Ga base region was formed using low concentration doping-junction depth process-high concentration doping method, and the NPN transistor sample was prepared. The experimental result and the theoretical analysis indicate that the linear slowacting distribution of Ga and diffusion under SiO2 film strongly heighten the withstand voltage level and all round electrical performance; negative-resistance effects in IC-VCE curve is caused by the concentration of Ga of the former surface becoming exhausted state by reoxidation. It can be improved with Si3N4/SiO2/Si composite structure; In brief, tube-open diffusion of Ga is a unexampled new approach to prepare high voltage device.