We have developed a sensitive, ultrahigh speed photodetector which has a structure of a modulation‐doped AlxGa1−xAs/GaAs field‐effect transistor. In spite of a large gate‐drain spacing of >8 &mgr;m and a gate length of >20 &mgr;m, this detector exhibited a rise time of 12 ps and a full width at half‐maximum of 27 ps. When tested by a 8200‐A˚ GaAs injection laser, the detector showed an ac (>20 MHz) external quantum efficiency of >630%, i.e., 9 times more sensitive than apinphotodiode. In view of its high sensitivity, ultrahigh speed, and compatibility with modulation‐doped field‐effect transistors, this detector has promise for a variety of high‐speed optical applications.