  Sub-threshold swing (S) defines the sharpness of ON-OFF switching of a Field
Effect Transistor (FET) with S=0 corresponding to abrupt switching
characteristics. While thermodynamics dictates S to be greater than or equal to
60mV/dec for classical FETs, "Landau switches" use inherently unstable gate
insulators to achieve abrupt switching. Unfortunately, S=0 switching is always
achieved at the expense of an intrinsic hysteresis, making these switches
unsuitable for low-power applications. The fundamental question therefore
remains: Under what condition, hysteresis-free abrupt switching can be achieved
in a Landau switch? In this paper, we first provide an intuitive classification
of all charge based switches in terms of their energy landscapes and identify
two-well energy landscape as the characteristic feature of Landau switches. We
then use nanoelectromechanical field effect transistor (NEMFET) as an
illustrative example of a Landau switch and conclude that a flat energy
landscape is essential for hysteresis-free abrupt switching. In contrast, a
hysteresis-free smooth sub-60mV/dec switching is obtained by stabilizing the
unstable gate insulator in its unstable regime. Our conclusions have broad
implications and may considerably simplify the design of next charge based
logic switch.
