Photodetectors　based　on　two-dimensional　(2D)/　three-dimensional　(3D)　semiconductor　heterojunction　structures　are　emerging　as　appealing　candidates　for　high-sensitivity　applications.　The　performances　of　these　hybrid　photodetectors　are　closely　correlated　with　their　current　gain　mechanism.　Carrier　recirculation　is　the　most　commonly　reported　mechanism.　Recently,　a　Fermi　level　alignment　mechanism　was　proposed　for　2D　graphene/0-dimensional　(0D)　quantum　dot　heterostructures　because　of　the　easy　Fermi　level　tunability　of　the　quantum　dot.　In　this　article,　an　interface-induced　gain　mechanism　using　this　Fermi　level　alignment　process　is　proposed　and　identified　based　on　a　2D　graphene/3D　GaAs　hybrid　structure　with　comparative　measurement　configurations.　Because　of　the　high　surface　state　density　of　GaAs,　the　photo-excited　holes　tend　to　become　trapped　at　the　graphene/GaAs　interface,　which　can　easily　lower　the　interface　Fermi　level　and　the　Fermi　level　in　graphene　via　an　alignment　process.　When　combined　with　the　high　carrier　mobility　characteristics　of　graphene,　a　maximum　current　gain　of　2520　and　responsivity　of　1321　A　W-1　are　achieved　in　the　devices.　This　study　clarifies　the　role　of　the　interface　states　in　the　gain　characteristics　of　some　2D/3D　hybrid　devices,　with　results　that　are　instructive　for　optimal　device　design.