The operation of junction transistors at high currents and in saturation

Abstract

The lumped-model characterization for junction transistors has been extended to current ranges where the transistor operation may no longer be considered linear. A number of effects are considered which contribute to the nonlinear behavior. In all cases it is shown that the self-bias cutoff effect plays an important part in the deterioration of performance at high currents. In alloy transistors, both normally and inversely biased, an exact solution for the form of the current gain as a function of collector current is given for the case where the injected density is small compared with the equilibrium base majority-carrier density. This solution is shown to be applicable to normally biased diffused-base units. A decrease in current gain is predicted due to the self-bias effect. The sources of lateral base current are bulk recombination and injection into the emitter, the relative amount being immaterial since both are proportional to the injected density. In the small-area alloy devices, it is seen that the injected density becomes large compared with the base majority-carrier density before the transistor performance is appreciably degraded. In such cases the lateral base current is seen to be normally dominated by non-unity emitter efficiency. In all cases, surface recombination is assumed to be the dominant source of total base current, and the first-order correction to the linear theory is shown to exhibit a 1/(1 + Ki_c) dependence. At higher currents all cases follow the observed 1/i_c proportionality. The effect of nonlinear phenomena on saturation voltage is considered in some detail. The saturation voltage is shown to increase with drive current, again emphasizing the importance of avoiding large overdrive currents. The use of alloy transistors as low-level signal switches is discussed, indicating the necessity for high values of β and low values of K.

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