Photodiodes with a p-ZnO/oxide/n-Si substrate structure were fabricated. The N-In codoped p-type ZnO films were deposited by ultrasonic spray pyrolysis on a (111)-oriented silicon substrate with a thin oxide layer. A photocurrent of ~ 4.99 × 10-5 A was measured at a reverse bias of 1 V, and a photocurrent to dark current contrast ratio of almost five orders of magnitude was found. The photodiode responses exhibited three regions of behaviour: around 400 nm, between 400 nm–700 nm, and between 700 nm–1000 nm, denoted as regions A, B, and C, respectively. Region A corresponds to band-to-band absorption in the ZnO film, region B to band-to-deep level absorption in the ZnO film, and region C to band edge absorption in the Si substrate.