The　effects　of　different　optical　window　positions　and　different　optical　window　areas　on　the　optical　response　characteristics　of　SiGe/Si　Heterojunction　Phototransistor　(HPT)　are　analyzed.　HPTs　with　emitter　optical　window　can　generate　more　photo-generated　carriers　due　to　the　longer　absorption　region,　and　then　bring　out　a　larger　optical-generated　voltage　at　emitter　junction　interface,　which　is　beneficial　for　electronsto　inject　from　the　emitter　into　the　base.　Therefore,　a　larger　collector　current　and　optical　gain　are　obtained.　When　the　optical　window　area　is　10　μm×10　μm,　the　maximum　optical　gain　of　SiGe/Si　HPT　can　reach　9.24　with　650　nm　incident　light　wavelength　and　2.0　V　collector　voltage.　HPTs　with　base　optical　window　get　larger　photo-generated　carrier　density　in　the　absorption　region　when　incident　power　become　larger,　then　the　possibility　of　rapid　relaxation　for　photo-generated　holes　increases,　which　relieves　the　limitation　of　the　operating　speed　from　hole＇s　low　mobility　to　some　extent,　so　the　optical　characteristic　frequency　is　increased.　When　the　optical　window　area　is　10　μm×10　μm,　the　optical　characteristics　frequency　of　SiGe/Si　HPT　can　reach　16.75　GHz　with　650　nm　incident　light　wavelength　and　2.0　V　collector　voltage.　For　the　SiGe/Si　HPTs　with　emitter　optical　window　that　can　achieve　higher　optical　gain　and　optical　characteristic　frequency　merit,　when　the　optical　window　area　gradually　increases　from　3　μm×10　μm　to　50　μm×10　μm,　the　effective　injection　area　of　electrons　at　emitter　junction　interface　gradually　increases.　However,　at　the　same　time　the　emitter　junction　capacitance　and　collector　junction　capacitance　increase　and　results　the　RC　delay　time　increasing,　so　the　optical　characteristic　frequency　gradually　decreases.　The　optical　gain　and　optical　characteristic　frequency　merit　increase　gradually　with　the　increase　of　the　optical　window　area,　but　the　rate　of　increase　slows　down　and　the　optical　gain　and　optical　characteristic　frequency　merit　tends　to　become　saturated.　©　2020,　Science　Press.　All　right　reserved.