Plasmonic harvesting of hot carriers (HCs) in metal–semiconductor (M–S) junctions has stimulated intensive research activities for sub-bandgap photodetection, in particular the development of silicon-based infrared photodetectors. Here, a copper–silicon heterojunction was investigated both theoretically and experimentally in comparison to the commonly used gold–silicon ones. A 1-order-of-magnitude higher responsivity and a longer cutoff wavelength over 2000 nm were observed in experiments in the sub-bandgap wavelength range of silicon with a copper–silicon junction. A phenomenological model was developed to analyze the dynamic processes of HCs and attributed the advanced photodetection performance of copper–silicon devices to the relatively higher electron density of state above the Fermi level and the higher ejection probability. Such a complementary metal–oxide–semiconductor-compatible and low-cost HC photodetection platform shows promising potential in silicon-based optoelectronic applications.