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