News: Microelectronics
11 December 2025
Cornell develops HEMTs on single-crystal AlN substrate for RF power amplifiers
Cornell University has developed a new transistor architecture for high-power wireless electronics that addresses supply chain vulnerabilities for gallium (‘XHEMTs on Ultrawide Bandgap Single-Crystal AlN Substrates’, Advanced Electronic Materials, 29 November 2025, https://doi.org/10.1002/aelm.202500393).
The single-crystal high-electron-mobility transistor (XHEMT) includes an ultra-thin pseudomorphic gallium nitride (GaN) channel layer sandwiched between AlN layers, fabricated on a bulk single-crystal aluminium nitride substrate, whose low defect densities and ultrawide bandgap allow it to withstand higher temperatures and voltages while reducing electrical losses.
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 in 5G and emerging 6G wireless networks, as well as radar systems for national defense applications. Pushing large amounts of electrical power at high frequencies generates heat and degrades performance.
“Because we’re using an aluminium nitride substrate with much higher thermal conductivity, the channel temperature is lower compared to other technologies,” notes Kim. “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 one-million-fold fewer crystalline defects than traditional GaN-based devices grown on silicon, silicon carbide or sapphire.
“These defects can propagate all the way through a device, whereas our new aluminium nitride substrate basically eliminates them,” Xing says. “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 in applications ranging from smartphone chargers to cell towers. Reducing dependence on gallium is becoming increasingly important for US research and manufacturing, notes Jena.
“More than 90% of all gallium is produced outside the US, and the critical need for it in semiconductor technology has attracted export restrictions,” notes Jena. “The supply chain has been highly disrupted but, with this particular aluminium nitride XHEMT, we use very, very little gallium, cutting down on its usage by several orders of magnitude,” he adds.
The aluminium nitride single crystal used in the research was produced in collaboration with Crystal IS of Albany, NY, USA (one of only a few manufacturers in the world capable of growing it with the quality required for the XHEMT).
“Aluminium nitride substrates have been used for photonics, but this research really opens the door for electronics applications,” Jena says. “We’re showing that we can take semiconductor materials that were produced here in the US and create new value and new markets for them.”
The technology’s progress toward commercial readiness was highlighted on 1 December in APL Materials, which showed wafer-scale growth of the XHEMT structure on 3-inch AlN wafers – work supported through the Northeast Regional Defense Technology Hub 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 Corp. Parts of the research were performed at the Cornell NanoScale Facility and the Cornell Center for Materials Research, both supported by the National Science Foundation.
Cornell upgrades lab with MOCVD system for next-gen nitride materials
Cornell’s Huili Grace Xing to receive 2025 University Research Award in Technology from SIA and SRC
