In　this　paper,　the　output　current　characteristics　within　a　large　range　of　incident　light　power　of　a　double　heterojunction　uni-travelling-carrier　phototransistor　(UTC-DHPT)　were　analyzed　in　detail　and　compared　with　a　single　heterojunction　phototransistor　(SHPT)　simultaneously.　Firstly,　the　output　current　of　UTC-DHPT　is　smaller　than　that　of　SHPT　under　small　power　incident　light　because　the　UTC-DHPT　＇　s　absorption　section　is　only　the　base　region,　which　is　smaller　than　SHPT＇　s　absorption　section　consisting　of　base　and　collector.　Secondly,　benefiting　from　the　double　hetero-junction,　the　photogenerated　carriers　are　only　generated　in　base　in　UTC-DHPT,　and　there　are　few　photogenerated　holes　accumulated　in　the　collector　junction　interface.　The　space　charge　effect,　which　is　seriously　in　SHPT,　is　effectively　alleviated.　Furthermore,　the　saturation　of　the　output　current　under　small　power　incident　light,　which　always　occurs　in　SHPT,　is　avoided.　Therefore,　UTC-DHPT　has　a　larger　quasi　linear　range　than　SHPT.　Lastly,　the　uni-travelling　transportation　in　UTC-DHPT　make　photogenerated　holes　in　the　base　region　reach　to　the　emitter　junction　interface　by　the　rapid　relaxation,　and　thereby　effectively　reduce　the　emitter　junction　barrier.　This　would　increase　the　number　of　electrons　transmitting　from　the　emitter　to　the　base　in　the　unit-time,　and　the　emitter　junction　injection　efficiency　is　improved.　Thus.,　UTC-DHPT　can　obtain　higher　output　current　than　SHPT　under　the　high　power　incident　light.