Effect of Ge composition on the ballasting resistor of RF power SiGe HBTs

A new expression for the thermal stability factor S of power HBTs is presented in this paper, considering the temperature dependence of collector saturation current. Based on the expression, the accurate minimum emitter ballasting resistance (_REmin) of the HBTs that is necessary in the thermal stability condition is determined. It is found that for SiGe HBTs, the R_Emin decreases with the increasing operating temperature in the high temperature region due to the existence of Ge composition in SiGe base whereas for Si BJTs, the R_Emin increases monotonously with the operation temperature. Hence for high-temperature-operation SiGe HBTs, the additional emitter ballasting resistance can be decrease so as to minimize adverse effect of the ballasting resistance as possible. These results provide a good guide to design emitter ballasting resistance of power HBTs and high-temperature-operation HBTs.