The sensitivity of the detector is the core technical indicator of the infrared detector. Short-wave infrared detector has low dark current and the sensitivity will be limited by the inherent read-out circuit noise of the detection system. Therefore, it is an effective way to further enhance the sensitivity by introducing internal gain into the detector. Heterogeneous phototransistor takes advantages of high gain, low operating bias, and low excess noise, which provides novel approach for high-sensitive detection. This paper mainly focuses on the simulation design of InGaAs/GaAsSb type-II superlattice short-wave infrared phototransistor, and studies the dependence of the device size on the optoelectronic characteristics. The results show that a higher gain, a lower dark current, and a faster response can be achieved by a smaller base size. Based on the optimization design of size structure, a sensitivity with a noise equivalent photon lower than 10 can be achieved, which provides a new technical approach to achieve high-sensitive heterogeneous phototransistor detector.