Al-doped n-ZnO/p-Si heterojunctions were fabricated using a sol-gel dip coating technique at 700 degrees C, in a nitrogen ambient. The structural, optical, and electrical properties of ZnO:Al thin films, and the heterojunction properties of ZnO:Al/p-Si were investigated with respect to the effects of Al doping concentration. Hexagonal nanostructured ZnO: Al thin films with a 1.2% and a 1.6 at.% Al concentration exhibited high optical transmittance in visible ranges. Electrical resistivity changed with respect to Al doping concentration, and minimum resistivity was detected at a 1.2 at.% Al concentration. The ZnO: Al/p-Si heterojunction properties were analysed using current-voltage (I-V) measurements at four different Al concentrations, ranging from 0.8 to 1.6 (at.%). The ZnO: Al/p-Si heterojunctions exhibited diode-like rectifying behaviour. Under UV illumination, the photoelectric behaviour observed for the ZnO: Al/p-Si heterojunctions was diode.