  We present a physical model for electronic switching in cantilever based
nano-electro-mechanical field effect transistors, focusing on the steepness of
its switching curve. We find that the subthreshold swing of the voltage
transfer characteristic is governed by two separate considerations - the
ability of the charges to correlate together through dipolar interactions and
amplify the active torque, versus the active pull-in forces that drive an
abrupt phase transition and close the air gap between the tip of the cantilever
and the drain. For small sized relays, dipolar and short-range Van Der Waals
'sticking' forces dominate, while for longer cantilevers the capacitive energy
acquires a major role. The individual pull-in and pull-out phases demonstrate a
remarkably low subthreshold swing driven by the capacitive forces, sharpened
further by dipolar correlation. The sharp switching, however, comes at the
expense of strong hysteresis as the metastable and stable states interchange
along the forward and reverse phases of the voltage scan.
