Transparent n-type metal-oxide semiconductor of MoO_x was applied on a p-type Si substrate for high-performing heterojunction photodetector. The formation of MoO_x on Si spontaneously established a rectifying current flow with a high rectification ratio of 1,252.3%. Under light illumination condition, n-type MoO_x/p-type Si heterojunction device provided significantly fast responses (rise time : 61.28 ms, fall time : 66.26 ms). This transparent metal-oxide layer (MoO_x) would provide a functional route for various photoelectric devices, including photodetectors and solar cells.