forward biased, similar to the active region. However, the base-emitter junction is heavily forward biased, allowing a large amount of current to flow from the emitter to the base. The base-collector junction is also forward biased, allowing a large amount of majority charge carriers to flow from the base to the collector. In this region, the BJT operates as a switch, with a low resistance between the collector and emitter.

3. Cut-off Region:
In the cut-off region, both junctions of the BJT are reverse biased. No current flows through the device, and it behaves like an open circuit. The BJT is effectively turned off in this region.

Operation with one junction forward biased and either the collector or emitter open:

When one junction of the BJT is forward biased and either the collector or emitter is open, the device operates in the pinch-off region. This region is also known as the reverse active region or the reverse active mode.

Pinch-off Region:
In the pinch-off region, the base-emitter junction is forward biased, allowing the flow of electrons from the emitter to the base. However, the base-collector junction is reversed biased, preventing the flow of majority charge carriers from the base to the collector. The reverse bias causes the depletion region to widen, creating a barrier for the majority carriers. As a result, the BJT operates in a pinch-off mode, where the collector current is significantly reduced or cut off.

Therefore, when one junction of the BJT is forward biased and either the collector or emitter is open, the BJT operates in the pinch-off region.