The n-type ultrathin fully depleted silicon-on-in-sulator(FDSOI)metal-oxide-semiconductor field-effect transistors(MOSFETs),with a Hf0.5Zr0.5O2 high dielectric permittivity(high-k)dielectric as gate insulator,were fabri-cated.The total ionizing dose effects were investigated,and an X-ray radiation dose up to 1500 krad(Si)was applied for both long-and short-channel devices.The short-channel devices(0.025-0.100 μm)exhibited less irradiation sensi-tivity compared with the long-channel devices(0.35-16 μm),leading to a 71％reduction in the irradiation-induced drain current growth and a 26％decrease in the shift of the threshold voltage.It was experimentally demonstrated that the OFF mode is the worst case among the three working conditions(OFF,ON and All0)for short-channel devices.Also,the determined effective electron mobility was enhanced by 38％after X-ray irradiation,attributed to the different compensa-tions for charges triggered by radiation between the high-k dielectric and buried oxide.By extracting the carrier mobility,gate length modulation,and source/drain(S/D)parasitic resistance,the degradation mechanism on X-ray irradiation was revealed.Finally,the split capacitance-volt-age measurements were used to validate the analysis.