Total ionizing dose effects on NMOS devices have been studied as a function of the irradiation dose. The gate oxide thickness of NMOS devices for experiments was 85 A, the gate width was 10 mu m and the gate length was 5 fun. These devices were irradiated Using 1-MeV protons and 1-MeV electrons with total irradiation doses of 1, 5 10, 50 100 and 500 kGy, respectively. The changes in the electrical characteristics were measured using HP4155A semiconductor parameter analyzer. The changes after 1-MeV electrons irradiation were large compared to those after 1-MeV protons irradiation. The radiation induced electron-hole pair generation in MOS devices was also numerically simulated Using a GEANT4 code. (c) 2005 Elsevier B.V. All rights reserved.