The demand for high-sensitivity short-wave infrared (SWIR) detection technology is urgent in frontier fields such as lidar and quantum communication. Heterojunction phototransistor (HPT), benefiting from its internal gain mechanism, provides an effective solution for breaking the physical limit of conventional PIN photodetector in sensitivity. This paper focuses on the base-size effect of InP/GaAsSb/InGaAs HPTs with type-II barrier structure. Devices with two different base structures were fabricated by controlling the etching process. Measurement results show that maintaining an intact base structure significantly improves device performance: at a bias voltage of ?2 V, the responsivity and internal current gain reach 141 A/W and 160, respectively—superior to those of devices with etched bases. Temperature-dependent analysis and size-effect studies further reveal that the dark current of the intact-base device is dominated by diffusion mechanisms and exhibits better dimensional stability, whereas the etched-base device suffers from pronounced generation–recombination current and surface leakage current caused by sidewall defects. Under low-temperature and weak-light conditions, carrier trapping by these defects leads to severe degradation of photo response. This study clarifies the critical influence of base structural design on HPT performance and provides valuable theoretical and experimental guidance for optimizing high-performance SWIR detectors.