We demonstrate a Ga0.47In0.53As/Al0.48In0.52As modulation‐doped photoconductive detector. The detector has a built‐in electric field to separate photogenerated electron‐hole pairs. This allows the electron lifetime to be precisely determined by electrode spacings. When tested by a 200‐ps, &lgr;=1.3  &mgr;m laser pulse, the detector shows an internal gain of 18, a peak external quantum efficiency of 300%, a rise time of 80 ps, a full width at half‐maximum of 250 ps, and an internal gain‐bandwidth product of 72 GHz. Typical devices have 4‐&mgr;m spacings between electrodes and 94‐&mgr;m widths. The detector has a spectral response between 0.6 and 1.65  &mgr;m with a dip at 0.86  &mgr;m. We also measured the noise power of the detector at room temperature. With its capability to be integrated with a modulation‐doped field‐effect transistor, this detector can be useful in a monolithic integrated photoreceiver.