Abstract: Because of the unique advantages of silicon-on-insulator (SOI) technology in both radiation and high temperature environments, it is meaningful to investigate the characteristics of SOI device with different top silicon film thicknesses (tSi), which will be of great value to further enhance the performance of high temperature radiationhardened SOI complementary metal oxide semiconductor (CMOS) device. Firstly, a model of Nchannel metaloxidesemiconductor fieldeffect transistor (NMOSFET) is constructed and analyzed by processlevel simulation. Based on the simulation data, actual devices with different tSi are manufactured by using 0.15 μm radiationhardened SOI CMOS process that employs design and materials optimized for high temperature applications. The results indicate that both thin and thick tSi NMOSFET show approximate hardness performance under 150 krad(Si) total dose radiation, while the former has smaller leakage current at 225 ℃, making the NMOSFET with thinner tSi a better candidate for high-temperature electronics.