The radiation response of MOS transistors in a 22 nm Fully Depleted Silicon -On -Insulator (FDSOI) technology exposed to ultra -high total ionizing dose (TID) was investigated. Custom structures including n- and p -channel devices with different sizes and threshold voltage flavours were irradiated with X-rays up to a TID of 100 Mrad(SiO2) with different back -gate bias configurations, from -8 V to 2 V. The investigation revealed that the performance is significantly affected by TID, with the radiation response being dominated by the charge trapped in the buried oxide.