Ionizing radiation effects on the electrical properties of HfO2, Gd2O3, and HfO2/SiO2 based metal-oxide-semiconductor (MOS) capacitors have been studied. High-k dielectrics grown by atomic layer deposition and high-pressure Sputtering were exposed to photon radiation (18 MeV photons). Capacitance-voltage curves, deep-level transient spectroscopy, conductance and flat-band voltage transients, and current-voltage techniques were used to characterize the samples. An increment in bulk dielectric trap densities has been observed when the samples were exposed to the ionizing radiation. These traps give rise to a flat-band voltage displacement, the extent of which depends on the gate dielectric used. High-k/silicon interface quality becomes worse after irradiation. An increment in the gate leakage current was also observed when irradiating the samples. Disorder-induced gap state density inside the insulator increases in the case of Gd2O3 MOS based samples, which seems to be the most affected by ionizing radiation. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3021040]