Transistor Alpha and Beta

Transistor alpha (α) and beta (β) parameters represent the current gain, also known as forward current transfer ratio, of a BJT transistor. These parameters and associated formulas are used in semiconductor calculations.

In a common-base configuration of a NPN transistor, the collector current (I_C) is the input and emitter current (I_E) is the output, hence the ratio I_C/I_E is the dc alpha (α_dc). In transistor documentation, this is the h_FB parameter.

Here is the current gain formula for α_dc.

Alpha and beta share this very simple relationship formula, which is usually useful in general semiconductor calculations.

In common-emitter configuration of a NPN transistor, the input current I_B is through the base junction, and the output is the collector current I_C, hence the ratio I_C/I_B is the dc beta (β_dc). In transistor documentation, this is the h_FE parameter.

Here is the current gain formula for β_dc.

Semiconductor equations rely heavily on these identities and therefore it is very important to have a clear idea of their relationships.