Abstract: The primary performance degradation of SOI device in the total dose irradiation is the back-channel leakage current caused by gate threshold voltage shift.The trapped charge density in the buried oxide of SOI NMOS resulting from irradiating has been analyzed using a 2D finite element simulation.A model for radiation induced back channel threshold voltage shift in SOI NMOS transistors is presented,which enables the extraction of parameters for the radiation-induced parasitic MOSFET created at the back of the buried oxide.The modeling result agrees well with measured post-radiation characteristics devices fabricated with SIMOX SOI.