Current gain (hFE) and cutoff frequency (fT) falloff at high current densities

A theoretical and experimental study of bipolar transistor performance at high current densities has been performed. The analytical treatment is based upon the existence of a high level injection condition in the collector. Two models which describe the high current behavior of the collector junction space-charge region are discussed. The first deals with the formation of a current-induced base region at high current densities; the width of this current-induced base depends on the current density. The second model assumes that two-dimensional effects are predominant; at sufficiently high current densities lateral injection of carriers takes place. Theoretical curves for the current density necessary to bring about the space-charge limitation have been computed.