Tsunenobu Kimoto, a professor of electronic science and engineering at Kyoto University, literally wrote the book on silicon carbide technology. Fundamentals of Silicon Carbide TechnologyPublished in 2014, it covers the properties, processing techniques, theory, and practical device analysis of SiC materials.
Developing better manufacturing technologies through silicon carbide research, Kimoto has improved wafer quality and reduced defects. His innovations in making silicon carbide semiconductor devices more efficient, reliable, and commercially viable have had a major impact on modern technology.
Tsunenobu Kimoto
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Kyoto University
title
Professor of Electronic Engineering
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person
my school
Kyoto University
For his contributions to silicon carbide materials and power devices, the IEEE Fellow received this year's IEEE Andrew S. Grove Award, sponsored by the IEEE Electron Devices Society.
Silicon Carbide’s humble beginnings
Decades before the Tesla Model 3 rolled off the assembly line with SiC inverters, a handful of researchers, including Kimoto, foresaw the potential of silicon carbide technology. In obscurity, they studied it and improved the technology to manufacture power transistors with characteristics superior to the silicon devices then mainstream.
Today, MOSFETs and other silicon carbide transistors significantly reduce on-state losses and switching losses in power conversion systems, such as inverters in electric vehicles, which are used to convert direct current from batteries to alternating current that drives motors. Lower switching losses improve vehicle efficiency, reducing the size and weight of power electronics and improving powertrain performance. Silicon carbide-based chargers that convert alternating current to direct current provide similar improvements in efficiency.
But the tool didn't just appear. “We first had to develop basic techniques, such as how to dope materials to create n-type and p-type semiconductor crystals,” says Kimoto. NThe atomic structure of a type crystal is arranged so that negatively charged electrons can move freely through the material's lattice. Conversely, the atomic arrangement is blood‘-type crystals’ contain positively charged holes.
Kimoto's interest in silicon carbide began during his Ph.D. from Kyoto University in 1990.
“At the time, very few people were working on silicon carbide devices,” he says. “And the main target for silicon carbide in those people’s minds was blue LED.
“There has been little interest in silicon carbide power devices such as MOSFETs and Schottky barrier diodes.”
Kimoto started by studying how SiC could be used as the basis for blue LEDs. However, he had read B. Jayant Baliga's 1989 paper “Power Semiconductor Device Figures of Performance for High Frequency Applications”. IEEE Electronic Devices LetterHe attended a presentation on the topic by 2014 IEEE Medal of Honor recipient Baliga.
“I was convinced that silicon carbide was very promising for power devices,” says Kimoto. “The problem was that there was no wafer or substrate material.” Without this, it was impossible to manufacture the device commercially.
To obtain silicon carbide power devices, “researchers like me had to develop basic technologies, such as methods for doping materials. blood-type and N– It's a tangible crystal,” he says. “There was also the issue of forming high-quality oxides on silicon carbide.” Silicon dioxide is used in MOSFETs to isolate the gate and prevent electrons from flowing into the gate.
The first challenge Kimoto solved was to produce pure silicon carbide crystals. He decided to start with carborundum, a form of silicon carbide commonly used as an abrasive. Kimoto took some factory waste (small silicon carbide crystals measuring about 5 mm x 8 mm) and polished them.
He found himself highly doped N– Type of decision. But he realized he was just highly doped. N-type SiC is rarely used in power applications unless a lightly doped (high purity) product can be produced. N-type and blood-SiC type.
Connecting two material types creates a depletion zone that spans the joint. N-type and blood– Tangible aspects meet. In this region, free-moving charges are lost due to diffusion and recombination with opposite charges, creating an electric field that can be exploited to control the flow of charge across the boundary.
“Silicon carbide is a family with many brothers.”
Kimoto was able to grow high-purity silicon carbide using an established technology, chemical vapor deposition. This technique grows SiC as a layer on a substrate by introducing gas into a reaction chamber.
At the time, silicon carbide, gallium nitride, and zinc selenide were all contenders in the race to produce practical blue LEDs. Kimoto says silicon carbide has just one advantage. That said, making silicon carbide is relatively easy. blood–N join. to make blood–N With the other two options, splicing was still difficult to perform.
But in the early 1990s it started to become clear that SiC was not going to win the blue LED sweepstakes. The inescapable reality of the laws of physics overwhelmed SiC researchers' belief that the material's unique properties could somehow be overcome. Because SiC has a so-called indirect bandgap structure, when charge carriers are injected, the probability that the charges will recombine and emit photons is low, making it less efficient as a light source.
While the exploration of blue LEDs has been making headlines, many low-profile advancements have been made using SiC for power devices. In 1993, a team led by Kimoto and Hiroyuki Matsunami demonstrated the first 1,100 V silicon carbide Schottky diode. IEEE Electronic Devices Letter. The diodes produced by our team and others achieved fast switching that was not possible with silicon diodes.
“With silicon blood–N The diode requires approximately 0.5 microseconds to switch. “But with silicon carbide, it only takes 10 nanoseconds.”
The ability to quickly turn devices on and off increases the efficiency of power supplies and inverters because less energy is wasted as heat. High efficiency and low heat allow for smaller, lighter designs. This is a big problem for electric vehicles, where lighter weight means less energy consumption.
Kimoto's second innovation was to identify the most useful form of silicon carbide material for electronic applications.
“Silicon carbide is a family with many, many siblings,” says Kimoto, noting that more than 100 variants exist with different silicon-carbon atomic structures.
Type 6H silicon carbide has been the default standard phase used by researchers targeting blue LEDs, but Kimoto discovered that type 4H has much better properties for power devices, including higher electron mobility. Now, all silicon carbide power devices and wafer products are manufactured in 4H type.
Silicon carbide power devices in electric vehicles can improve energy efficiency by about 10 percent compared to silicon, Kimoto says. For electric trains, the power needed to propel the cars can be reduced by up to 30 percent compared to trains using silicon-based power devices, he says.
He admits challenges still remain. Silicon carbide power transistors are used in Teslas, other EVs and electric trains, but their performance is still less than ideal due to defects present in the silicon dioxide-SiC interface, he says. Since interface defects degrade the performance and reliability of MOS-based transistors, Kimoto et al. are working to reduce them.
When Kimoto grew up as an only child in Wakayama, Japan, near Osaka, his parents insisted that he study medicine and expected him to live with their family as an adult. His father was a garment factory worker. His mother was a housewife. Moving to Kyoto to study engineering “was disappointing on both counts,” he says.
His interest in engineering, he recalls, began when he was in middle school, when Japan and the United States were competing for supremacy in the semiconductor industry.
He received bachelor's and master's degrees in electrical engineering from Kyoto University in 1986 and 1988. After she graduated, she joined the R&D center of Sumitomo Electric Industries in Itami. He worked with silicon-based materials there, but was dissatisfied with the center's research opportunities.
He returned to Kyoto University in 1990 to obtain his Ph.D. While studying power electronics and high-temperature devices, he also gained understanding of defects, breakage, mobility, and luminescence in materials.
“My corporate experience was invaluable, but I didn't want to go back to the industry,” he says. In 1996, around the time he earned his doctorate, the university hired him as a research associate.
Since then, he has introduced innovations that have helped make silicon carbide an integral part of modern life.
Growing the Silicon Carbide Community at IEEE
Kimoto joined IEEE in the late 1990s. An active volunteer, he has helped grow the global silicon carbide community.
He is an editor. IEEE Transactions on Electronic DevicesHe has served on program committees for conferences including the International Symposium on Power Semiconductor Devices and ICs and the IEEE Workshop on Wide Bandgap Power Devices and Applications.
“Now when we hold a silicon carbide conference, we get over 1,000 people,” he says. “At IEEE conferences such as the International Electron Devices Meeting (ISPSD), we always see well-attended sessions on silicon carbide power devices because more IEEE members are now paying attention to this field.”