Thin gate oxides, irradiated under conditions similar to those experienced in X-ray lithography, exhibit a large increase in the leakage current. The current-voltage characteristics of the radiation-induced leakage current (RILC) and the electrical stress-induced leakage current (SILC) are very similar. Both currents comprise a de component due to trap-assisted tunneling, and a transient component attributed to the tunnel charging/discharging of carriers. Current-voltage and current-time data suggest essentially the same degradation mechanisms for both the RILC and SILC in ultra-thin oxides. A quadratic relationship between the X-ray dose and the equivalent charge fluence that induces the same amount of degradation is established. (C) 2000 Elsevier Science Ltd. All rights reserved.