Switching limits in nano-electronic devices

Present day CMOS transistors operate by thermionic emission of electrons over a gate tunable barrier. The fundamental switching energy for each such switching event can be derived from equilibrium thermodynamic considerations. While clever ways can sometimes mitigate the power budget, more often than not, this involves trade-offs with short channel effects (variability), on-off ratio (reliability) and mobility (switching speed). We discuss switching paradigms that venture beyond the near-equilibrium operation of transistors involving the absence or presence of charges as the digital switching bits. To this end, a few case studies are presented. Dipolar switching is invoked as an example to show how gating non-electronic degrees of freedom can reduce the subthreshold swing below the textbook limit by acting as an added cut-off filter on the current. We discuss how new state variables may be engineered into a CMOS platform to enable such non-electronic switching. Another, completely different direction involves non-equilibrium switching, such as a ratchet that allows us to move charges unidirectionally without a drain bias, by using instead an always present AC clock signal adiabatically coupled with the gate.