Engineered crystals can help computers run on less power

Engineered crystals can help computers run on less power

Engineers at the University of California, Berkeley, have created engineered crystal structures that exhibit an unusual physical phenomenon known as negative capacitance. By incorporating this material into advanced silicon transistors, computers can become more energy efficient. Credit: Ella Maru Studio, University of California – Berkeley

Computers may be getting smaller and more powerful, but they require a lot of energy to function. The total amount of energy the US spends on computers has increased dramatically over the past decade and is rapidly approaching that of other major sectors, such as transportation.

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

“We’ve been able to demonstrate that our gate oxide technology is better than commercially available transistors: what the trillion dollar semiconductor industry can do today, we can essentially beat it,” said senior author Sayeef Salahuddin, the TSMC Distinguished Professor of Electrical Engineering and Computer Science. at UC Berkeley.

This increase in efficiency is made possible by an effect called negative capacitance, which helps reduce the amount of stress 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 new study shows how negative capacitance can be achieved in an engineered crystal composed of a layered stack of hafnium oxide and zirconia, which is easily compatible with advanced silicon transistors. By incorporating the material into model transistors, the study shows how the negative capacitance effect can significantly lower the amount of voltage needed to drive transistors, and as a result, the amount of energy consumed by a computer.

“Over the past 10 years, the energy used for computers has grown exponentially, already accounting for single-digit percentages of the world’s energy production, growing only linearly, with no end in sight,” Salahuddin said. “Usually, when we use our computers and our cell phones, we don’t think about how much energy we consume. But it’s a huge amount, and it’s only going to increase. Our goal is to reduce the energy needs of this basic building block of computing, because that lowers the energy requirement for the entire system.”

Bringing Negative Capacity to Real Technology

State-of-the-art laptops and smartphones contain tens of billions of tiny silicon transistors, 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 electrical charge, which then switches the transistor.

Negative capacitance can increase the performance of the gate oxide by decreasing the amount of voltage required to achieve a given electrical charge. But the effect cannot be achieved in every material. Creating negative capacitance requires careful manipulation of a material property called ferroelectricity, which occurs when a material exhibits a spontaneous electric field. Previously, the effect was only achieved in ferroelectric materials, called perovskites, whose crystal structure is not compatible with silicon.

In the study, the team showed that negative capacitance can also be achieved by combining hafnium oxide and zirconia in an artificial crystal structure called a superlattice, leading to simultaneous ferroelectricity and antiferroelectricity.

“We found that this combination gives us an even better negative capacity effect, showing that this negative capacity phenomenon is much broader than originally thought,” said co-first author Suraj Cheema, a postdoctoral researcher at UC Berkeley. “Negative capacitance doesn’t just appear in the conventional image of a ferroelectricity with a dielectric, which has been studied over the past decade. You can make the effect even stronger by designing these crystal structures to exploit antiferroelectricity in conjunction with ferrous -electricity.”

The researchers found that a superlattice structure consisting of three atomic layers of zirconia sandwiched between two single atomic layers of hafnium oxide, totaling less than two nanometers thick, produced the best negative capacitance effect. Since most advanced silicon transistors already use a 2-nanometer gate oxide composed of hafnium oxide on top of silicon dioxide, and because zirconia is also used in silicon technologies, these superlattice structures can be easily integrated into advanced transistors.

To test how well the superlattice structure would perform as a gate oxide, the team made short-channel transistors and tested their capabilities. These transistors would require approximately 30% less voltage, while maintaining semiconductor industry benchmarks and without loss of reliability, compared to existing transistors.

“One of the problems we often see with this kind of research is that we can show different phenomena in materials, but those materials are not compatible with advanced computing materials, so we can’t bring the advantage to real technology,” Salahuddin says. said. “This work transforms negative capacitance from an academic subject into something that can actually be used in an advanced transistor.”

‘Negative capacitance’ could yield more efficient transistors

More information:
Suraj S. Cheema et al, Ultrathin Ferroic HfO2–ZrO2 super grid gate stack for advanced transistors, Nature (2022). DOI: 10.1038/s41586-022-04425-6

Provided by University of California – Berkeley

Quote: Engineered crystals can help computers run on less power (2022, April 8), retrieved April 8, 2022 from

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