New semiconductor material shows promise for more efficient power electronics

July 6, 2017

Every year it seems like we have another device to charge — laptops, smartphones, tablets, wearable electronics, electric cars and so on. Charging processes ubiquitously use power electronic switches to convert electricity from one form to another so that it can be used in these devices. As power electronics become increasingly part of how we move and consume electricity, the U.S. Department of Energy (DOE) is looking for new ways to make these devices more powerful, more efficient and more compact.

Semiconductors are a critical part of power electronic devices, which today, are most commonly made of silicon semiconductor. However, silicon gets less efficient as power demands increase, so new materials are needed as power electronics capabilities and performance continue to advance. Gallium nitride (GaN) is a promising new wide-bandgap (WBG) semiconductor material with properties that allow it to operate at higher voltages, frequencies and temperatures at higher efficiencies than silicon. Yuji Zhao holds up a gallium nitride wafer Professor Yuji Zhao is working to advance fundamental knowledge of selective area doping processes for gallium nitride wide-bandgap semiconductors. Photographer: Pete Zrioka/ASU Download Full Image

To achieve semiconductor power devices, specific impurities are added to the semiconductor materials to achieve structures called p-n junctions through a process called doping. However, the lack of effective doping processes for GaN WBG semiconductor materials currently obstructs the progress in creating these new and better power electronics devices based on these materials.

Yuji Zhao, assistant professor of electrical and computer engineering at Arizona State University’s Ira A. Fulton Schools of Engineering, is working to advance fundamental knowledge of selective area doping processes for GaN WBG semiconductors through a $1.5 million project selected by the DOE Advanced Research Projects Agency-Energy (ARPA-E) as part of its Power Nitride Doping Innovation Offers Devices Enabling SWITCHES (PNDIODES) program. The PNDIODES program aims to address technological gaps in GaN doping, a critical technical step to achieve high performance GaN power devices.

“We need to get a fundamental understanding of the critical issues for the p-n junction doping, why it’s not effective, and what effective approach we can use,” Zhao said. “We’re also developing a new method to achieve a GaN vertical power transistor that can meet ARPA-E’s target including current, breakdown voltages, efficiency and more.”

 in his lab

Yuji Zhao

Zhao’s team — comprised of co-principal-investigators Stephen Goodnick, professor of electrical engineering, as well as Robert Nemanich and David Smith, both ASU Regents’ Professors of physics, and Fernando Ponce, professors of physics — is taking an innovative, advanced materials science research approach to this problem. Their project, “Effective Selective Area Doping for GaN Vertical Power Transistors Enabled by Innovative Materials Engineering,” is one of seven projects from academia and industry selected for the new ARPA-E PNDIODES program launched in June.

The GaN WBG semiconductor device the team is developing has a new vertical format, and thus has different p-n junction structure. Zhao believes their approach to GaN WBG semiconductor development is what impressed the DOE.

“We have a lot of innovative ideas that have never been explored,” Zhao said. “We have a very comprehensive approach that covers physics to materials to the device.”

He also credits his “dream team” of ASU researchers to their success in earning ASU’s award from the program.

“We have a very strong team with a recognized track record in GaN materials and devices and organized effort from experts all around,” Zhao said.

The School of Electrical, Computer and Energy Engineering, one of the six Fulton Schools of which Zhao and Goodnick are faculty members, has heavily invested in Zhao’s research, including the multi-million-dollar Metal-Organic Chemical Vapor Deposition Lab on ASU’s Tempe campus.

“This is a very big investment from the Fulton Schools, and we have been given support from the Fulton Schools and ASU to organize this effort so we can put people like Bob Nemanich, Fernando Ponce and David Smith together so we could submit this proposal.”

With several other proposals pending in GaN research, Zhao and the team are looking to continue their momentum beyond this ARPA-E project.

“We think this is going to be something big, and this is a good starting point,” Zhao said. “We’re going to take advantage of this winning proposal and we hope to turn this into a new research center in the near future.”

Monique Clement

Communications specialist, Ira A. Fulton Schools of Engineering


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Finding a new path with ASU Online

July 7, 2017

Combining digital and hands-on learning leads this Sun Devil to success

Imagine you’re on a 28-mile journey that takes you across icy rivers and up steep jungle trails. After a grueling two days, you and your research team catch your first glimpse of the mountaintop archaeological site Ciudad Perdida, the “Lost City.”

It may sound like the opening scene of a summer blockbuster, but it was a recent reality for one mother of three, military spouse and Arizona State University online student.

Kristin Keckler-Alexander spent last summer at a field school in Colombia. Although the trek to the site itself was long, her journey really began several years before, when she was working toward an associate’s degree in history.

During that time, a “crazy-amazing professor” made anthropology come alive for her, she said, and got her interested in the connections between history and archaeology.

At an adviser’s recommendation, she later transferred to ASU Online and is now in the process of earning an undergraduate degree in anthropology, a minor in global health and a 4+1 master’s degree in history through the School of Human Evolution and Social Change and the School of Historical, Philosophical and Religious Studies.

“ASU Online gave me — a military spouse with three children who doesn’t live anywhere for very long — the opportunity to pursue my degrees, and that’s not something I would have at a brick-and-mortar institution,” she said.

“A lot of people seem to think that getting an online education is super easy,” she added, “but this learning style comes with its own set of challenges.”

Besides self-motivation, organization and an openness to unique communication workarounds with professors, online students also require a willingness to seek out their own extracurricular experiences.

"You dive into something, and you find out new information which leads you to new, bigger questions. And that’s part of the fun of it."
— Kristin Keckler-Alexander

Not wanting to miss out on the grit and real-world adventure of archaeological research, Keckler-Alexander found a program that allowed students to work in the field — in this case, at Ciudad Perdida.

Located high in the Sierra Nevada on the north coast of Colombia, the sprawling, terraced site was built by the Tairona people around AD 800.

“Getting to the site is pretty intense, but you fall in love with it as soon as you’re there,” she said. “It was definitely one of those life-changing experiences.”

Keckler-Alexander and the team with her camped on the site for a month. During that time, tasks such as historic reconstruction, archaeological survey, excavation and topographic mapping cemented a passion for fieldwork and taught her that each discovery only leads to more curiosity.

“You dive into something, and you find out new information which leads you to new, bigger questions,” she said. “And that’s part of the fun of it.”

This summer, in addition to continuing her online studies, Keckler-Alexander is taking advantage of several other opportunities to work in her field, including an ongoing project with the Society for American Archaeology to advise the Boy Scouts on the archaeology merit badge. She’ll also be heading to the Kansas State Historical Society field school with one of her sons, and she is anticipating the publication of a scholarly paper she wrote as a capstone project for her history degree.

The paper describes a little-known, WWI-era women’s civic organization that nonetheless had a big impact in the Midwest. Members of the Military Sisterhood of America provided financial charity, negotiated discounts for goods sold to soldiers, and even organized lawyers in Kansas and other Midwestern states to offer free legal services to military families.

“I became very passionate about making sure that people knew who these remarkable women were,” Keckler-Alexander said. “In my opinion, they were the precursor to a lot of the standard Army community service activities that we have today.”

Once she completes her master’s degree, Keckler-Alexander plans to apply to an archaeology doctoral program, though she may take some time off beforehand to work in the field. Her hope is to someday return to the sites in Colombia and bring her daughter with her, so that she too can be inspired by all the female scientists there.

“My daughter is very much in that age where she’s starting to think about what she wants to be when she grows up,” Keckler-Alexander said. “If that means she wants to be a shovel bum like her mom, then she can be a shovel bum. There really are no limits.”

Top photo: Kristin Keckler-Alexander (center-right) hikes across a river in Colombia with her research team. Photo courtesy of Kristin Keckler-Alexander

Mikala Kass

Communications Specialist , ASU Knowledge Enterprise