Optimum design methodology for thermally stable multi-finger power SiGe HBTs

The two-dimensional temperature profile of a multi-finger power SiGe HBT is studied with the electrothermal model, which shows that there is an uneven temperature profile over the device finger for HBT with uniform finger length. Because of the positive current-temperature feedback, the uneven temperature profile will leads to an anomalous current distribution, which eventually caused the thermal instability. To improve the uneven temperature profile and enhance the thermal stability, the HBT with non-uniform finger length is designed. Considering that designing multiple finger length values becomes trivial and time-consuming for the HBT with dozens of emitter fingers, a new thermal design methodology namely Grouping and Adjusting (GA) method is proposed to shorten design time. Taking 30-finger HBT for example, a detailed design procedure is present. The calculated results show both significant improvement on the peak temperature and the uniformity of SiGe HBT with non-uniform finger length.