Optimum Gate Workfunction for $V_{rm th}$-Controllable Four-Terminal-Driven Double-Gate MOSFETs (4T-XMOSFETs)—Band-Edge Workfunction Versus Midgap Workfunction

摘要

We investigated the optimum gate workfunction (phim) for four-terminal-driven double-gate MOSFETs (4T-XMOSFETs) using device simulation. Threshold voltage (Vth) controllability for the 4T-XMOSFETs was investigated in relation to the initial Vth in the double-gate mode (VthDG) based on comprehensible modeling of the devices. It was shown that I--V characteristics for the 4T-XMOSFETs are categorized into two states while VthDG forms a boundary. If Vg2 is less than VthDG, i.e., VthSG is larger than VthDG, subthreshold-slope (S) keeps low value. If Vg2 is larger than VthDG, i.e., VthSG is less than VthDG, S significantly deteriorates. As a result, setting VthDG, i.e., phim at a low value and thus using VthSG larger than VthDG, is preferable for improving the 4T-XMOSFET performance. To confirm it, both static and dynamic characteristics for CMOS with low (band-edge) phim (phimn=4.17 eV for NMOS, phimp=5.25 eV for PMOS) were compared with that with high (mid-gap) phim(phimn=phimp =4.71 eV) DGs. It was found that CMOS 4T-XMOSFET with low (band-edge) phim DGs showed a higher Ion and a shorter inverter delay than that with high (midgap) phim DGs