Effect of base potential distribution on thermal runaway and the power limitation of the heterojunction bipolar transistor with circular dot geometry

The dc power limitation of heterojunction bipolar transistor (HBT) with dot geometry is studied theoretically using combined electro-thermal and transmission line models. In most cases, thermal runaway occurs at a power level lower than the limit set by the intrinsic electronic property of the device. The inter-relation among the maximum operational power, emitter ballast resistance and emitter dot radius are established and discussed. The discussion is based on the theory of junction-temperature-rise-threshold for the thermal runaway and the base potential distribution in the emitter-base junction.