Transient drain currents caused by proton microprobe irradiations in partially-depleted (PD) silicon-on-insulator (SOI metal-oxide-semiconductor field-effect transistors (MOSFETs) were analyzed for soft-error issues. Transient currents of the body-tied MOSFETs can be lowered compared to those of the floating body SOI MOSFETs by suppression of the floating body effect. The effectiveness of the body-tie structure was analyzed by device simulation. Increase in the body potential by proton irradiation is suppressed efficiently in the narrow-channel body-tied SOI MOSFETs due to the low body resistance to excess carrier extraction. On the other hand, the body potential of narrow-channel floating body SOI MOSFETs increase to higher levels than those of the wide-channel MOSFETs due to the lower body capacitance, It is indicated that narrow-channel body-tied SOI MOSFETs are suitable for highly reliable devices. Moreover, a more reliable body-tied structure with high impurity concentration in the body regions to reduce the body resistance in the structure is proposed. The collected drain charge was able to be reduced by utilizing this structure. These devices are expected to be applied to highly reliable LSI's used for satellite systems, server and mainstream LSI applications of the multimedia era.