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The subthreshold softening characteristic of MOSFET's due to the narrow-gate effect has been investigated based on the two-dimensional (2-D) numerical solution of the Poisson equation and device physics. Numerical results taken on stepped-oxide MOSFET's with different gate widths show that a narrower gate width device tends to give higher cut-off voltage. Two parameters account for the softening of the subthreshold characteristics: the subthreshold slope of the drain conductance-gate voltage characteristic and the effective channel width. Both parameters can be extracted easily from the theoretical 2-D computed or experimental drain conductance-gate voltage characteristics. A two-parameter analytical approximation formula for narrow-gate MOSFETs operating in the subthreshold range is thus proposed and tested against exact 2-D numerical results, showing good accuracy. This model is the first one ever reported.