N-ZnO nanorods/n-silicon heterojunction was fabricated by growth of ZnO nanorods on a n-type silicon (111) wafer with a low-temperature aqueous solution method. Capacitance-voltage measurements revealed that after annealing at 900 °C in O2 ambient for 1 h, the heterojunction changed from abrupt N-ZnO nanorods/n-silicon to graded P-ZnO/n-silicon junction. The annealed diode showed good photoresponse in both the ultraviolet and visible regions with responsivity around 0.3 and 0.5 A/W without bias. The photoresponses toward ultraviolet and visible light were enhanced when the diode was under reverse and forward bias, respectively. The results were discussed in terms of phosphorus diffusion process and the band diagrams of the heterojunctions in this work.