Although computers are becoming smaller and more powerful they still require significant amounts of energy to function. Over the past decade, the U.S. has spent a lot of energy on computing. It is now approaching the level of transportation and other major sectors. Researchers from the University of California, Berkeley published a breakthrough in the design of a transistor component. These tiny electrical switches are the building blocks of computers. The study was published online in the Nature journal on April 6, 2022. It could dramatically reduce the energy consumed without compromising speed, size, or performance. The transistor’s gate oxide is a crucial component.
“We have been able show that our gateoxide technology outperforms commercially available transistors,” said Sayef Salahuddin (UC Berkeley TSMC Distinguished Professor of Electrical Engineering and Computer Sciences).
Negative capacitance is a property that helps to reduce the voltage required to store charge in a material. This is what makes efficiency possible. Salahuddin predicted the existence negative capacitance in 2008. was the first to demonstrate the effect on a ferroelectric cristal in 2011.
A new study has shown that negative capacitance can also be achieved in an engineered crystal made of a layered stack containing hafnium oxide as well as zirconium dioxide. This is compatible with advanced silicon transistors. The study shows how the negative capacitance effect can dramatically lower the voltage needed to control transistors and the energy required by computers.
Salahuddin stated that the energy consumed for computing has increased exponentially in the past 10 years. This accounts for single digit percents of the world’s energy production. However, this growth is only linear and there is no end in sight. We don’t often think about how much energy our phones and computers are using. It is huge, and it will only get bigger. We want to lower the energy consumption of the basic building block of computing because it reduces the energy requirements for the whole system.
Real technology can be made negative by bringing in capacitance
Smartphones and laptops with the most advanced technology contain millions of tiny silicon transistors. Each one must be controlled by applying voltage. A thin layer of material called the gate oxide converts voltage into an electrical charge that then switches the transistor.
The performance of gate oxide can be boosted by negative capacitance. This is because it reduces the voltage needed to produce an electrical charge. However, this effect cannot be achieved in any material. Negative capacitance can only be created by carefully manipulating a property of ferroelectricity. This is when a material exhibits an electrical field spontaneously. The effect was previously only possible in ferroelectric materials, called perovskites. Their crystal structure is not compatible to silicon.
In the study, the team showed that negative capacitance can also be achieved by combining hafnium oxide and zirconium oxide in an engineered crystal structure called a superlattice, which leads to simultaneous ferroelectricity and antiferroelectricity.
“We discovered that this combination actually gives rise to an even greater negative capacitance effect, and this shows that the negative capacitance phenomenon is much broader than initially thought,” Suraj Cheema (a postdoctoral researcher at UC Berkeley), co-first author of the study. Negative capacitance is not limited to the traditional picture of a ferroelectric and a dielectric. This has been the most widely studied in the last decade. This effect can be made even more powerful by engineering crystal structures that combine antiferroelectricity and ferroelectricity.
Researchers found that the most effective negative capacitance structure was a superlattice structure consisting of three layers of zirconium dioxide sandwiched between two layers of hafniumoxid. This layer had a thickness of less than 2 nanometers. These superlattice structures are easily integrated into advanced silicon technology, as they can be used on top of the 2-nanometer gate of silicon oxide.
The team created short channel transistors to test the effectiveness of the superlattice structure as a gateoxide. These transistors require 30% less voltage, but maintain semiconductor industry benchmarks. They also have no loss in reliability when compared to the existing transistors.
