Investigation of the current gain of a composite transistor

An expression is derived for the complex current gain of a composite transistor. It is shown that the current gain of a composite transistor is influenced by the low-frequency current gain and cutoff frequency of the individual transistors. The emitter and collector depletion-layer capacitances and the extrinsic base resistances are also shown to influence the current gain of a composite transistor. It is found that there is a considerable difference between the current-gain characteristic of a composite transistor predicted by the equation for ¿ based on one-dimensional diffusion theory and that predicted by the first-order approximation. Measurements are made of the steady-state frequency response and the transient response of a composite transistor constructed from two alloy-junction transistors of the same polarity. Close agreement is found between the measured characteristic and the characteristic predicted by the equation for ¿ based on one-dimensional diffusion theory where the effects of extrinsic components have been taken into account.