Damage in thin (10 nm) oxide layers of metal-oxide-semiconductor varactor cells caused by exposures with ions, electrons, and x rays has been electrically characterized by constant-current stress time-dependent dielectric breakdown and capacitance-voltage measurements. The time-dependent dielectric breakdown investigations show measurable damage if the ions (H+; 74 keV) are allowed to penetrate the resist (300 nm PMMA) and the poly-Si layer (250 nm) to reach the oxide. This is not the case if the ions (H-2(+); 74 keV) are stopped inside the poly-Si layer by use of a thicker (450 nm) resist. Also for electrons and x rays the exposure with lithographic doses did not change the time-dependent dielectric breakdown curves. The capacitance-voltage measurements demonstrate that electrons and x rays which do penetrate the gate oxide cause a flatband voltage shift of nearly 400 mV. This shift could not be observed if the H-2(+) ions were stopped in the poly-Si buffer layer. Ions seem to create no secondary effects like x-ray generation which reach beyond the penetration depth. The measurements were performed in the unannealed state to detect all possible damage effects including those which can be removed by annealing.