Thermal effects on the characteristics of AlGaAs/GaAs heterojunction bipolar transistors using two-dimensional numerical simulation

Two-dimensional numerical simulations incorporating the spatial distributions of the energy band and temperature were used to study AlGaAs/GaAs heterojunction bipolar transistor characteristics. It was found that the negative differential resistance and the reduction of the base-emitter voltage for a constant base current in the active region are due to thermal effects. The differential current gain and cutoff frequency decrease when the transistor is operated at high power levels. The temperature distribution of the transistor operated in the active region shows a maximum at the collector region right beneath the emitter mesa. When the transistor is operated in the saturation region, the emitter contact region may be at a slightly lower temperature than the heat sink temperature. This thermoelectric cooling effect results from the utilization of the thermodynamically compatible current and energy flow formulations in which the energy band discontinuities are part of the thermoelectric power