A criterion is developed to select the ratio of silicon film thickness to gate oxide thickness between two limits to set the threshold voltage independent of channel length. The concept of short channel immunity factor (v) and critical length (L-critical) is evolved, which serves as the basic tool for device optimization for enhanced short channel immunity. The model shows new viewpoints for realizing future ULS1 circuits with VSG MOSFETs. (C) 2002 Elsevier Science Ltd. All rights reserved.