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.　©　2006　IEEE.