Design and optimization of Ge profiles for improved thermal stability of SiGe HBTs

Abstract

The impact of the three state-of-the-art germanium (Ge) profiles (box, trapezoid and triangular) across the base of SiGe heterojunction bipolar transistors (HBTs) under the condition of the same total amount of Ge on the temperature dependence of current gain β and cut-off frequency fT, as well as the temperature profile, are investigated. It can be found that although the β of HBT with a box Ge profile is larger than that of the others, it decreases the fastest as the temperature increases, while the β of HBT with a triangular Ge profile is smaller than that of the others, but decreases the slowest as the temperature increases. On the other hand, the fT of HBT with a trapezoid Ge profile is larger than that of the others, but decreases the fastest as the temperature increases, and the fT of HBT with a box Ge profile is smaller than that of the others, but decreases the slowest as temperature increases. Furthermore, the peak and surface temperature difference between the emitter fingers of the HBT with a triangular Ge profile is higher than that of the others. Based on these results, a novel segmented step box Ge profile is proposed, which has modest β and fT, and trades off the temperature sensitivity of current gain and cut-off frequency, and the temperature profile of the device.