A nonlinear variance equation has been derived and applied to the threshold voltage of a 0.1 µm SOI MOS device. The equation enabled an analysis of the effects of process-parameter fluctuations to be made. The analysis showed that the effect of the nonlinear terms (15.48%) is more important than the effect of the mixed term (0.02%), and almost as important as the contribution of the second most dominant input-process parameter (21.98%). This illustrates the importance of the proposed nonlinear equation.