A function-fit model for the hard breakdown current-voltage characteristics of ultra-thin oxides in metal-oxide-semiconductor structures based on the smoothing function concept is presented. The model is intended to capture the diode-like and resistance-like behaviours observed at low and high applied biases, respectively, by means of a simple, continuous and derivable function. These features make the proposed expression suited for circuit simulation environments. The effect of temperature on the model parameters is also analysed. © 2004 Elsevier Ltd. All rights reserved.