An　effective　method　for　enhancing　thermal　stability　of　a　multi-emitter　power　heterojunctlon　bipolar　transistor　(HBT)　has　been　presented.　The　method　employs　the　segmented　emitter　structure　rather　than　the　traditional　non-segmented　structure　to　suppress　the　junction　temperature　rise　and　reduce　the　thermal　resistance.　A　suitable　thermal　model,　which　includes　various　thermal　resistances　of　the　different　components,　is　built　for　the　segmented　multi-emitter　HBT.　Using　this　model,　the　thermal　simulation　for　a　ten-finger　power　SiGe　HBT　with　segmented　emitter　structure　is　performed　by　ANSYS　software.　Considering　the　precision　of　the　simulation　and　the　restriction　of　ANSYS　software,　this　study　proposes　two　steps　simulation　method:　first　substrate　simulation　and　second　active　region　simulation.　The　three-dimensional　temperature　distribution　on　emitter　fingers　is　obtained.　Compared　with　non-segmented　structure,　the　maximum　Junction　temperature,　thermal　resistance　of　the　power　HBT　with　improved　structure　is　significantly　lower　as　expected,　and　the　thermal　stability　is　enhanced.