The　effect　of　bandgap　engineering　on　I-C-V-BE　fly-back　characteristic　of　power　SiGe　heterojunction　bipolar　transistor　is　studied　through　theoretical　analysis,　computer　simulations,　and　experimental　measurements.　It　is　found　that　because　of　the　existence　of　Ge　composition,SiGe　Heterojunction　Bipolar　Transistors　have　better　thermal　stability　compared　with　homojunction　bipolar　transistors　under　the　same　operating　condition　which　will　be　beneficial　to　decrease　the　emitter　ballast　resistance　of　HBT　and　improve　the　performance　of　transistor.　And　the　greater　Ge　composition　is,　the　more　stable　the　HBT　is