Simulations and measurements of SOI MOSFET's are presented with analytical insight to reveal the severe limitation of current-drive enhancement caused by carrier velocity saturation in the deep-submicrometer fully depleted device. For L = 0.1 mum, the enhancement, which tends to result from the suppressed body charge and electric field in the thin-film device, is virtually negated by the velocity saturation driven by the high longitudinal electric field.