ZnO:Ga (GZO)/Si photodiodes having different thicknesses of GZO layer are fabricated by the sol-gel spin-coating method. The photoelectric properties of the devices are investigated by measuring the current-voltage (I-V) characteristics. The photodiodes exhibit high rectification ratio (RR) and relatively low leakage current. Moreover, the best performance of the photodiodes is achieved by the heterojunction with GZO thickness of 35 nm, which has a transmittance over 87% in the visible range, compared to others. The resulting device shows a current RR of 1275 at +/- 3 V, a low saturation current (I-s) of 3 x 10(-8) A, and a photocurrent-to-dark-current ratio of 4639 at -3 V under 100 mW/cm(2).