Determination of the thermal resistance and current exponent of heterojunction bipolar transistors for reliability evaluation

The reliability of Heterojunction Bipolar Transistors (HBT's) depends on parameters such as junction temperature and current density. For the description of the failure behaviour of HBT devices, an extended half-empirical Arrhenius model is used introducing a current acceleration exponent. The thermal resistance (Rh) of the device has to be determined which is itself temperature dependent. The novel approach presented in this paper allows the accurate determination of the device junction temperature and thermal resistance of the HBT under actual working conditions at high collector currents. The current exponent is estimated to be in the range between 1... 2, based on prior knowledge gained on laser diodes, which leaves a huge range of uncertainty. In this paper, this current exponent is determined by direct measurement using two independent approaches with good correlation of results. (C) 2001 Elsevier Science Ltd. All rights reserved.