n-ZnO/p-Si heterojunction photodiodes have been fabricated by sputter deposition of n-ZnO films on p-Si substrates. Substrate temperatures of 300, 400, 500 and 600 degreesC were used for the n-ZnO film deposition using various Ar/O-2 ratios from 1:1 to 6:1. All the diodes, except for one obtained at a high substrate temperature of 600 degreesC, show strong rectifying behavior as characterized by current-voltage (I-V) measurement in a dark room. Photoelectric effects from the diodes have been observed under illumination using monochromatic red light with a wavelength of 670 nn. High levels of photocurrent or responsivity were obtained under reverse bias conditions when the stoichiometry of n-ZnO films was improved by optimizing the process conditions, such as the substrate temperature and Ar/O-2 ratio. The n-ZnO/p-Si structure prepared at 300 degreesC was found to be unsuitable because photoelectric effects were not apparent from this structure. A heterojunction diode with n-ZnO deposited at 600 degreesC was also found to be unsuitable because the dark leakage current was too high, even though the diode showed photoelectric effects.