The　effects　of　buried　oxide　(BOX)　layer　on　the　capacitance　of　SiGe　heterojunction　photo-transistor　(HPT),　including　the　collector-substrate　capacitance,　the　base-collector　capacitance,　and　the　base-emitter　capacitance,　are　studied　by　using　a　silicon-on-insulator　(SOI)　substrate　as　compared　with　the　devices　on　native　Si　substrates.　By　introducing　the　BOX　layer　into　Si-based　SiGe　HPT,　the　maximum　photo-characteristic　frequency　f(t,opt)　of　SOI-based　SiGe　HPT　reaches　up　to　24.51　GHz,　which　is　1.5　times　higher　than　the　value　obtained　from　Si-based　SiGe　HPT.　In　addition,　the　maximum　optical　cut-off　frequency　f(beta,opt),　namely　its　3-dB　bandwidth,　reaches　up　to　1.13　GHz,　improved　by　1.18　times.　However,　with　the　increase　of　optical　power　or　collector　current,　this　improvement　on　the　frequency　characteristic　from　BOX　layer　becomes　less　dominant　as　confirmed　by　reducing　the　3-dB　bandwidth　of　SOI-based　SiGe　HPT　which　approaches　to　the　3-dB　bandwidth　of　Si-based　SiGe　HPT　at　higher　injection　conditions.