Editing Bipolar junction transistor (section)

==== Common-base current gain ====
Another useful characteristic is the ''[[common-base]] current gain'', {{mvar|α}}<sub>F</sub>. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.980 and 0.998. It is less than unity due to recombination of charge carriers as they cross the base region.

Alpha and beta are related by the following identities:
: <math>\begin{align}
  \alpha_\text{F} &= \frac{I_\text{C}}{I_\text{E}}, & \beta_\text{F} &= \frac{I_\text{C}}{I_\text{B}}, \\
  \alpha_\text{F} &= \frac{\beta_\text{F}}{1 + \beta_\text{F}} & \iff \beta_\text{F} &= \frac{\alpha_\text{F}}{1 - \alpha_\text{F}}.
\end{align}</math>

Beta is a convenient figure of merit to describe the performance of a bipolar transistor, but is not a fundamental physical property of the device. Bipolar transistors can be considered voltage-controlled devices (fundamentally the collector current is controlled by the base–emitter voltage; the base current could be considered a defect and is controlled by the characteristics of the base–emitter junction and recombination in the base). In many designs beta is assumed high enough so that base current has a negligible effect on the circuit. In some circuits (generally switching circuits), sufficient base current is supplied so that even the lowest beta value a particular device may have will still allow the required collector current to flow.