We found that tensile stress actually increases both the on current and the subthreshold off current for n-channel metal-oxide-semiconductor transistors because of an increase in mobility. Our theory is that stress engineering works because the increase in the subthreshold off current can be easily offset by a slight increase in the saturation threshold voltage, while the increase in the on current can be offset only by a much larger increase in the saturation threshold voltage. In this paper, experimental variation in the saturation threshold voltage is achieved by the statistical variation in gate length and short-channel effect. Thus, the overall effect is an improvement.