THERMAL EFFECTS ON THE CHARACTERISTICS OF ALGAAS/GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS USING 2-DIMENSIONAL NUMERICAL-SIMULATION

Simulations incorporating the spatial distributions of the energy band and temperature have been used to study AlGaAs/GaAs heterojunction bipolar transistor characteristics. The coupled heat transfer and conventional device carrier transport equations were solved numerically in two dimensions using both the voltage and current boundary conditions applied on the base contact. An equivalent thermal resistance model was used to establish the temperature boundary condition between the electrically active device and the relatively large thermally active semi-insulating substrate. The negative differential resistance and the reduction of the base-emitter voltage for a constant base current in the active region are caused by the thermal effects. The differential current gain and cutoff frequency are decreased when the transistor is operated at high power levels. The temperature distribution of the transistor operated in the active region shows a maximum temperature occurring at the collector region right beneath the emitter mesa. When the transistor 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 reults from the utilization of the thermodynamically compatible current and energy flow formulations in which the energy band discontinuities are part of the thermoelectric power.