For custom integrated circuit design, the use of a reliable device model parameter set as input for the circuit simulator is very important. The curve fitting algorithm in our parameter extraction program SIMPAR is based on a modified Marquardt fitting routine for nonlinear least squares. Classical parameter extraction programs rely only on the minimization of the relative current deviation. But, especially in analog applications (e.g., gain), a correct value for the slope of the I /sub ds/ - V /sub ds/ curves in the saturation region is at least equally important. The new generalized error criterion implemented in SIMPAR extracts a parameter set which minimizes both the current and slope residuals in every point. In doing this, the accuracy of the output conductance modeling has improved considerably, and in some cases even the current residual itself decreases. Although any analytical current-voltage relationship can be used, the SPICE MOS level 3 was chosen as a test vehicle. To illustrate the effect of this new fitting strategy, the gain of a CMOS invertor was calculated.