Effect of non-uniform emitter current distribution on power transistor stability

Abstract

The concentration of current to the edges of an emitter in a power transistor is known to be due to the transverse voltage drop along the emitter face caused by base current flow. The calculation of this ‘crowding’ phenomenon is usually made assuming an isothermal emitter surface. It is shown that in general, when a transistor is dissipating power, that the emitter current can crowd either to the center portion or edge of the emitter, depending on the value of operating collector voltage and current. The non-uniform current distribution results from a temperature peak at the center of the emitter causing a concentration of current to the emitter center; this is counteracted by the transverse base voltage drop causing crowding to the emitter edge. The effect of this non-uniform emitter current distribution on transistor thermal stability is investigated for two transistor designs—one simulating an alloy transistor and the other a diffused-base transistor. The results indicate that the wide-base alloy device is more stable for a given power level. Values of external emitter or base resistance necessary for thermal stabilization at high power levels are calculated for the two different transistor designs. In both cases for a given power level, stabilization is more easily achieved at high current, low collector voltage rather than low current, high voltage.