With　the　aid　of　the　thermal-electrical　model　of　the　0.12　mu　m　SiGe　heterojunction　bipolar　transistor　(HBT)　with　shallow　trench　isolation　(STI),　the　transient　thermal　behavior　of　the　device　is　studied.　Both　thermal　time　constant　(r)　and　thermal　capacitance　(C-TH)　are　adopted　to　represent　the　transient　thermal　behavior.　Furthermore,　the　influence　of　pulse　power,　thickness　and　depth　of　shallow　trench　isolation　on　r　and　C-TH　are　studied.　It　is　shown　that　both　r　and　Cut　are　increased　with　the　increase　of　pulse　power.　With　the　increase　of　shallow　trench　thickness　(t,),　tau　is　almost　increased　linearly,　which　leads　to　the　prolongation　of　time　to　reach　thermal　steady-state　for　STI　device.　At　the　same　time,　with　the　increase　of　shallow　trench　depth　(d(s)),　C-TH　is　decreased　obviously,　which　leads　to　the　weakness　of　ability　to　store　heat　of　HBT　with　STI.　In　addition,　r　is　increased　slightly　with　the　increase　of　d.　and　C-TH　is　insensitive　to　variation　of　t(s).　The　results　above　shed　light　on　the　improvement　of　the　transient　thermal　behavior　of　shallow　trench-isolated　SiGe　fIBTs.