This paper presents the design and fabrication process of an a-Si:H/a-SiC:H heterojunction n–i–p photodiode developed for low-level light detection applications. The critical fabrication issues associated with deposition of device-quality materials, tailoring of defects at the i–p interface, film patterning, junction passivation, and contact formation are discussed. A significant reduction of the leakage current down to ∼10pA/cm 2 at reverse bias of 1V has been achieved by the introduction of ∼2nm graded and ∼4nm a-SiC:H buffer layers between the i- and p-layers. To preserve interface integrity, a semi-transparent Cr film with a-SiN x anti-reflection coating is used as a front contact. It is found that such contact induces lower leakage than transparent conductive oxide (TCO) contacts, which can cause a degradation of the p–i interface. A drawback of the semi-transparent metal contact is the optical loss, which can be minimized by thinning the metal layer and optimizing the anti-reflection coating. Quantum efficiency up to 52% is achieved for the optimized photodiode.