Comparison of short-channel effect and offstate leakage in symmetric vs. asymmetric double gate MOSFETs

Double gate MOSFETs (DGFETs) in the sub-0.1 μm regime are unlikely to use channel doping to set the threshold voltage, V_t . Therefore, work function engineering is required to properly set V_t. Asymmetric DGFETs use one n⁺ and one p⁺ -poly gate to achieve a reasonable threshold voltage (Tanaka et al., 1994), whereas symmetric DGFETs use the same near-midgap material for both gates. This results in significantly different energy-band diagrams. The on-state drive currents in these two structures have been shown to be comparable to each other if off-state leakage currents are balanced; in the on-state, the asymmetric DGFET matches the inherent two-channel advantage of the symmetric DGFET with a single dynamic-threshold channel (Kim and Fossum, 1999). However, their short-channel effects should differ, since the two structures have different leakage paths. In this paper, the MEDICI 2D device simulator is used to study off-state leakage current in NMOS DGFETs as a function of channel length. The short-channel leakage behavior is explained by analyzing the change in the channel potential barrier