Engineered crystals could help computers run on less power

When we use our computers and phones, we usually aren’t thinking about the amount of energy they’re using. But as computers continue to grow smaller and more powerful, they require more and more energy to operate.

Now, a major breakthrough in the design of a transistor component – the tiny electrical switches that form the building blocks of computer chips – could significantly reduce their energy consumption without sacrificing speed, size, or performance.

A new study has shown that an engineered crystal – composed of a layered stack of hafnium oxide and zirconium oxide – can lower by approximately 30% the amount of voltage required to control transistors, and as a result the amount of energy a computer consumes.

The engineered crystal is used in a component of transistors known as a gate oxide – a thin layer of material that converts the applied voltage into an electric charge, which then switches the transistor on or off. 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 the material.

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

State-of-the-art laptops and smart phones contain tens of billions of tiny silicon transistors, and while negative capacitance can reduce the amount of voltage required to control them, the effect can’t be achieved in just any material.

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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 and so can’t be used with current silicon transistors.

Now, researchers have shown that negative capacitance can also be achieved by combining hafnium oxide and zirconium oxide in an engineered crystal structure called a superlattice. Composed of three atomic layers of zirconium oxide sandwiched between two single atomic layers of hafnium oxide, the resulting crystal is less than two nanometres in thickness.

To test how well this structure performs as a gate oxide, the researchers fabricated transistors and tested their capabilities, finding they required approximately 30% less voltage (while maintaining semiconductor industry benchmarks) and were just as reliable compared to existing transistors.

Because most state-of-the-art silicon transistors already use a 2nm 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.

“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,” says Salahuddin. “This work transforms negative capacitance from an academic topic to something that could actually be used in an advanced transistor.”

According to Salahuddin, the energy used for computing has increased exponentially in the past decade, “already accounting for single digit percentages of the world’s energy production”.

But this new material could help reduce the energy needs of a basic building block of computing, bringing down the total energy required for the entire system. The research was published in Nature.