The effects of lambda -ray irradiation on GaAs MESFETs were studied. The lambda -ray irradiation was generated from a Co-60 source. DC characteristics of the MESFETs were used to monitor the effects of radiation, which included source-drain I-V characteristics, gate leakage current, saturation drain current, sub-threshold gate leakage current, and sheet resistance. These changes of device characteristics provided us a useful method to determine the radiation damage coefficient. (C) 2001 Elsevier Science Ltd. All rights reserved.