To　improve　the　responsivity　and　working　speed　of　an　InP-based　optical　detector,　an　uni-traveling　carrier　heterojunction　phototransistor　with　gradual　coupled　ridge　waveguide　is　designed.　The　optical　transmission　mode　of　the　gradual　coupled　ridge　waveguide　is　analyzed　through　effective　index　method　and　beam　propagation　method,　and　the　waveguide　width　and　length　are　optimized　as　2.6　μm　and　250　μm　respectively　to　achieve　single　mode　transmission　and　high　absorption　efficiency.　Because　the　direction　of　optical　transmission　is　perpendicular　to　the　direction　of　carrier　motion,　the　absorption　efficiency　and　speed　of　the　phototransistor　are　optimized,　and　the　output　photocurrent　and　characteristic　frequency　of　the　device　are　improved.　The　responsivity　is　33.83　A/W,　the　saturation　output　photocurrent　reaches　90　mA　and　the　highest　characteristic　frequency　is　87　GHz.　Compared　to　the　vertical-illuminated　uni-traveling　carrier　heterojunction　phototransistor,　the　saturated　output　current　and　characteristic　frequency　of　the　uni-traveling　carrier　heterojunction　phototransistor　with　gradual　coupled　ridge　waveguide　are　increased　by　20%　and　24%　respectively.　However,　the　uni-traveling　carrier　heterojunction　phototransistor　with　gradual　coupled　ridge　waveguide　has　a　bigger　absorption　volume,　and　then　the　input　light　power　at　which　it　gets　the　saturation　current　is　larger　too,　therefore　its　responsivity　is　slightly　smaller　than　that　of　the　vertical-illuminated　uni-traveling　carrier　heterojunction　phototransistor.　©　2019,　Science　Press.　All　right　reserved.