The electrical properties of Ge-doped CdO (CdO:Ge) films grown on p-Si were studied in this work. The focussing was on the effect of optoelectrical properties of CdO:Ge film on the optoelectrical properties of the constructed p–n heterojunction, as well as to study the dependence of the optosensitivity and optoresponse of the heterojunction on the Ge% doping level. The characterisation of the transparent conducting oxide CdO:Ge layer was performed by the X-ray diffraction, SEM, electrical measurements, and spectral photometry. A strong optosensitivity was found especially for the p–n heterojunction that used 0.16 wt% Ge-doped CdO, attaining a great value of about 60,000% comparing to undoped CdO. In addition, a good value of optical response of 319.4 mA/W for the p–n heterojunction constructed with 0.25 wt% Ge-doped CdO. The results show that CdO:Ge/p-Si heterojunctions act as very good candidates for constructing high-efficiency photodetectors.