  We present a numerical model that simulates the current-voltage (I-V)
characteristics of materials that exhibit percolation conduction. The model
consists of a two dimensional grid of exponentially different resistors in the
presence of an external electric field. We obtained exponentially non-ohmic I-V
characteristics validating earlier analytical predictions and consistent with
multiple experimental observations of the Poole-Frenkel laws in non-crystalline
materials. The exponents are linear in voltage for samples smaller than the
correlation length of percolation cluster L, and square root in voltage for
samples larger than L.
