We investigated the effect of 0.65 MeV electron irradiation on the transport parameters of GaAs FATFETs with an S-doped active channel of thickness 0.2 μm to 5 μm. We measured mainly the drift mobility and carrier concentration profile at severall temperatures from 170 K to 300 K before and after irradiation. A decrease of the mobility is observed of the mobility profiles as a function of temperature by introduction of different scattering mechanisms gives the scattering parameters and particularly the scattering rate by ionized impurities and space charge before and after irradiation. DLTS and DLOES measurements indicated three electron traps and four hole traps. The results show that the two main electron traps E2 and E3 are affected by an intermediate donor type electronic stateT x The nature ofTx is not known, except that it behaves like a donor.