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A compact analytical model of variable-threshold-voltage CMOS (VTCMOS) is proposed to study the active on current, linking it with the standby off-current characteristics. Comparisons of modeled results to both numerical simulations and experimental data are made with an excellent agreement. It is clearly demonstrated using the model that speed degradation due to low supply voltage can be compensated by the VTCMOS scheme, even with smaller power. Influence of the short channel effect (SCE) on the performance of VTCMOS is investigated in terms of a new parameter, dS/d/spl gamma/, both qualitatively and quantitatively. It is found that the SCE degrades the VTCMOS performance. Issues on the optimum conditions of VTCMOS and the performance of series-connected VTCMOS circuits are also discussed.