Abstract: Novel Si phototransistors with a punchthrough base have been fabricated with regular planar technology. High gain and low noise have been achieved in this kind of device due to the fully depletion of base. The optical conversion gains larger than 150,000 have been observed when incident light power is 0.1nW. In addition to very high gain, the unique structure of the device also results in low output noise. The measured output noises at different currents well fit the wellknown relation \overlinei_n^2=2 q I_c \Delta f.