Surface phosphidization and bulk hydrogenation of defects in GaAs epilayers grown on Si substrate (GaAs/Si) was realized simultaneously by exposure to a metal-organic chemical vapor deposition (MOCVD) reactor-associated PH3 + H-2 RF plasma generator. X-ray photoelectron spectroscopy analysis reveals that a phosphidized layer was formed which removed the native As oxide from the GaAs/Si surface. Minority carrier lifetime measurement proved that both P and H atoms play important roles in this defect passivation process. The passivation process was applied to GaAs solar cells grown on Si substrate. The saturation current density J(0) of the PH3 + H-2 plasma-passivated cell was significantly decreased, as a result. the photovoltaic characteristics were improved.