A seven-parameter nonlinear current-voltage (I-V) characteristics model for sub-micrometre range GaAs MESFETs has been developed. In this regard, Ahmed et al. model [IEEE Trans. Electron Devices 44 (1997) 360] for submicron GaAs MESFETs has been modified. In this modification, the effects of both drain-to-source voltage, V-DS, and gate-to-source voltage, V-GS, on the output conductance, g(d), have been incorporated. The developed model has been compared with Ahmed et al. model. For this comparison, an improved mean square error (MSE) technique has been employed instead of root mean square (RMS) error technique. An algorithm has also been developed for the optimization of empirical constants of the model. Sub-mum range MESFETs of different aspect ratios have been modeled with greater accuracy by the modified model. The modified model should be a useful tool for the designing of future integrated circuits with sub-micron gate length GaAs MESFETs. (C) 2004 Elsevier Ltd. All rights reserved.