In　this　paper,　the　working　mechanism　and　high　characteristics　of　an　InP/InGaAsP　uni-traveling-carrier　(UTC)　double　hetero-junction　phototransistor　(DHPT)　has　been　analyzed.　The　UTC-DHPT　employs　a　heavy　doping　base　and　two　hetero-junctions　to　realize　UTC　structure　to　ease　the　contract　between　HPT　responsivity　and　HPT　working　speed.　Optical　responsivity　and　optical　transit　frequency　are　analyzed　physically　through　illustrated　several　kinds　of　paramistic　capacitors　in　BE　junction　and　BC　junction　under　different　electrical　bias,　optical　bias　and　combined　electrical　and　optical　bias　in　this　work.　The　results　show　that　three-terminal　(3T)　working　mode　with　combined　electrical　bias　and　optical　bias　in　base　achieves　a　highest　maxim　fT　of　120GHz　at　collector　current　of　30mA　and　keeps　the　same　optical　responsivity　(37.5A/W)　compared　with　two-terminal　(2T)　working　mode.　©　(2015)　Trans　Tech　Publications,　Switzerland.