(Invited) Negative Capacitance Transistors
Authors: Salahuddin Sayeef 
Published: February 2020

Negative capacitance is a new state within ferroelectric materials that exploits a region of thermodynamic space that is otherwise forbidden. In the conventional thinking, a material that goes through a ferroelectric phase transition, develops an energy landscape such that two degenerate states are separated by an energy barrier. This gives the energy landscape the shape of a ‘W’. By itself, the ferroelectric sits in one of the degenerate states where its energy is minimized. Given that these states correspond to the minima of the energy function, G, the curvature, i.e., the permittivity is positive. Note also, that because of the ‘W’ landscape, these energy minima occur at non-zero polarization values, meaning a ferroelectric material has non-zero charge in its equilibrium state. However, this very well-known and well understood situation changes when a dielectric is brought in series connection with the ferroelectric. The parabolic energy landscape of the dielectric, when added to the ‘W’ landscape of the ferroelectric, can lead to a situation where the overall energy landscape of the composite system looks parabolic. This means that the energy minimum of the composite system now occurs at P~0, where, locally, the ferroelectric has an energy minima. Thus, the competing forces of electrostatics can lead to a situation, remarkably, where a ferroelectric can be stabilized in equilibrium at a state where its own energy landscape has a maximum, i.e., a negative curvature , and therefore a negative permittivity.
The negative permittivity state thus stabilized has important implications for transistors. A transistor looking from the gate essentially acts as a series combination of two capacitors: the gate oxide capacitor and the channel capacitor. When the gate oxide is replaced by an appropriate ferroelectric, this series combination looks exactly like the ferroelectric dielectric combination discussed in the previous paragraph and can stabilize the ferroelectric material at a state of negative capacitance. At this state, the total capacitance of the series combination is enhanced, leading to more charge at the channel at the same voltage. This boost of charge, in turn, leads to larger current at the same voltage. In fact, this boost makes it possible to reduce supply voltage of transistors below the traditional Boltzmann limit --- often termed as the Boltzmann tyranny. In this talk, we shall briefly discuss the thermodynamic underpinnings that make it possible to go beyond the Boltzmann limit.
The recent advent of HfO2 based ferroelectric materials has enabled integration of thin ferroelectric gate dielectrics with high performance transistors. Exploiting this capability, many groups around the world, both in academia and in the industry, have demonstrated the fundamental effect and the Negative Capacitance Transistors. We shall describe our current understanding of the recent experimental work and also discuss some of the salient features by which negative capacitance transistors are very different from conventional transistors. We will emphasize the fact that negative capacitance effect should not be viewed either as a new high-K material problem or a booster technology such as strain. Rather, it is necessary to realize that negative capacitance transistor is, in fact, a completely new device, that operates on a new physical principle, and to extract the most benefit from this technology, it is important to go back to device design from the scratch, rather simply following the classical scaling rules. I shall discuss some of our recent results that promise to significantly improve the energy efficiency of the latest technology node and also provide a viable pathway to extend scaling beyond what is currently believed to be the end of the roadmap.