Turn the Heat Up: New Approaches To Energy Conversion

Srabanti Chowdhury joins UC Davis to help transform the semiconductor industry

Published in the Summer 2016 issue of Engineering Progress.

Meeting global energy demands will require much more than simply expanding renewable sources; it’s also necessary to improve the efficiency of how energy is used. Taking a new look at the materials presently used in electronics, Srabanti Chowdhury is working to transform the way energy is converted to run power electronics, spanning from data servers and hybrid cars to laptops and smart phones.

Prof. Srabanti Chowdhury of the Department of Electrical and Computer Engineering at UC Davis.

Prof. Srabanti Chowdhury of the Department of Electrical and Computer Engineering at UC Davis.

Among the newest faculty members in the UC Davis Department of Electrical and Computer Engineering, Chowdhury previously held an assistant professor position at Arizona State University’s School of Electrical, Computer and Energy Engineering.

Chowdhury earned her master’s degree (2008) and doctorate (2010) in electrical engineering at UC Santa Barbara. As part of her PhD work, she developed gallium nitride (GaN)-based vertical devices for power conversion, and demonstrated the first vertical GaN high voltage power-switching device — a current aperture vertical electron transistor (CAVET) — with a record-high breakdown electric field.

After nearly three years in industry, furthering her work with gallium nitride at a California- based start-up, Chowdhury found herself missing the university environment. She is happy to be back in academia, and these days concentrates on her new lab at UC Davis.

“My primary goal is to make a vertically integrated lab, where we can address materials, devices and circuits for next generation electronics, all in one place,” she says. “I want to ensure that as a team at UC Davis we excel within each of these fields.”

“My branch of research focuses on solid-state devices, specifically the tiny switches at the heart of a converter. Without such switches, you cannot ‘condition’ or convert your power from one form to another. Such ‘switches’ use semiconductors as the medium to convert power from one form to another: from one voltage or current level to another; from one level of frequency to another; AC to DC and vice versa, and so forth,” she says. “All of this occurs in a black box called the “converter,” where the conversion takes place via a power-electronic ‘switch’ that is made of a semiconductor.” Exploring different semiconductor materials to optimize the performance of such switches and maximize their efficiencies, Chowdhury and her lab colleagues are looking into diamond and gallium oxide in addition to GaN — popularly known as “ultra-wide bandgap semiconductors.”

The recent challenge, however, involves the need to keep up with increased demand and expectations from end users. Chowdhury points out that almost all of the devices and gadgets used today have been made possible by silicon semiconductors. However, there are increased demands for more functionality, smaller footprint, higher efficiencies, faster speed and longer lifetime from these applications. “Silicon has hit its saturation due to its ‘materials limit,’” explains Chowdhury. “Any improvement with Si is now incremental. This calls for new semiconductors: hence my work with gallium nitride and diamond, which have the potential to improve power conversion efficiency by unprecedented values.”

Besides commercial power electronics applications, these semiconductors are well suited for high frequency applications. GaN is used in radar technology that relies on high power density and high frequency at the same time. Chowdhury is investigating means to increase the power density of radio frequency amplifiers while managing the unwanted ‘heat’ that is generated during operation at such high power.

Prof. Srabanti Chowdhury of the Department of Electrical and Computer Engineering at UC Davis.

Prof. Srabanti Chowdhury of the Department of Electrical and Computer Engineering at UC Davis.

That work already has garnered Chowdhury considerable acclaim. In 2015 alone, she received a National Science Foundation CAREER Award, an Air Force Office of Scientific Research (AFOSR) Young Investigator Program (YIP) Award, and a Defense Advanced Research Projects Agency (DARPA) Young Faculty Award. These programs are allowing her to explore the high-speed world with frequencies ranging from gigahertz to terahertz.

Another one of her projects involves collaboration with PowerAmerica, a U.S. Department of Energy organization based at North Carolina State University with the goal of enhanced energy efficiency by making wide bandgap (WBG) semiconductor technologies cost competitive with the currently used silicon-based power electronics. UC Davis is now a member of PowerAmerica, which will allow collaboration with other universities and industries to address next generation power electronics.

Chowdhury indicates that roughly 10 percent of electricity generated in the U.S. is lost via inefficient power conversion, manifested in the form of heat. “A year’s worth of this wasted energy could supply the annual energy needs for a country such as Singapore, Malaysia or our entire West Coast,” she says. “This problem demands a revolution: Even if we fully convert from fossil fuels to renewables, we cannot fully implement sustainability without addressing electronics.”

Nor is it simply a matter of efficiency. As silicon components are heated repeatedly over time, they degrade and become less effective, until eventually they die. But, Chowdhury says, wide bandgap and ultra-wideband gap semiconductors can run at higher temperatures.

“This is the huge advantage that people are trying to harness: We could run devices hotter and efficiently without having to cool the system by various means,” she says. Such “systems” include the vehicles we drive. Hybrid cars require coolant technology to maintain the electronics at a “safe” temperature, at a huge cost and enormous loss of space within the vehicle.

“Toyota, which funded my PhD project, wants to reduce or eliminate the cooling requirements to the point where the system can rely wholly on air cooling. This innovation alone leads to significant savings in terms of cost and ‘much-needed’ space, leading to widespread adoption of hybrid vehicles.

“The next generation of scientists and technologists will grow up with wide bandgap and ultra-wideband gap semiconductors,” says Chowdhury.

“These materials address more than one field creating opportunities for interdisciplinary research. The timing is perfect to explore and develop solutions with these semiconductors and I am happy to be in the right place at the right time.”

Engineering Progress Staff