12.3 : Working Principle of BJT

A Bipolar Junction Transistor (BJT), specifically a PNP transistor in a common-base configuration, effectively amplifies or switches electronic signals by controlling the flow of charge carriers. This discussion focuses on its operation in the active mode.

In the PNP configuration, the emitter is heavily doped with positive charge carriers (holes), while the base is lightly doped with negative carriers (electrons). This setup allows for a forward bias across the emitter-base junction, diminishing the potential barrier and permitting holes to diffuse from the emitter into the base. These holes then attempt to traverse towards the collector. During this journey, some holes recombine with electrons in the base, reducing the number of carriers reaching the collector and contributing to the base current. Simultaneously, the collector's thermally generated electrons move towards the base, adding to the collector current.

The primary component of the emitter current are these diffusing holes. The base current arises from the difference between the emitter current and the collector current, which is crucial in maintaining charge neutrality in the base. The transistor's efficiency is gauged by its current gain, which includes two critical factors: the emitter efficiency and the base transport factor. The emitter efficiency indicates the fraction of carriers injected from the emitter that contribute to the output current. In contrast, the base transport factor reflects the proportion of these carriers that reach the collector. Ideally, both values should approach unity, signifying efficient carrier transport and minimal recombination.

Moreover, any leakage current between the collector and base when the emitter-base junction is open, is included in the calculation of the collector current in terms of current gain. This leakage denotes an operational inefficiency, influencing the transistor's performance in electronic circuits.