"Mind the gap” is an expression that’s frequently heard on the London Underground. It also applies to manufacturers that build products equipped with battery chargers, converters, inverters, regulators, transformers, transistors and other types of power electronics.
In England, “mind the gap” is a polite way to warn people to be careful when entering and exiting trains. In the manufacturing world, it means engineers should pay attention to wide bandgap (WBG) semiconductors.
“Bandgap” refers to the amount of energy required to make electrons leap off atoms and begin conducting electricity. WBG semiconductors require more energy to excite an electron into the conduction band than other semiconductor materials. They allow electronic components to be smaller, faster and more efficient than semiconductors made from silicon.
The technology has the potential to transform the way engineers design and build products such as cell phones, electric vehicles, laptop computers, LED lighting, motors and drives, and solar panels. It offers new opportunities to achieve unprecedented performance while using less electricity.
Earlier this year, the Obama Administration and the U.S. Department of Energy established the Next Generation Power Electronics National Manufacturing Innovation Institute to develop large-scale production of WBGs. The consortium includes five universities, the National Renewable Energy Laboratory, the U.S. Naval Research Laboratory, and 18 manufacturers, including ABB, Deere, Delphi and Toshiba.
The Japanese government is funding a similar R&D program involving local universities and automakers such as Honda, Nissan and Toyota.
“Most of today’s power conversion is accomplished with a silicon-based technology that has reached the limits of its capability to convert power efficiently,” says Srabanti Chowdhury, an assistant professor in the School of Electrical, Computer and Energy Engineering at Arizona State University, which is one of the schools participating in the new consortium. “The inefficiency of the current power conversion process results in enormous amounts of wasted power.”
WBG technology eliminates up to 90 percent of the power losses that occur when converting from alternating current to direct current, and vice versa. It can handle voltages more than 10 times higher than silicon-based devices, greatly enhancing performance in high-power applications. And, it can operate at more than twice the temperature of silicon-based devices, resulting in better reliability and efficiency.
According to a study conducted by Oak Ridge National Laboratory (ORNL), approximately 30 percent of all power generation today utilizes power electronics between the point of generation and its end use. By 2030, this is expected to jump to 80 percent of generated electricity—supporting greater renewable energy integration and increased grid reliability. WBG semiconductor-based power electronics will be able to better withstand the power loads and switching frequencies required by next-generation technologies.
New semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), can operate at higher temperatures and have greater durability and reliability at higher voltages and frequencies. They also are smaller, more efficient and cost less.
For instance, a WBG semiconductor-based inverter, which switches electricity from direct current (DC) to alternating current (AC), could be four times more powerful, half the cost and one-fourth the size and weight of a traditional inverter. Toyota Motor Corp. claims that power electronic devices that use SiC semiconductors will raise the fuel efficiency of hybrid vehicles by as much as 10 percent.
According to IDTechEx, a market research company that specializes in emerging technology, WBG semiconductors will transform the electric vehicle industry by making it easier and cheaper to mass-produce power electronic components. They promise to reduce the size of a vehicle cooling system by 60 percent and cut the size of a 60-kilowatt DC charging station to the size of a kitchen microwave.
“Wide bandgap semiconductors enable devices that can operate at higher frequencies; provide more output power in a smaller footprint; operate at higher temperatures and harsher environmental conditions; provide more efficiency; lower switching losses; and allow faster switching,” says Eric Higham, director of advanced semiconductor applications at Strategy Analytics, a market research firm. “These devices are more efficient, and this ‘green’ aspect [is] a big reason why devices with WBG materials are gaining market share.”
“At the component level, WBG technologies offer higher current densities, higher breakdown voltages, faster switching rates, and in some cases, improved thermal characteristics as compared with standard silicon technologies,” adds Roland Kibler, chief technologist at NextEnergy, a nonprofit organization that promotes advanced energy investment in Michigan. “All of these characteristics translate directly into improved final product performance.”
Myths and Misperceptions
Before WBG materials can replace silicon, however, engineers must develop devices and packaging that can handle the technology’s higher temperatures and voltages. And, they need to redesign current manufacturing systems to integrate WBG devices. They also have to address several myths and misperceptions surrounding the technology.
“The biggest misperception revolves around cost and reliability,” claims Higham. “Reliability is a nonissue. Where WBG materials have been adopted, the reliability results have been good and are getting better.
“[However], cost will always remain an issue,” adds Higham. “When we talk about a WBG material like GaN, we must also think about the carrier substrate. In radio frequency applications, this is most commonly SiC, but there are activities on silicon, as well as diamond.”
In the power electronics market, the majority of GaN devices are on silicon substrates. This material stack leads to challenges with processing and this, along with the cost of the fundamental SiC or diamond wafer, adds to the cost of the final device.
In addition, production volume for WBG devices is nowhere near that of traditional silicon technologies. “It’s taken the industry a long time to develop a scalable process to make devices at large volumes,” says Pallavi Madakasira, energy electronics research analyst at Lux Research Inc.
“Bulk GaN is very expensive today, costing about $1,900 or more for a two-inch substrate, compared with $25 to $50 for a far larger six-inch silicon substrate,” Madakasira points out. “In addition, only five companies in the world currently make WBG materials and market economics discourage start-ups.”
“SiC material still suffers from many primary material defects,” adds Richard Eden, senior analyst for semiconductor supply chain at IHS Inc., a market research company. “Many applications, such as automotive components and industrial motors, [operate] in environments where personal health and safety is a legal requirement. These applications cannot consider adopting SiC devices until there is proven reliability and life-test data available.
“For power devices, GaN is still in the very early stages of development,” explains Eden. “There are very few GaN commercial products on the market. It will need several stable and able manufacturers to bring products to market before the perceived risk in choosing GaN will diminish.”
Small Devices, Big Potential
Within the next decade, WBG technology will find its way into numerous applications. “WBG semiconductors allow engineers to make smaller, lighter and more efficient power conversion devices, from power supplies to motor drives to HVDC converter stations,” says Le Tang, Ph.D., vice president and head of the U.S. corporate research center at ABB Inc.
“WBG semiconductors will first be adopted in applications demanding higher voltage, efficiency, temperature and switching frequency by replacing silicon power devices,” predicts Tang. “Once they are mature and their cost is reduced, they will be expanded to more applications, and could be the mainstream power semiconductors.”
But, engineers first need to tackle several challenges, such as improving yield rate, including process improvement and optimization, and improving device reliability. According to Tang, the industry must “educate engineers to better understand WBG semiconductor performance characteristics and how to properly use them in power conversion devices, such as DC-AC inverters and DC-DC converters.”
“The largest WBG application today, and for the foreseeable future, is in LED lighting,” claims William Stanchina, Ph.D., chairman of the Department of Electrical and Computer Engineering at the University of Pittsburgh. “Some WBG materials effectively emit and absorb optical radiation in the ultraviolet wavelength spectrum. They serve as the electric-to-optic converters within most of today’s white light LEDs.”
The U.S. military is also very interested in using WBG technology for power management systems in ground-based vehicle systems for managing heat generation and packaging advantages, as well as improved efficiency. High-power applications include armor and weapons systems.
“Military applications and cellular base stations will utilize the high power density, high frequency capability of WBGs,” says Stanchina. “These will lead to more compact systems that can tolerate higher operating temperatures without cooling.”
Automakers and suppliers are also eager to harness the benefits of WBG technology, especially when it comes to electric and hybrid vehicle applications. “Battery chargers and battery management systems will be the first areas of integration,” predicts NextEnergy’s Kibler. “Traction motor drive controllers (inverters) also offer an attractive area for application and these will follow, as the reliability and durability of WBG devices are demonstrated.”
In conventional vehicles, Kibler says any application requiring higher power conversion would be attractive, including electric power steering, cooling, supercharging, valve train control and fuel-injection systems.
Engineers at ORNL recently developed a power inverter made from SiC material that could make next-generation electric vehicles lighter, more powerful and more efficient. Additive manufacturing helped them create complex geometries, increase power densities, and reduce weight and waste while building a 30-kilowatt inverter.
“[We] optimized the inverter’s heat sink, allowing for better heat transfer throughout the unit,” says Madhu Chinthavali, an electrical engineer in the lab’s Power Electronics and Electric Machinery Research Group. “This construction technique allowed [us] to place lower-temperature components close to the high-temperature devices, further reducing electrical losses and reducing the volume and mass of the package.”
Half the parts in the liquid-cooled SiC traction drive inverter were made via 3D printing. Initial evaluations confirmed an efficiency of nearly 99 percent, surpassing DOE’s power electronics target.