Thermal management in strongly non-planar microwave HBTs

We have systematically investigated technological parameters of Al _0.3Ga_0.7As/GaAs power heterojunction bipolar transistors (HBTs) using finite-element modelling, taking into account the temperature dependence of the semiconductor materials involved and a realistic nonplanar mesa structure. Our calculations include a multilayer structure and raise the question of the magnitude of heat transfer via top metal contacts. Equally important, for the case of multielement power HBTs with close element spacing, the lateral spread of the heat flow is inhibited by the neighbouring elements and the dependence of the thermal resistance on element spacing and substrate thickness can be approximated by a very simple analytical relationship