The total ionizing dose response of bulk nFinFETs with multiple gate lengths and multiple fins is investigated for on-state bias condition. Experiments and Technology Computer Aided Design simulations were performed to analyze the effect of the trapped charges in the gate oxide and shallow trench isolation (STI) oxide on the threshold voltage and transconductance of the devices. The increases in the threshold voltage and transconductance are observed after X-ray irradiation. The positive shift of the threshold voltage is caused by the net negative charges trapped in the gate oxide. The simulation results show that the trapped holes in the STI oxide reduce the electric field and increase the electron mobility in channel near the fin bottom, which is the major contribution to the increased transconductance. An interesting phenomenon was also observed that the threshold voltage continues to increase during the annealing process, whereas the transconductance decreases. These results suggest that there may also be a small amount of trapped holes in gate oxide during irradiation, and those trapped holes are compensated by electrons transporting from the silicon during the anneal, leading to further positive shift of the threshold voltage. Moreover, the decrease in transconductance is mainly introduced by the neutralization of the trapped holes at STI/silicon interface.