Aluminum nitride transistor advances next-gen RF electronics

Cornell researchers have developed a new transistor architecture that could reshape how high-power wireless electronics are engineered, while also addressing supply chain vulnerabilities for a critical semiconductor material.

The device, called an XHEMT, includes an ultra-thin layer of gallium nitride built on bulk single-crystal aluminum nitride, a semiconductor material with low defect densities and an ultrawide bandgap - properties that allow it to withstand higher temperatures and voltages while reducing electrical losses.

The device was detailedNov. 29 in the journal Advanced Electronic Materials and the research was co-led by Huili Grace Xing, the William L. Quackenbush Professor, Debdeep Jena, the David E. Burr Professor - both in the School of Electrical and Computer Engineering, the Department of Materials Science and Engineering, and the Kavli Institute at Cornell for Nanoscale Science - and doctoral student Eungkyun Kim.

The XHEMT is designed for radio frequency power amplifiers, a critical component of 5G and emerging 6G wireless networks, as well as radar systems for national defense applications. These amplifiers push large amounts of electrical power at high frequencies, conditions that generate heat and degrade performance.

"Because we're using an aluminum nitride substrate with much higher thermal conductivity, the channel temperature is lower compared to other technologies," Kim said. "This opens the possibility of operating at even higher power, extending the present communication range or radar capability."

The XHEMT's material layers are lattice-matched from top to bottom, resulting in about a 1 million-fold fewer crystalline defects than traditional gallium nitride-based devices grown on silicon, silicon carbide or sapphire.

"These defects can propagate all the way through a device, whereas our new aluminum nitride substrate basically eliminates them," Xing said. "While this needs to be studied in more detail, I think it will translate to a tremendous advantage in the upcoming iterations of this device."

Reducing reliance on gallium

As demand for high-performance electronics grows, so does demand for materials like gallium nitride, which is used widely in everything from smartphone chargers to cell towers. Jena said reducing dependence on gallium is becoming increasingly important for U.S. research and manufacturing.

"More than 90% of all gallium is produced outside the U.S. and the critical need for it in semiconductor technology has attracted export restrictions," Jena said. "The supply chain has been highly disrupted, but with this particular aluminum nitride XHEMT, we use very, very little gallium, cutting down on its usage by several orders of magnitude."

The aluminum nitride single crystal used in the research was produced in collaboration with Crystal IS, a company based in Albany, New York, and one of only a few manufacturers in the world capable of growing it with the quality required for the XHEMT.

"Aluminum nitride substrates have been used for photonics, but this research really opens the door for electronics applications," Jena said. "We're showing that we can take semiconductor materials that were produced here in the U.S. and create new value and new markets for them."

The technology's progress toward commercial readiness was highlightedDec. 1 in the journal APL Materials, which showed wafer-scale growth of the XHEMT structure on 3-inch aluminum nitride wafers - work supported through the Northeast Regional Defense Technology Hub, also known as NORDTECH.

The material layers used in the XHEMT were developed at Cornell by doctoral student Yu-Hsin Chen and research associate Jimy Encomendero. Their atomic structures were investigated by doctoral student Naomi Pieczulewski and David Muller, the Samuel B. Eckert Professor of Engineering in the School of Applied and Engineering Physics.

The research was supported by the Army Research Office, the Defense Advanced Research Projects Agency and the Asahi-Kasei Corporation. Portions of the research were performed at the Cornell NanoScale Facility and the Cornell Center for Materials Research, both supported by the National Science Foundation.

Syl Kacapyr is associate director of marketing and communications for Cornell Engineering.