The most effective and potential approach to improve the performance of heterojunction photodetectors is to obtain favorable interfacial passivation by adding an insertion layer. In this paper, MoO _x /Al ₂ O ₃ /n-Si heterojunction photodetectors with excellent photocurrents, responsivity and detectivity were fabricated, in which alumina acts as a tunneling passivation layer. By optimizing the post-annealing treatment temperature of the MoO _x and the thickness of the ultra-thin Al ₂ O ₃ , the photodetector achieved a ratio of photocurrent to dark current of 3.1 × 10 ⁵ , a photoresponsivity of 7.11 A W ⁻¹ (@980 nm) and a detective of 9.85 × 10 ¹² Jones at −5 V bias. Besides, a self-driven response of 0.17 A W ⁻¹ and a high photocurrent/dark current ratio of 2.07 × 10 ⁴ were obtained. The result demonstrated that optimizing the interface of heterojunctions is a promising way to obtain a heterojunction photodetector with high-performance.