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

We investigated the optimum gate workfunction (phi_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 Vth 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., phi_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) phi_m (phi_mn=4.17 eV for NMOS, phi_mp=5.25 eV for PMOS) were compared with that with high (mid-gap) phi_m(phi_mn=phi_mp =4.71 eV) DGs. It was found that CMOS 4T-XMOSFET with low (band-edge) phi_m DGs showed a higher I_on and a shorter inverter delay than that with high (midgap) phi_m DGs