The effects of different optical window positions and different optical window areas on the optical response characteristics of SiGe/Si Heterojunction Phototransistor (HPT) are analyzed. HPTs with emitter optical window can generate more photo-generated carriers due to the longer absorption region, and then bring out a larger optical-generated voltage at emitter junction interface, which is beneficial for electronsto inject from the emitter into the base. Therefore, a larger collector current and optical gain are obtained. When the optical window area is 10 μm×10 μm, the maximum optical gain of SiGe/Si HPT can reach 9.24 with 650 nm incident light wavelength and 2.0 V collector voltage. HPTs with base optical window get larger photo-generated carrier density in the absorption region when incident power become larger, then the possibility of rapid relaxation for photo-generated holes increases, which relieves the limitation of the operating speed from hole's low mobility to some extent, so the optical characteristic frequency is increased. When the optical window area is 10 μm×10 μm, the optical characteristics frequency of SiGe/Si HPT can reach 16.75 GHz with 650 nm incident light wavelength and 2.0 V collector voltage. For the SiGe/Si HPTs with emitter optical window that can achieve higher optical gain and optical characteristic frequency merit, when the optical window area gradually increases from 3 μm×10 μm to 50 μm×10 μm, the effective injection area of electrons at emitter junction interface gradually increases. However, at the same time the emitter junction capacitance and collector junction capacitance increase and results the RC delay time increasing, so the optical characteristic frequency gradually decreases. The optical gain and optical characteristic frequency merit increase gradually with the increase of the optical window area, but the rate of increase slows down and the optical gain and optical characteristic frequency merit tends to become saturated.