In this study, a InGaAs/InP passive unit pixel with an optical-sensitivity-improved heterojunction phototransistor (HPT) for array imaging applications has been designed and characterized for low-light signal detection considering light-dependent shunt resistance. Using the proposed devices, a 1 x 256 highly sensitive linear array chip with suitable shunt resistance has been fabricated and characterized. The designed passive unit pixel consists of one photodetector and one select transistor with a collector-base terminal tied configuration for a wide dynamic range. We also present epitaxial structures and an equivalent model to optimize optical gain and shunt resistance characteristics. The device operation mechanism and experimental results are discussed. The experimental results show that our device has an optical sensitivity of 118 A/W, which is significantly higher than that of a conventional PIN photodetector with the same light-absorbing area. This high sensitivity originates from the optical gain-enhanced device structure. A typical optical gain is approximately 236, which means HPTs are 236-fold more sensitive than PIN photodetectors. The proposed HPT also has tens of k Omega shunt resistance with high optical sensitivity under low illumination, which is sufficient for effective signal conversion through a transimpedance amplifier circuit.