A new model for the degradation and breakdown of thin SiO2 films is presented. The oxide degradation is due to the fact that part of the kinetic energy of the injected electrons is converted into the generation of some sort of defects that behave as electron traps. From the experimental evolution of the stress conditions, the breakdown times are predicted as a function of the applied current in constant-current stress experiments. From the constant-current stress breakdown characteristics, the experimental I-V characteristic and the time-to-breakdown of a MOS structure submitted to a voltage-ramp stress are also well predicted. Due to the generated negative charge density in the SiO2 bulk, the energy barrier becomes parabolic. The I-V tunnelling characteristic through such a barrier has been studied in detail. © 1989.