  The degradation of ultrathin oxide layers in the presence of a stress voltage
is modeled in terms of two antagonist percolation processes taking place in a
random resistor network. The resistance and leakage current fluctuations are
studied by MonteCarlo simulations for voltages below the breakdown threshold.
An increase of excess noise together with a noticeable non-Gaussian behavior is
found in the pre-breakdown regime in agreement with experimental results.
