A study of current crowding in bipolar transistors

The existing theory which considers an equipotential emitter was extended to include the effects of the distributed emitter resistances along and perpendicular to the emitter base junction. It was found that the resistance along the junction causes the emitter current to crowd towards both, the emitter and base contacts, unlike the existing theory where it crowds only towards the base contact. The resistance normal to the junction, which simulates a distributed ballust resistance was found to reduce crowding. The reduction in crowding was more pronounced at high currents. The effects of voltage dependent collector multiplication and distributed collector resistance were studied. Transient current crowding was studied and found to be considerably different from dc crowding. Thermal effects on current crowding are discussed, in order to determine the susceptibility of a transistor to second breakdown, and the stabilizing effects of distributed emitter and collector resistances.