In this work, the influence of the underlap region on the electrical behavior of a SOI-nFinFET transistor has been studied with the purpose of radiation sensing. The analysis was performed by evaluating the impact of variations in the underlap region on the on-state current and by studying its sensitivity. The impact of the underlap region on the drain current and, consequently, on the devices’ sensitivity was explained by the analysis of series resistance, the fringing field and electron density. Considering the main impact of radiation in these devices, the study of sensitivity was also performed taking into consideration the variation of oxide trapped charges density. When applying the transistor to a harsh environment, the Underlapped FinFET showed to be a quite respectable radiation sensor, since the results performed with very good sensitivities when using long and narrow spacer oxide with low permittivity oxide. With thicker spacer oxide in the underlap region, the charge concentration makes the spreading field high enough to overcome the series resistance effect, which results in a less sensible device. Once presented the on-state current variation of the Underlapped FinFET, the study turns radiation-sensing purpose applicable using the excellent characteristics of this device, which is shown in detail throughout this work.