In this work, the study of heterojunction Zinc Oxide on Silicon to enhance UV photodetector performance has been conducted. The study was carried out using a SILVACO ATLAS device simulator. There are two parameters studied in this project are: to find the best thickness of ZnO and suitable doping concentration of ZnO on the silicon substrate as a photodetector device. There are three different thicknesses of ZnO (0.5 µm ,1 µm and 2 µm) and four different doping concentrations of ZnO applied (1x1015 cm-3, 1x1019 cm-3, 1x1020 cm-3 and 1x1021 cm-3). The performance of the ZnO on Si as a photodetector was evaluated by dark and photo current-voltage (I-V) characteristics. It was found that the optimum thickness of ZnO is at 1 µm and the suitable doping concentration of ZnO is at 1x1021 cm-3 since with these values, the photocurrent response is high, and in spectral response, the peak value for both is depicted in a visible region, which is suitable as UV photodetector device criteria.