We report an ex-situ phosphorus diffusion doping for germanium integrated photonic devices on silicon chip. Here, phosphorus oxychloride (POCl3)-based phosphosilicate glass is chosen for n-type diffusion. As an alternative process to the in-situ P doping during Ge epitaxy so far reported for Ge laser prototyping, the presented external P-diffusion method demonstrates photoluminescence (PL) emission enhancement of Ge-on-Si. The PL enhancement, along with the P secondary ion mass spectroscopy profile in Ge, clearly indicates that our ex-situ diffusion method to form n-type Ge has a significant potential for Ge active device fabrication as an enabling technology. It should be also noted that PL quenching is observed at high temperature diffusion processes which is induced by intermixing at the Ge and Si interface. The presented ex-situ P-diffusion process can serve as a template to monolithically integrate Ge devices such as not only light sources but modulators and photodetectors on Si complementary metal-oxde-semiconductor platform, as it may tailor device-specific pn junctions.