Negative capacitance for low power transistors to efficient super capacitors

Rajesh Uppal October 20, 2022 Electronics & EW, Energy & Propulsion Comments Off on Negative capacitance for low power transistors to efficient super capacitors 245 Views

As we increasingly rely on computers for daily tasks, the energy needed to run these systems is becoming substantial. The total amount of energy the U.S. dedicates to computing has risen dramatically over the last decade and is quickly approaching that of other major sectors, like transportation. Studies show that the total electricity consumption by the world’s data centers is equivalent to 10 percent of all electricity used in the United States.

State-of-the-art laptops and smartphones contain tens of billions of tiny silicon transistors, and each of which must be controlled by applying a voltage. The gate oxide is a thin layer of material that converts the applied voltage into an electric charge, which then switches the transistor.

“This is where a new physical phenomenon such as negative capacitance could provide a completely new set of tools to improve the energy efficiency of our computers,” said Salahuddin. This boost in efficiency is made possible by an effect called negative capacitance, which helps reduce the amount of voltage that is needed to store charge in a material. Salahuddin theoretically predicted the existence of negative capacitance in 2008 and first demonstrated the effect in a ferroelectric crystal in 2011.

The capacitor is a key element of electronic devices that can store an electrical charge. Their capacitance, or ability to store electrical energy, is determined by how much the capacitor’s charge changes when it is connected to a voltage source, like a battery. Negative capacitance occurs when a change in charge causes the net voltage across material to change in the opposite direction; so that a decrease in voltage leads to an increase in charge.

Negative capacitance can boost the performance of the gate oxide by reducing the amount of voltage required to achieve a given electrical charge. But the effect can’t be achieved in just any material. Creating negative capacitance requires careful manipulation of a material property called ferroelectricity, which occurs when a material exhibits a spontaneous electrical field. Previously, the effect has only been achieved in ferroelectric materials called perovskites, whose crystal structure is not compatible with silicon.

Owing to the energy barrier that forms during phase transition and separates the two degenerate
polarization states, a ferroelectric material could show negative differential capacitance while in
non-equilibrium1-5. The state of negative capacitance is unstable, but just as a series resistance
can stabilize the negative differential resistance of an Esaki diode, it is also possible to stabilize a
ferroelectric in the negative differential capacitance state by placing a series dielectric capacitor

In this configuration, the ferroelectric acts as a ‘transformer’ that boosts up the input voltage.
The resulting amplification could lower the voltage needed to operate a transistor below the limit
otherwise imposed by the Boltzmann distribution of electrons. Due to this reason, the possibility of a transistor that exploits negative differential capacitance has been widely studied in the recent years

However, despite the fact that negative differential capacitance has been predicted by the standard Landau model going back to the early days of ferroelectricity, a direct measurement of this effect has never been reported, severely limiting the understanding and potential application of this effect for electronics

Researchers capture an image of negative capacitance in action reported in Jan 2019

For the first time ever, an international team of researchers imaged the microscopic state of negative capacitance. This novel result provides researchers with fundamental, atomistic insight into the physics of negative capacitance, which could have far-reaching consequences for energy-efficient electronics. The team, led by scientists at the University of California, Berkeley, describes their results in a paper published in the January 14 issue of Nature.

“The upshot is that the opposite relation between charge and voltage could locally enhance the voltage across the common dielectric material,” said Sayeef Salahuddin, professor of electrical engineering and computer sciences, who led the overall effort. “The voltage ‘amplification’ gained could be used to reduce the supply voltage requirement in a transistor, thus making computers and other electronic devices more energy-efficient.”

The work in this paper directly captured negative capacitance in an atomically perfect superlattice of ferroelectric-dielectric heterostructure, synthesized by the group of Ramamoorthy Ramesh, professor of physics and of material science and engineering. Using state-of-the-art imaging techniques, the researchers mapped out the polarization as well as the electric field with atomic resolution. This allowed them to estimate the local energy density, which clearly showed regions where the curvature of the energy density is negative, indicating stabilization of the steady-state negative capacitance.

Breakthrough in Engineered Crystals reported in April 2022

In a study published online in the journal Nature on April 6, 2022, University of California, Berkeley, engineers describe a major breakthrough in the design of a component of transistors — the tiny electrical switches that form the building blocks of computers — that could significantly reduce their energy consumption without sacrificing speed, size or performance. The component, called the gate oxide, plays a key role in switching the transistor on and off.

The new study shows how negative capacitance can be achieved in an engineered crystal composed of a layered stack of hafnium oxide and zirconium oxide, which is readily compatible with advanced silicon transistors. By incorporating the material into model transistors, the study demonstrates how the negative capacitance effect can significantly lower the amount of voltage required to control transistors, and as a result, the amount of energy consumed by a computer.

“We have been able to show that our gate-oxide technology is better than commercially available transistors: What the trillion-dollar semiconductor industry can do today — we can essentially beat them,” said study senior author Sayeef Salahuddin, the TSMC Distinguished professor of Electrical Engineering and Computer Sciences at UC Berkeley.

“We believe that the microscopic insight of negative capacitance obtained in this work will allow researchers to design highly energy-efficient transistors that can exploit the negative capacitance in the most optimum manner,” said Salahuddin. “The implication of our work, however, goes well beyond transistors. Negative capacitance could find use in batteries, super capacitors and non-conventional electromagnetic applications.”

Supercapacitors

The increasing demand for efficient storage of electrical energy is one of the main challenges in the transformation toward a carbon-neutral society. The storage of electrical energy has only been possible since the invention of the capacitor in 1745.

When a voltage is applied to a capacitor, energy is stored in the electric field in the dielectric material which separates the two conducting electrodes. The major advantages of the energy storage in capacitors are a high energy storage efficiency, temperature, and cycling stability as well as high power densities. On the other hand, regular dielectric capacitors cannot compete with the orders of magnitude higher energy storage densities of, e.g., batteries or fuel cells.

However, so-called supercapacitors, which combine the high power density of capacitors with much higher energy densities, are ideal for applications where a large amount of energy has to be stored and released in a relatively short time.

Currently, high energy density electrochemical supercapacitors, which are mostly based on the double-layer capacitance and pseudocapacitance effects, are used, e.g., to stabilize the power grid, recover braking energy in electric vehicles or provide a backup power supply for critical electrical systems. Recently, there has been increasing interest in purely electrostatic solid state supercapacitors based on highly polarizable materials, e.g., ferroelectrics and antiferroelectrics

Negative capacitance, which is present in ferroelectric materials, can be used to improve the energy storage of capacitors beyond fundamental limits. While negative capacitance was previously mainly considered for low power electronics, it is shown that ferroelectric/dielectric capacitors using negative capacitance are promising for energy storage applications.

Michael Hoffmann and others have proposed that by combining a Negative Capacitance (NC) layer (e.g., a ferroelectric) with a regular positive capacitance layer (e.g., a dielectric), it is possible to build an NC supercapacitor, which can store large amounts of electric energy with high efficiency. For an optimal NC supercapacitor design, the capacitances of both layers should be closely matched (aC ≈ –1), and the NC layer should be in a positive capacitance state when no voltage is applied (ΔQ-shift). Furthermore, the capacitor should be operated close to Vmax, to obtain the highest benefit due to the NC effect.

This new concept has three main advantages: (1) By engineering a nonlinear Q–V curve with positive curvature, the stored energy can be increased compared to a regular capacitor even at identical voltage. (2) The breakdown voltage and leakage currents of the capacitor are improved by the addition of the NC layer, thus enabling much higher energy storage densities. 3) Since there is no hysteresis, the theoretical efficiency is 100%.