An a-IGZO thin-film phototransistor incorporating graphene absorption layer was proposed to enhance the responsivity and sensitivity simultaneously for photodetection from ultraviolet to visible regime. The spin-coated graphene dots absorb incident light, transferring electrons to the underlying a-IGZO to establish a photochannel. The 5 A/W responsivity and 1000 photo-to-dark current ratio were achieved for graphene phototransistor at 500 nm. As compared with <1% absorption, the graphene phototransistor indicates a >2700 transistor gain. The highest responsivity and photo-to-dark current ratio is 897 A/W and 10 6 , respectively, under 340-nm light illumination.