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　beta　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,　beta　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.