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Gallium oxide and aluminum gallium oxide have drawn increased interest as potential ultrawide bandgap semiconductors, due to their exceptional electrical transport properties. The catch: both have low thermal conductivity, resulting in challenges when dissipating heat during operation.

One way to mitigate this challenge is to utilize the metal contacts used to power the transistors to remove the heat. But anisotropy of the crystals can occur, as well as chemical reactions between metals and the oxides, impacting thermal resistance and affecting heat removal.

The phenomenon has, until now, not been well-understood. But a new paper by Maryland Engineering’s Dean Samuel Graham, Jr. and collaborators at multiple institutions sheds light on the factors that cause resistance, and presents approaches to minimizing it.

“We’re showing the different reactions that occur when metals are deposited onto gallium oxide semiconductors, as well as their dependence on processing conditions, and finding ways to stabilize these interfaces. The goal is to link the microstructural morphology and chemistry to both heat flow and electrical charge transport, since these contacts will be used to carry heat and current into and out of the device,” Graham said of the paper, titled “Thermal Transport Across Al-(AlXGa1-X)2O3 and Al-Ga2O3 Interfaces.”

“When metals are deposited onto gallium oxide, there is a possibility that the metal can steal the oxygen away from the semiconductor crystal, leaving behind a complex layer underneath the metal contact with defects and different chemical composition than in the bulk crystal,” Graham said. “This defective and non-homogenous region gives rise to the thermal resistance between the metal and semiconductor by scattering phonons that carry the thermal energy.”

“In addition, the crystallographic orientation also plays a role in the level of phonon scattering. Ideally, we want to get rid of that resistance in order to maximize power density while also understanding the anisotropic nature of the resistance. This, in part, can be controlled by manufacturing techniques and by using a slightly different chemistry near the interface to stabilize the chemical reactions,” he said.

Graham’s paper, with co-authors from the Georgia Institute of Technology, Ohio State University, Pennsylvania State University, UC Santa Barbara, UCLA, and the University of Virginia, was named Best Paper this fall at InterPACK 2021—the flagship conference of the ASME Electronic and Photonic Packaging Division, and the leading international conference in the field of electronics packaging and heterogeneous integration.

Keynote speech at InterPack highlights novel approaches

In addition to receiving the Best Paper award, Graham also was selected as Keynote Speaker at the event. His address, “Developing Wide Bandgap Electronics for Future Power and RF Applications,” centered on the development of new materials based on gallium oxide as well as gallium nitride (GaN). While these materials are expected to yield many improvements—including greater efficiency, higher operational frequency, and smaller form factors—they also necessitate the development of new manufacturing techniques and metrology methods.

In his talk, Graham detailed some of the manufacturing approaches being developed in order to ensure efficient heat dissipation from gallium nitride and gallium oxide devices, highlighting the use of surface activated bonding to combine these semiconductors with efficient heat dissipation layers like diamond, silicon carbide (SiC), and aluminum nitride (AlN). He also presented thermal and stress metrology methods that enable the measurement of temperature and stresses, and discussed an actively cooled power substrate that is being developed for packaging power devices.

Graham became Nariman Farvardin Dean at UMD’s A. James Clark School of Engineering on October 1. He previously served as professor and Eugene C. Gwaltney, Jr. Chair of the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. He also holds a joint appointment with the National Renewable Energy Laboratory and is a Distinguished Visiting Professor at Nagoya University in Nagoya, Japan.

Graham is a Fellow of ASME, a member of the Engineering Sciences Research Foundation Advisory Board of Sandia National Laboratories, and a member of the Emerging Technologies Technical Advisory Committee of the US Department of Commerce.



December 6, 2021


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“We’re showing the different reactions that occur when metals are deposited onto gallium oxide semiconductors, as well as its dependence on processing conditions, and finding ways to stabilize these interfaces."

Dr. Samuel Graham, Jr., Dean, A. James Clark School of Engineering, University of Maryland

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