Optimum gate workfunction for v _th-Controllable four-terminal-driven double-gate MOSFETs (4T-XMOSFETs) - Band-edge workfunction versus midgap workfunction

We investigated the optimum gate workfunction (φ _m) for four-terminal-driven double-gate MOSFETs (4T-XMOSFETs) using device simulation. Threshold voltage (V _th) controllability for the 4T-XMOSFETs was investigated in relation to the initial V _th in the double-gate mode (V _thDG) based on comprehensible modeling of the devices. It was shown that I-V characteristics for the 4T-XMOSFETs are categorized into two states while V _thDG forms a boundary. If V _g2 is less than V _thDG, i.e., V _thSG is larger than V _thDG, subthreshold-slope (S) keeps low value. If V _g2 is larger than V _thDG, i.e., V _thSG is less than V _thDG, S significantly deteriorates. As a result, setting V _thDG, i.e., φ _m at a low value and thus using V _thSG larger than V _thDG, is preferable for improving the 4T-XMOSFET performance. To confirm it, both static and dynamic characteristics for CMOS with low (band-edge) φ _m (φ _mn = 4.17 eV for NMOS, φ _mp = 5.25 eV for PMOS) were compared with that with high (mid-gap) φ _m(φ _mn= φ _mp = 4.71 eV) DGs. It was found that CMOS 4T-XMOSFET with low (band-edge) φ _m DGs showed a higher I _on and a shorter inverter delay than that with high (midgap) φ _m DGs.