12.5 : BJT の動作モード

A bipolar junction transistor has four modes of operation, depending upon the voltage polarities at the emitter-base junction and the collector-base junction.

In the active mode, the emitter-base junction is forward-biased, allowing electron injection from the emitter to the base. The base-collector junction is reverse-biased, blocking the flow of majority carriers.

In this mode, the transistor operates as an amplifier, with a small base current controlling a larger collector current.

In the saturation mode, both junctions are forward-biased. This mode corresponds to a small biasing voltage and a large output current, effectively acting as a closed switch.

The cut-off mode occurs when both junctions are reverse-biased, resulting in no current flow between the terminals. The transistor acts as an open switch.

In the inverted mode,  the emitter-base junction is reverse-biased, while the collector-base junction is forward-biased. It is often called the inverted active mode, where the collector becomes the emitter and vice versa.

This mode has lower current gain due to reduced "emitter efficiency" caused by lower collector doping compared to base doping.