Investigation of advanced SiGe heterojunction bipolar transistors at high power densities

Advanced SiGe bipolar technologies can now even be used for very challenging applications like 77 GHz automotive radar oscillators with high output power levels. Such ICs must operate close to the limits of the technology to achieve the desired performance, demanding high current densities and high voltages especially in the output transistors. Consequently, self-heating must be accurately taken into account. In this paper we investigate the behavior of advance 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.