To explain different doping effects in a buffer layer,thermally annealed interface,and upper epilayers of GaAs/Si films grown by Metalorganic Chemical Vapor Deposition (MOCVD),the behaviors of unintentional doping in GaAs/Si films are investigated in detail.A third doping mechanism of arsine impurity incorporation during the growth process of GaAs/Si films,apart from conventional mechanisms of gas phase reaction and diffusion from the silicon substrate,is proposed.The experimental results reveal that the doping behavior in the buffer layer studied is determined by the three types of doping mechanisms together.However in the thermally annealed interface and upper epilayers,the third doping mechanism is dominant.According to the third mechanism,the background carrier concentration in GaAs/Si films grown by MOCVD could be properly controlled through the arsine flow rate.