Layered transition metal dichalcogenide materials grown over a conventional 3D semiconductor substrate have ignited a spark of interest in the electronics industry. The integration of these 2D layered materials extensively addresses the formidable challenges faced by a new generation of opto-electronic and photovoltaic devices. Herein, we have demonstrated a 2D/3D heterojunction type photodetector by depositing MoS2 on a GaN substrate with a mass-scalable sputtering method. Spectroscopic and microscopic characterizations expose the signature of the highly crystalline, homogeneous and controlled growth of a deposited few-layer MoS2 film. The greater light absorption of few-layer MoS2 results in the high performance of the MoS2/GaN photodetector. Our device shows high external spectral responsivity (∼103 A W-1) and detectivity (∼1011 Jones) with a very fast response time (∼5 ms). Our obtained results are significantly better than previous MoS2- and GaN-based photodetectors. This work unveils a new perspective in MoS2/GaN heterojunctions for high-performance optoelectronic applications. © 2018 IOP Publishing Ltd.