Thermal instability of power SiGe heterojunction bipolar transistor (HBT) at high current over a wide temperature range restricts the applications of the device in RF and microwave circuits. In order to improve the thermal instability, the influences of Ge profile in a base region on the electrical and thermal characteristics of microwave power SiGe HBT are studied with the aid of the model of multi-finger power SiGe HBT established by SILVACO TCAD. It is shown that for the HBT with graded step Ge profile, a higher cut-off frequency fT can be achieved due to the accelerating electric field caused by the graded step Ge concentration in the base region when compared with the device with uniform Ge profile. The influences of temperature on current gain β and fT are weakened, which avoids the drift of electrical characteristics over a wide temperature range. Although the temperature of device is lowered, the temperature of each emitter finger is still non-uniform. Considering the difference in heat dissipation among emitter fingers, a new device with non-uniform emitter finger spacing in layout and a graded step Ge profile in base region is designed. For the new device, the uniformity of temperature among emitter fingers is achieved, higher fT is kept, β and fT are less sensitive to temperature variation. Hence the thermal instability is obviously improved compared with the device with uniform emitter finger spacing and uniform Ge profile in base region, indicating the superiority of the new device at high current over a wide temperature range.