Cross sections that cause single event upsets by heavy ions are sensitive to doping concentration in the source and drain regions, and the structure of the raised source and drain regions especially in FDSOI. Due to the parasitic bipolar effect (PBE), radiation-hardened flip flops with stacked transistors in FDSOI tend to have soft errors, which is consistent with measurement results by heavy-ion irradiation. Device-simulation results in this study show that the cross section is proportional to the silicon thickness of the raised layer and inversely proportional to the doping concentration in the drain. Increasing the doping concentration in the source and drain region enhance the Auger recombination of carriers there and suppresses the parasitic bipolar effect. PBE is also suppressed by decreasing the silicon thickness of the raised layer. C_gg-V_gs and I_ds-V_gs characteristics change smaller than soft error tolerance change. Soft error tolerance can be effectively optimized by using these two determinants with only a small impact on transistor characteristics.