Design Guideline of Multi-Gate MOSFETs With Substrate-Bias Control

A device design guideline of multi-gate MOSFETs with both short-channel effect immunity and a large body factor gamma is developed considering threshold-voltage control by a substrate bias. A sufficiently large gamma, at least 0.04-0.05, is essential for suppressing a subthreshold leakage current and die-to-die characteristic variation by a substrate bias. It is experimentally evaluated that gamma decreases with decreasing channel width. Channel thickness and width design space is explored by means of three-dimensional device simulations, and a thin and wide channel structure is found to be the best design for a threshold-voltage control. Thin buried oxide is advantageous for obtaining a large gamma. When the channel doping concentration is high, channel-structure design window shifts to a thinner and wider region compared to the undoped channel due to the modulation of carrier distribution in the channel. However, due to within-die random variations, highly doped design is not practical, and undoped channel design is only the solution. Required accuracy of structural parameters is also discussed. The thin and wide channel design is also advantageous in the viewpoint of the process variation