© 2020 Elsevier GmbHHeterojunction photodiodes of assembling Ag nanoparticles (NPs) onto Al-Ga co-doped ZnO (AGZO)/Si were fabricated via a sol-gel spin coating method. The current-voltage characteristics of the devices measured in the dark and under illuminated conditions demonstrated that the performance of photodiodes was remarkably enhanced via decoration of Ag NPs. This was attributed to the scattering effect of Ag NPs because of their large size. Ultimately, the Ag NPs-decorated devices achieved a rectification ratio of 4513, a photoresponsivity of 0.30 A/W and a photo-to-dark-current ratio of 1016. Compared to the bare device, a substantial improvement of 58 % in the photoresponsivity and 147 % in the photo-to-dark-current ratio were observed for the Ag NPs-decorated device. These results render Ag NPs-decorated AGZO/Si heterojunction photodiodes as promising building blocks of future optoelectronic applications.