This paper proposes two methods for enhancing thermal stability of a multi-finger power heterojunction bipolar transistor (HBT). The methods adjust either the spacing or length of emitter fingers to improve thermal stability. The temperature rise at the device center of the space-adjusted HBT is suppressed by reducing heat flow from adjacent fingers, and that of the length-adjusted HBT is suppressed by reducing heat generation in the region. A five-finger AlGaAs/GaAs HBT is considered for numerical and experimental investigations. The results show that the method is very effective in enhancing thermal stability: the temperature of the device center is reduced and the power level for thermal regression is increased significantly. When compared with an HBT which has emitter fingers of equal spacing and length, the power level For thermal regression is increased by 34% for the space-adjusted HBT and by 67% for the length-adjusted HBT,