A SELF-CONSISTENT DC-AC 2-DIMENSIONAL ELECTROTHERMAL MODEL FOR GAALAS/GAAS MICROWAVE-POWER HBTS

A 2D self-consistent model has been developed to investigate the coupled electrothermal problem in GaAlAs/GaAs HBT's under dc and ac conditions. Both the electrical and thermal aspects of device behavior are simultaneously investigated by means of a nodal analysis of the distributed and physics-based model using a circuit simulator. In particular, the effects of base widening and the temperature dependence of the thermal conductivity are taken into account. Rigorous simulations are compared to the results given under different approximations (uniform temperature, constant conductivity, etc.). In particular, the sensitivity to the temperature dependence of the electron mobility is used to highlight the beneficial role of the negative temperature coefficient of the current gain in determining the thermal stability of the device. A HF performance criterion (Maximum Stable Gain) is calculated for the first time by considering the junction temperatures and current distributions and then compared to the results derived from the isothermal approximation. Finally, as an example of the applicability of the proposed approach to optimizing the thermal performance of power HBT's, the effects of varying the emitter spacing in a multiple-finger structure and the thickness and nature of the substrate are briefly considered.