Modes of Operations of BJT

In active mode, the emitter-base junction is forward-biased, allowing electrons to be injected from the emitter to the base. Simultaneously, the base-collector junction is reverse-biased, blocking majority carriers. This configuration enables the transistor to function as an amplifier, where a small base current controls a significantly larger collector current.

In saturation mode, both the emitter-base and base-collector junctions are forward-biased. The transistor acts as a closed switch with minimal biasing voltage and maximum output current. This mode is essential for switching operations where the transistor must conduct high current across its terminals.

In cut-off mode, both junctions are reverse-biased, resulting in zero current flow and acting as an open switch. Active mode, by contrast, has a forward-biased emitter-base junction and reverse-biased collector-base junction, enabling amplification. Cut-off mode is critical in digital circuits for representing an 'off' state.

In inverted mode, the collector-base junction is forward-biased while the emitter-base junction is reverse-biased, reversing the roles of collector and emitter. Current gain is reduced due to lower emitter efficiency, caused by the collector's lower doping levels compared to the base. This asymmetry in doping concentrations limits the transistor's amplification capability.

A BJT's operating mode depends on emitter-base and collector-base junction biasing. Active mode requires forward-biased emitter-base and reverse-biased collector-base junctions. Saturation has both forward-biased; cut-off has both reverse-biased. Inverted mode features reverse-biased emitter-base and forward-biased collector-base junctions, reversing normal transistor operation.

In saturation mode, the transistor conducts maximum current, functioning as a closed switch for 'on' states. In cut-off mode, no current flows, acting as an open switch for 'off' states. These two modes enable BJTs to perform digital switching operations, making them essential components in logic circuits and power switching applications.

During active mode, the forward-biased emitter-base junction enables electron injection from the emitter into the base region. These injected electrons are collected at the reverse-biased collector-base junction, creating a collector current much larger than the base current. This electron injection mechanism is fundamental to the transistor's amplification capability and small signal analysis of BJT amplifiers.