Effect of bandgap engineering on thermal characteristic of radio frequency power SiGe heterojunction bipolar transistor

As is well known, direct-current (DC) characteristic, frequency characteristic and noise characteristic of SiGe heterojunction bipolar transistor(HBT) can be improved by "bandgap engineering" (by Ge composition). However, the effect of "bandgap engineering" on the thermal characteristic of HBT has not been reported. In this paper, the effect of "bandgap engineering" is analyzed by the use of 3D thermal-electric feedback model. Considering the temperature dependence of emitter junction voltage and current gain, the expression of the minimum emitter ballasting resistance (R-Emin), which is necessary for SiGe HBT thermal stability, is presented. Furthermore, non-uniform ballasting resistance design is given so as to further enhance the thermal stability of device. It is found that the surface temperature of the device decreases with the increase of Ge composition in SiGe base. This is because SiGe HBT internally possesses the thermal-electrical negative feedback. For the same dissipated power, the R-Emin decreases as Ge composition increases, which is beneficial to the improvment of the performance of radio frequancy(RF) power SiGe HBT. These results provide a good guide to further optimization of RF power SiGe HBT performance, especially thermal design.