Approximations to α for diffusion transistors

The paper reviews, from the viewpoint of design engineers, various approximations to the common-base intrinsic current gain of junction transistors. The basis of this comparison is the familiar expression for ¿ derived directly from the semiconductor diffusion and continuity equations. A 2-pole approximation is considered in detail. It is shown that both the sinusoidal and transient response predicted using this expression is in good agreement with that predicted using the exact theory. For most practical transistors the parameters of this approximation remain reasonably constant. In the response to a step function of emitter current, the 2-pole equation closely describes the initial time delay arising from the transit time of minority carriers across the base region. The collector response to unit charge inserted into the base in the form of ¿ function also gives good agreement with the results obtained from the exact theory. The various approximations are compared by graphical presentation of the results.