Current gain and cutoff frequency falloff at high currents

A theoretical and experimental study of the effects of high-level injection of carriers into a reverse-biased collector-base junction has been performed. Two models which describe the high-current behavior of the junction space-charge region are discussed. The first deals with the formation of a current-induced base region at space-charge-limited current densities. The second model assumes that two-dimensional effects are predominant; at current densities corresponding to the onset of space-charge-limited current, lateral injection of carriers takes place. These phenomena were studied experimentally using silicon double-diffused transistor structures. The existence of space-charge-limited current in the reverse-biased collector depletion layer manifests itself in significant changes in the ac and dc parameters of the transistor. In particular, it is shown that the cutoff frequency (fT) and large-signal current gain (hFE) begin to decrease rapidly with increasing current at the onset of the space-charge limitation. A comparison of experimental results with predictions of the above theories indicates that, while both the formation of a current-induced base region and lateral injection do take place, the latter mechanism controls conventional device performance.