Hybrid　graphene/semiconductor　phototransistors　have　attracted　great　attention　because　of　their　ultrahigh　responsivity.　However,　the　specific　detectivity　(D*)　for　such　hybrid　phototransistors　obtained　from　source-drain　electrodes　is　assumed　to　be　1/f　noise.　In　this　paper,　D*　of　~1.82×1011　Jones　was　achieved　from　source-gate　electrodes.　Compared　with　the　same　device　which　was　measured　from　source-drain　electrodes,　D*　was　improved　by　~500　times.　This　could　be　attributed　to　the　carrier　trapping　and　detrapping　processes　having　been　screened　by　the　Schottky　barrier　at　the　interface.　The　rise　and　decay　times　were　4　ms　and　37　ms,　respectively.　The　temporal　response　speed　also　correspondingly　improved　by　~2　orders　of　magnitude.　This　work　provides　an　alternative　route　toward　light　photodetectors　with　high　specific　detectivity　and　speed.　Copyright　©2021　Chinese　Optics.　All　rights　reserved.