Flash x‐ray measurements have been made on GaAs metal semiconductor field effect transistor structures fabricated in both Cr‐doped and undoped semi‐insulating substrate material in order to characterize the levels responsible for long term transients in these devices. Prominent electron and hole traps with activation energies in the 0.76–0.90‐eV range have been identified. The main electron trap, which is present in both Cr‐doped and undoped substrates, appears to be primarily responsible for the observed transient response. However, traps related to Cr‐doping also contribute to the transient behavior.