The discovery transforms negative capacitance from an academic topic to something that could be used in an advanced transistor.
More and more aspects of our lives are getting digitized, which means that ever more powerful computers are increasingly part of the way we socialize, conduct business and get entertained.
On the downside, all these state-of-the-art computers require vast amounts of energy. In fact, in the United States alone computing is on course to outpace other energy-dependent sectors like transportation.
Meanwhile, data centers for social media platforms like Facebook, too, pack an enegy punch, consuming well over 200 terawatt hours every year, or more than the energy demands of entire nations.
Technology advances at a rapid pace and in an exciting new development a team of engineers at the University of California, Berkeley, report that they have found a solution to how to reduce the energy demands of computers without compromising their performance or size.
They have done this by modifying their transistors with a new component called the gate oxide, which serves to switch the transistor on and off. “We have been able to show that our gate-oxide technology is better than commercially available transistors,” explains Sayeef Salahuddin, a professor of Electrical Engineering and Computer Sciences at UC Berkeley.
Salahuddin, and his team, has exploited an effect he discovered more than a decade ago called negative capacitance, which helps reduce the amount of voltage needed to store electric charge in a material.
Now in new research, whose results have been published in a study, the engineers demonstrate how this effect can be achieved in a specially made crystal consisting of a layered stack of hafnium oxide and zirconium oxide, which is compatible with advanced silicon transistors.
Smartphones, tablets, laptops and desktops contain billions of these tiny silicon transistors, each of which needs to be controlled through the application of voltage. The gate oxide is a thin layer of material that converts thatvoltage into an electric charge, which then switches the transistor on.
“Negative capacitance can boost the performance of the gate oxide by reducing the amount of voltage required to achieve a given electrical charge,” Berkeley News explains.
“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,” it elucidates.
“Previously, the effect has only been 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 zirconium oxide in an engineered crystal structure called a superlattice, which leads to simultaneous ferroelectricity and antiferroelectricity.”
Through a process of trial and error the engineers discovered that a superlattice structure composed of three atomic layers of zirconium oxide sandwiched between two single atomic layers of hafnium oxide, less than two nanometers in thickness, provided the best negative capacitance effect.
“Because most state-of-the-art silicon transistors already use a 2-nanometer gate oxide composed of hafnium oxide on top of silicon dioxide, and since zirconium oxide is also used in silicon technologies, these superlattice structures can easily be integrated into advanced transistors,” Berkeley says.
Tests showed that this superlattice structure performed well as a gate oxide and transistors containing them would require around 30% less voltage than current transistions even while maintaining the semiconductor industry benchmarks and reliability standards.
“One of the issues that we often see in this type of research is that we can demonstrate various phenomena in materials, but those materials are not compatible with advanced computing materials, and so we cannot bring the benefit to real technology,” Salahuddin says. “This work transforms negative capacitance from an academic topic to something that could actually be used in an advanced transistor.”
As a result of this engineering breakthrough, negative capacitance effect can significantly lower the amount of voltage required to control transistors and with that computers would require much less energy throughout their lifecycle.
“In the last 10 years, the energy used for computing has increased exponentially, already accounting for single digit percentages of the world’s energy production, which grows only linearly, without an end in sight,” Salahuddin observes.
“Usually, when we are using our computers and our cell phones, we don’t think about how much energy we are using,” the scientist adds. “But it is a huge amount, and it is only going to go up. Our goal is to reduce the energy needs of this basic building block of computing, because that brings down the energy needs for the entire system.”