We report fabrication and electro‐optic measurement of In0.52Al0.48As/In0.53Ga0.47As modulation‐ doped field‐effect transistors. The devices are monolithically integrated with coplanar stripline fixtures incorporating photoconductive switches. The switching time for a 0.35 &mgr;m T‐gate device is 4.2 ps and the delay is 3.2 ps. This is the fastest directly measured switching in a three‐terminal device reported to date. ©1995 American Institute of Physics.