Negative Capacitance in a Ferroelectric Material and Its Potential Use for Beyond Boltzmann Limit Transistors

Negative capacitance in a ferroelectric material is characterized by a state of the polarization where the polarization charge and the net electric field oppose each other. In a prototypical ferroelectric this situation occurs where the polarization is significantly suppressed. Conventionally, this is the region where the material experiences a polarization catastrophe; in other words left all by itself in this state, the material will spontaneously polarize. As a result, in an isolated ferroelectric material, this state can only be accessed in a time-dependent manner. In the first part of my presentation, I shall discuss how one could go beyond this and stabilize the ferroelectric material at a state of negative capacitance in equilibrium, i.e., at the steady state, with the help of another dielectric material placed in a series connection. Interestingly, this series connection emulates what happens at the gate of a transistor where the gate insulator and the semiconductor capacitors are connected in series. Therefore if the gate oxide is replaced by an appropriate ferroelectric, this series combination 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. The 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 the recent years, many groups around the world, both in academy and in the industry, have demonstrated the fundamental effect and the Negative Capacitance Transistors. In the second part of this presentation, I shall discuss the principle of operation and potential routes for optimization of the Negative Capacitance Transistors.

Speaker: Sayeef Salahuddin, UC Berkeley