Effects of Ge profile in base region on thermal characteristics of SiGe HBTs

As is well known, the base Ge composition can improve the DC characteristics, frequency characteristics and noise characteristics of SiGe HBTs. However, the reports about the effects of Ge profile on HBTs thermal characteristics are rare. In this paper, by use of SILVACO simulator, the effects of different Ge gradients on thermal and electrical characteristics of SiGe HBT are investigated. It is found that under the same total Ge amount condition, as Ge gradient increases, the f(T) of device increases significantly, the uniformity of temperature distribution becomes better, the influences of temperature on the beta and f(T) are weakened, but the gain beta becomes smaller. For the device with uniform Ge composition, the beta is high, but the influence of temperature on the beta is enormous, the uniformity of temperature distribution is poor. Based on these results, in order to make a tradeoff among thermal, gain and frequency characteristics, a novel Ge composition structure with the combination of the uniform and graded Ge composition is proposed. The results show that the novel Ge composition structure SiGe HBT has good performances lower peak temperature, better uniform temperature profile, smaller variabilities of beta and f(T) with temperature, sufficient high beta and f(T) compared with the uniform Ge composition device. These new results provide valuable reference for the device thermal design, and are supplemental to the research and application of SiGe HBTs.