Consider a PNP transistor in a common-base configuration, operating in active mode.

Forward biasing reduces the emitter-base potential, enabling the diffusion of holes from the emitter to the base and electrons from the base to the emitter. At the same time, the leakage current flows into the emitter.

The injected holes diffuse towards the collector, with a few undergoing recombination with electrons in the base region.

Additionally, the thermally generated electrons drift from the collector to the base, further contributing to the collector current.

The difference between the emitter and collector currents forms part of the base current, which helps maintain charge neutrality in the base region.

By utilizing the terminal currents, the common-base current gain for the BJT is determined.

Here, the first term signifies emitter efficiency, while the second term represents the base transport factor. For a well-designed transistor, both terms should approach unity.

When expressing the collector current in terms of current gain, the second term corresponds to the leakage current between the collector and the base.