A transparent ultraviolet (UV)-detector was fabricated using a high-quality pn-heterojunction diode composed of transparent oxide semiconductors, p-type NiO and n-type ZnO, and its UV-response was measured at room temperature. Transparent tri-layered oxide films of ZnO/NiO/ITO were heteroepitaxially grown on an YSZ (1 1 1) substrate by a pulsed-laser-deposition combined with a solid-phase-epitaxy technique and they were processed to fabricate a p-NiO/n-ZnO diode. The diodes exhibited a clear rectifying I-V characteristic with an ideality factor of similar to2 and a forward threshold voltage of similar to1 V. Although the photoresponsivity was fairly weak at the zero bias voltage, it was enhanced up to similar to0.3 AW(-1) by applying a reverse bias of -6 V under an irradiation of 360-nm light, which is comparable to that of commercial devices. (C) 2003 Elsevier B.V. All rights reserved.