We have investigated homogeneous, photoconductive semiconductors as very fast radiation detectors. We irradiated GaAs, Cr‐doped GaAs, and Fe‐doped InP crystals with 14 MeV neutrons to produce lattice defects that act as fast recombination centers for electrons and holes. Using short‐pulse lasers and 17 MeV linear‐accelerator electrons and bremsstrahlung x rays, we measured the temporal response and sensitivity of these photoconductors as functions of fluence ranging from 1012 to 1016 neutrons/cm2. The carrier lifetime and mobility decrease monotonically as the neutron fluence increases, resulting in faster detector response at the expense of sensitivity. A resolving time of less than 30 ps (full width at half maximum) was measured for the above photoconductors irradiated with ∼1015 neutrons/cm2.