Secondary ion mass spectra analysis (SIMS) has been employed to construct the diffusion model of Ga in SiO2/Si system during doping and to study the distribution of Ga in silicon. Based on the above model, conclusions have been drawn as following: (1) Ga has a linear distribution and enters Si due to the segregation of SiO2/Si interface. As a result, the diffusion model of Ga was established. (2) Through controlling the temperature and adjusting the time during pre-deposition, and combination of them, an idea purity distribution can be obtained. (3) The planar junction with high uniformity and good repetition will be obtained by the above process, indicating that Ga has ideally diffused into silicon through the perfect interface.