Despite its versatility in photocatalysis, the application of carbon nitride (CN) to optoelectronic devices, especially silicon (Si) optoelectronics, has been constrained by a lack of synthetic methods for producing large-scale, highly uniform and processable films. Here we report a large-scale synthesis of ultrathin amorphous carbon nitride (aCN) on Si, achieving high uniformity, ultralow surface roughness and a covalently bonded interface with Si. The ultrathin aCN on Si (aCN/Si) is synthesized via a two-step process involving a dual-heating-zone chemical vapour deposition and subsequent postannealing in a hydrogen atmosphere. During the postannealing process, the initially formed bilayer of polymeric CN and underlying aCN undergoes material transformations, including thinning of the CN film, increasing spatial uniformity and covalent bond formation between nitrogen and silicon atoms, producing a high-quality aCN/Si heterostructure. Based on this aCN/Si, we developed vertical photodiodes that function both as electrical diodes with high rectification ratio (3.8 × 108) and photodetectors with high specific detectivity (1.9 × 1012 Jones), fast photoresponse (6.7 µs) and a broad linear dynamic range (>130 dB). Integrating these aCN/Si vertical photodiodes with amorphous indium-gallium-zinc oxide switching thin-film transistors enables active-matrix-based multispectral imaging across the visible to near-infrared spectrum range.