With　the　aid　of　the　thermal-electrical　model　of　the　0.12μm　SiGe　heterojunction　bipolar　transistor　(HBT)　with　shallow　trench　isolation　(STI),　the　transient　thermal　behavior　of　the　device　is　studied.　Both　thermal　time　constant　(τ)　and　thermal　capacitance　(CTH)　are　adopted　to　represent　the　transient　thermal　behavior.　Furthermore,　the　influence　of　pulse　power,　thickness　and　depth　of　shallow　trench　isolation　on　τ　and　CTH　are　studied.　It　is　shown　that　both　τ　and　CTH　are　increased　with　the　increase　of　pulse　power.　With　the　increase　of　shallow　trench　thickness　(ts),　τ　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　(ds),　CTH　is　decreased　obviously,　which　leads　to　the　weakness　of　ability　to　store　heat　of　HBT　with　STI.　In　addition,　τ　is　increased　slightly　with　the　increase　of　ds　and　CTH　is　insensitive　to　variation　of　ts.　The　results　above　shed　light　on　the　improvement　of　the　transient　thermal　behavior　of　shallow　trench-isolated　SiGe　HBTs.　©　2018　IEEE.