Investigation of advanced SiGe heterojunction bipolar transistors at high power densities

In this paper, we investigate the behavior of advanced SiGe HBTs operating at very high power densities. Measurement results are presented and explained using a model that considers both impact ionization and self-heating. It is shown that even for small transistors, the temperature difference between the center and the ends of the emitter finger causes a significantly more nonuniform current density than expected solely from impact ionization. This requires special attention because the high temperatures that result from very large current densities can degrade the device lifetime.