Current gain in Bipolar Junction Transistors (BJTs).

βF is the forward-active current gain in a BJT. It is the ratio of collector current (IC) to base current (IB). It can also be calculated using αF, the ratio of collector current to emitter current (IE): βF = (αF / (1 - αF)).

In reverse-active operating conditions, βR may be calculated in a similar way, replacing the collector and emitter currents with each other.

A BJT designed to have a high current gain in forward-active conditions will have a poor gain when reverse voltages are applied.