We demonstrated an ultraviolet detector employing an in-plane transport channel of n-type MoS2 with out-of-plane p-type rGO, which acts as a sensitizer for underlying n-type MoS2 photodetector. A developed vertical built-in field from vertical p-n nano-heterojunction separates the photo-excited carriers at the rGO/MoS2 interface. Therefore, the rGO/MoS2 device showed a notably improved photo-responsivity of \sim 6.92 A)/W and an excellent detectivity of 1.26 \times 1012 Jones under the irradiation of ultraviolet light. Moreover, the device exhibited an excellent reproducibility and stability in ambient environment even after four months. © 2018 IEEE.