We have developed a novel mid-infrared photodetector that provides high responsivity and a low dark current. The device consists of a heterojunction phototransistor (HPT) with a strained InAs/InGaAs multiquantum-well (MQW) absorption layer inserted in the low electric field region of the collector. The basic layer structure was formed using the mature InGaAsP material system, which is employed in optical telecommunications devices. The strained InAs well under a low electric field enhances the quantum-confined effect. This provides a high absorption coefficient at a wavelength of 2.3 μm. The photocurrent generated in the MQW absorption layer is amplified by the current gain, resulting in a high responsivity of more than 10 A/W. In addition, the small volume of the MQW in the low electric field region provides a low dark current even at room temperature.