A heterojunction of n-type zinc oxide (ZnO) nanowires and p-type silicon has been successfully constructed to demonstrate ultraviolet (UV) photodiodes. The prototype device consists of naturally doped n-type ZnO nanowires grown on top of a (1 0 0) p-silicon substrate by the bottom-up growth process. The diameter of the nanowires is in the range of 70–120 nm, and the length is controlled by the growth time. The isolation is achieved by using spin-on glass (SOG) that also works as the foundation of the top electrode. The current–voltage (I–V) characteristics show the typical rectifying behavior of heterojunctions, and the photodiode exhibits response of ∼0.07 A/W for UV light (365 nm) under a 20 V reverse bias.