All Aboard The Silicon Carbide Train…Literally

SiC power semiconductors are a first step to installing Schottky devices on trains.

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There were numerous announcements regarding power semiconductors at Techno-Frontier 2014, which was held last month in Tokyo. This year, one of the big stories was power semiconductors—for power and batteries, motors, and thermal designs—and the debate around cost is becoming much more frequent for silicon carbide.

One feature article last month on the Kyoto economy in Nihon Keizai Shimbun (Japan Daily Economy) cited the SiC power semiconductor-related joint development program called “Kyoto Regional Super Cluster Program,” which is vigorously promoting itself using a grant from the Japan Science and Technology Agency. Participating companies include Omron Corp., Kyocera, Samco, Shimadzu Corp., Sumitomo Electric Industries, Nichicon Corp., Nippon Densan, Horiba Ltd., Murata Ltd., and Loam. Some universities such as Kyoto University, Kyoto Institute of Technology, Doshisha University, and Ritsumeikan University also participate.

Loam is one of earliest domestic Japanese companies to develop SiC semiconductors, and its research has been led by Hiryoyuki Matsunami, professor emeritus of Kyoto. Matsunami had been involved in SiC since the 1960s. In 1987, the development of the step-control epitaxy method created the first glimpse of potential practical uses as it can reduce the crystal defect caused by a transition. Matsunami stated that even though the Schottky Barrier Diode (SBD) was developed in 1995 and the development of FET in 1999, both were largely ignored by major Japanese s companies.

Around that time, U.S.-based Cree had begun selling an SiC circuit board. In addition, Germany’s Infineon Technologies put SBD on the market in 2001. Loam sent its researchers to Matsunami’s laboratory at Kyoto University and began its first domestic research of SiC semiconductor. Following that, Loam was able to begin its mass production, which was the first in Japan. Nowadays, companies such as Mitsubishi Denki, Fuji Denki, and Denso also develop SiC power semiconductors. And with the exception of Denso, those companies also have begun selling SiC chips. Of the two types of SiC power semiconductors—SBD and power FETs—SBD is the more popular. At least part of the reason stems from the assumption among train makers that power loss can be reduced by 20% by simply converting silicon fast recovery diodes to SiC’s SBD. Those chips were installed in new cars of the Eidanchikatetsuginza line and 8,000-type vehicles made by Hankyu Dentetsu. It also is planned for E235 type vehicles for the JR Yamanote line by the fall of 2015. Moreover, Odakyu Dentesu is planning to begin the service using 1,000-type vehicles that have SiC’s VVVF (variable voltage variable frequency) inverter from this December.

The advantage of SiC is that it has a higher dielectric breakdown voltage than Si. Therefore, it is able to increase the pressure resistance even though its impurity concentration is high (low resistivity). As a result, it can achieve high-pressure resistance while reducing on-resistance. It is unnecessary to adopt a bipolar structure such as an IGBT in order to increase the electrical current, and it can use fewer carrier devices. Many carrier type transistors such as SBD or MOSFET/JFET enable its high-speed operation.

By enabling high-speed switching, external coils or capacitors are not required to be very large. This can reduce the cost of the passive element. The cost of the SiC semiconductor itself is one figure higher, but the overall sysem cost would not increase due to downsizing the inverter system. There would be a difference in the timing for the introduction of SiC power transistors, depending on how much of a cost increase the system can allow. As for devices, there have been one announcement after another regarding full SiC modules that integrate both SiC SBDs and MOSFETs. Mitsubishi Denki announced a mini module for 600V, 20A household electronic items. In May, it also had produced a 1200V, 100A to 600A power module (MOSFET and SBD). Loam also has begun sample shipments of 1200V, 300A power modules (MOSFET and SBD).

As for devices, there have been one announcement after another regarding full SiC modules that integrate both SiC SBDs and MOSFETs. Mitsubishi Denki announced a mini module for 600V, 20A household electronic items. In May, it also had produced a 1200V, 100A to 600A power module (MOSFET and SBD). Loam also has begun sample shipments of 1200V, 300A power modules (MOSFET and SBD).