We report on the properties of a heterojunction photodiode composed of p-type NiO and n-type Si. This photodetector was fabricated by evaporating NiO powders on Si (1 0 0) substrate. Our deposited p-NiO film was found to have a bole concentration of similar to 10(19) cm(-3) according to Hall measurements. Current-voltage (I-V) characteristics of our photodiode were measured in the dark and under ultra-violet (UV)-vis light illuminations (290 nm, 325 nm, 460 nm, 540 urn, and 633 nm) at room temperature (RT). The photo-responsivity of our photodiode appeared almost saturated even at 0 V, showing good photo-voltaic properties. The responsivity was as high as similar to 0.36 A/W at 0 V and similar to 0.40 A/W at 30 V for visible light (633 nm) while it was 0.15 A/W at 0 V and 0.17 A/W at 30 V for UV (290 nm). (c) 2004 Elsevier B.V. All rights reserved.