For power bipolar transistors, the emitter-ballasting-resistor is usually used to improve the thermal stability. However, the ballasting-resistors degrade the output power, power gain, power-added-efficiency (PAE) of the transistor. In this paper, the thermal stability can be improved substantially by adjusting either the spacing or length of the emitter fingers without using ballasting resistors. 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