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This paper answers the frequently asked question, ldquoHow accurate are the approximate long-wide-channel thick-base MOS transistor baseline models that have been used to develop the compact models for computer-aided circuit designs?rdquo Three commonly used surface-potential-based(Us = qPsis / kT ) approximations of the ionized impurity bulk charge are evaluated as follows: Q B alpha (i) (Us)1/2; (ii) (Us -1)1/2; (iii)[Us -1 + exp(-U s )]1/2. The double-integral baseline model for comparison includes the self-consistent remote charge-neutrality boundary condition, minority carriers, and space-constant impurity-concentration and oxide thickness. Percentage deviations of the approximations from the baseline model are computed for the dc drain current. Approximation (i) show a significant deviation, which is ~ 16% at threshold voltage, diverging rapidly in the subthreshold range toward flatband. Approximations (ii) and (iii) show a few percent (1% to 2%) deviations in both inversion and subthreshold ranges but diverge widely below subthreshold and in accummulation. A new analytical model is tested and shows better than 10% accuracy below subthreshold and in accummulation.