Cornell-led team creates gallium nitride power diode

Making tiny switches do enormous jobs in a more efficient way than current technology allows is one of the goals of a research team led by Cornell engineering professor Huili (Grace) Xing.

Xing and her group – which includes her husband, Debdeep Jena, also an engineering professor at Cornell – have created gallium nitride (GaN) power diodes capable of serving as the building blocks for future GaN power switches. The group built a GaN power-switching device, approximately one-fifth the width of a human hair, that could support 2,000 volts of electricity.

With silicon-based semiconductors rapidly approaching their performance limits in electronics, GaN is seen as the next generation in power control and conversion. Applications span nearly all electronics products and electricity distribution infrastructure.

“With some of these new materials, it’s actually conceivable now to shrink medium-scale power-distribution systems onto a chip,” Jena said. “Looking into the future, this is one of the goals, and it’s not a moonshot. It’s possible, but the materials have to be right, the design has to be right.”

The team’s work was published Dec. 15 in the journal Applied Physics Letters, a publication of the American Institute of Physics. The group includes researchers from Cornell, the University of Notre Dame – from where Xing and Jena arrived at Cornell last year – and the semiconductor company IQE.

Xing said the key to her team’s discovery was building the device on a GaN base layer that contained relatively few energy-sapping defects, in comparison to traditional silicon-based substrates.

“We’re going to take the defects, some of them anyway, out of the equation,” said Xing, the Richard Lundquist Sesquicentennial Professor of Electrical and Computer Engineering and a professor of materials science and engineering. “Nothing can be 100 percent [free of defects], but we’re talking about improvements along an order of magnitude of up to 10,000 times.”

The team used a couple of indicators to determine the defect level in the GaN diode, including “diode ideality factor” as measured by the Shockley-Read-Hall recombination lifetime. The SRH lifetime is the average time it takes positively and negatively charged particles to move around before recombining at defects, which creates inefficiency.

The team’s work yielded near-ideal performance in all aspects, spawning hope for the future of GaN power diodes.

“Our results are an important step toward understanding the intrinsic properties and the true potential of GaN,” said Zongyang Hu, a Cornell postdoctoral associate and the paper’s co-lead author.

While much of energy-related research and development is focused on alternative energy sources, such as wind and solar, the Xing team’s efforts in power transmission are just as important, Jena said.

“Power generation gets a lot of press, and it should,” he said. “But once the power is generated, the amount of power that is lost because of inefficiencies is mind-bogglingly large. This problem is about conservation rather than generating power, which is really the same thing.

“And the scale of losses today actually far surpasses the total of renewable energies combined,” he said. “And it’s a clear and present solution; it’s not like we have to discover something fundamental.”

The team’s work is supported in part by the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) “SWITCHES” program. SWITCHES stands for Strategies for Wide Bandgap, Inexpensive Transistors for Controlling High-Efficiency Systems.

“Leading one of these projects, we at Cornell – in collaboration with our industrial partners – have established an integrated plan to develop three terminal GaN power transistors, package them, and insert them into circuits and products,” Xing said.

The team’s paper is titled “Near unity ideality factor and Shockley-Read-Hall lifetime in GaN-on-GaN p-n diodes with avalanche breakdown.” Cornell collaborators included Kazuki Nomoto and Vladimir Protasenko, research associates in the School of Electrical and Computer Engineering, and graduate students Bo Song and Mingda Zhu. The team also included Jena’s Ph.D. student Meng Qi at the University of Notre Dame, and engineers Ming Pan and Xiang Gao of IQE.


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