Prospects for the Application of SiC Power Electronic Devices in Traction Field
Xi'an Yongdian Electric Co., Ltd.
Abstract: Wide bandgap semiconductor SiC is the most promising power electronic material, meeting the development trend of lightweight, miniaturized, and high-efficiency traction converters. This paper describes the current application status of SiC power electronic devices in the traction field, introduces the advantages and characteristics of SiC SBD, SiC MOSFET, SiC JFET, and SiC IGBT, and discusses the challenges faced by SiC power electronic devices in the traction field.
1 Introduction
The continuous development of electric traction technology demands higher power density, higher operating temperature, lower power loss, and faster switching speeds from power electronic devices . Silicon-Si-based power electronic devices are maturing due to the application of new power electronic devices such as high-power field-effect transistors (MOSFETs) and insulated-gate bipolar transistors (IGBTs). With improvements in device structure design and manufacturing processes, the performance of current devices is approaching the theoretical limits of Si materials . Currently, third-generation wide-bandgap semiconductor power electronic devices, represented by silicon carbide ( SiC), have been commercialized and show great potential in the traction field .
The larger bandgap of SiC (3.26 eV) gives it significant advantages over Si (1.12 eV): its intrinsic carrier concentration is 20 orders of magnitude lower, its critical breakdown electric field is 10 times higher, its thermal conductivity is 3 times higher, and its electron saturation drift velocity is 1 times higher. These properties enable SiC power electronic devices to surpass the limits of Si materials in terms of high temperature, high frequency, and blocking voltage .
Currently, SiC power electronic devices have been industrialized in the low-to-medium voltage range of 600V-1700V. Companies such as Cree, Rohm, and Infineon can supply SiC SBDs and MOSFETs with a maximum current of 50A in bulk, and their applications have significantly improved system operating frequency and overall efficiency. High-voltage SiC devices were first reported in 2003; however, due to limitations in crystal quality and related processes, the industry is still developing. High-voltage SiC devices are currently being tested in small batches in fields such as locomotive traction and high-voltage direct current transmission , resulting in significant improvements in system performance.
China's high-speed rail construction has reached a world-leading level, but the application of SiC power electronic devices for traction is still in its early stages. Since 2013, CRRC Yongji Electric and Zhuzhou Times Electric have begun research on SiC device packaging and application, hoping to promote the development of the entire SiC power electronics industry chain and catch up with advanced foreign technologies.
2 SiC power electronic devices
SiC devices still have very low on-resistance even at high blocking voltages, so research on SiC devices has begun to focus on a few carrier devices such as Schottky barrier diodes (SBDs) and MOSFETs.
2.1 SiC SBD
SiC SBDs are unipolar devices, without minority carrier injection or free charge storage, and have almost ideal reverse recovery characteristics, making them suitable for operation under high voltage, high frequency and high temperature conditions.
Because the Schottky barrier of SiC is thinner than that of Si under high voltage , further increasing the reverse voltage of SiC SBDs is limited by the reverse leakage current caused by the tunneling barrier. In order to fully utilize the advantage of the high critical breakdown electric field of SiC, structures such as JBS and MPS are usually used to reduce the electric field strength at the Schottky contact, thereby obtaining better device characteristics.
SiC SBDs are the most mature SiC power electronic devices, suitable for a blocking voltage range of 600V-3300V. Companies such as Cree, Rohm, Microsemi, and Infineon have already applied SiC SBDs in frequency converters or inverters to replace Si-based fast recovery diodes, significantly improving operating frequency and overall system efficiency. However, due to the relatively lagging development of SiC switching devices, the common practice in traction and industrial frequency conversion fields is to package SiC SBDs and Si IGBT chips together to form high-power switching devices, thereby reducing switching losses.
2.2 SiC MOSFET
SiC is the only wide-bandgap semiconductor material with a thermal oxide layer, so the design, manufacturing experience, and production equipment of Si-based MOSFETs can be directly referenced. Furthermore, SiC MOSFETs are compatible with existing Si-based MOSFETs and IGBT drive circuits, making them the fastest-growing switching device.
Early development of SiC MOSFETs faced several challenges, such as low channel mobility and gate oxide reliability issues. Currently, the mobility problem has been largely resolved through design and process technologies such as buried trenching and high-temperature oxidation. Regarding reliability, the gate oxide layer still exhibits good reliability at 350 °C and is no longer a bottleneck limiting the development of SiC MOSFETs.
In 2012, Rohm of Japan improved crystal quality by optimizing process conditions and device structure, achieving for the first time an integrated package of SiC SBD and SiC MOSFET. This solved the problem of high power conversion losses caused by the use of Si IGBTs and FRDs (Fast Recovery Diodes) in 1200V inverters. This product reduced device operating losses by more than 70% while achieving higher operating frequencies above 100kHz, driving the miniaturization of peripheral components. It is expected that in the next 5-10 years, SiC MOSFETs will replace Si IGBTs as the mainstream power electronic switching device.
2.3 SiC JFET
Due to the inherent imperfections in the SiC MOSFET structure, SiC JFETs (junction field-effect transistors), which are also unipolar switching devices, have gained significant attention and were among the first to be commercialized, alongside SBDs and MOSFETs. SiC JFETs also offer advantages such as good threshold voltage stability with temperature and high-temperature reliability, making them one of the fastest-growing SiC switching devices currently available.
However, the characteristics of the gate PN junction's operating mode also bring many adverse effects to device applications, such as normally-on operation, high Miller capacitance effect, and high negative gate turn-off voltage. This means that SiC JFETs cannot directly replace Si MOSFETs and IGBTs; corresponding adjustments to the drive circuit are required to ensure safe and reliable device operation.
Currently, SiC JFET devices have achieved a certain degree of industrialization, mainly through products from Infineon, SiCED, and Semisouth. Product voltage ratings are 600V, 1200V, and 1700V, with single-transistor current reaching up to 20A and module current ratings exceeding 100A. In 2013, Rockwell Automation fabricated a 25A three-phase motor drive module using a 600V/5A SiC enhancement-mode JFET and a SiC SBD in parallel. Compared to the then-advanced Si IGBT modules, this module reduced the chip area by 40% for the same power output, while significantly reducing losses and issues with switching overvoltage and overcurrent.
2.4 SiC IGBT
Due to limitations imposed by the high resistivity of P-type substrates, low channel mobility, and gate oxide reliability issues, the development of SiC IGBTs started relatively late, with reports only appearing in 1999. After years of research, these problems have been gradually resolved. A 13kV N-channel SiC IGBT reported in 2008 achieved an on-state resistivity of 22 mΩ· cm² .
Compared with SiC MOSFETs, Si IGBTs, and thyristors, SiC IGBTs offer significant advantages in applications with blocking voltages above 15kV, combining fast switching speed with low power consumption. Therefore, by continuously improving the characteristics and reliability of SiC IGBT chips, they will become core components in smart grids.
3. Challenges of applying SiC power electronic devices in the traction field
3.1 High chip manufacturing costs
From a commercial perspective, SiC power devices have a large market share in power electronics, but whether SiC can successfully penetrate the traction market ultimately depends on its cost-effectiveness. Although 6-inch 4H-SiC substrate fabrication has been achieved, Cree took 13 years to scale up from 2-inch (1997) to commercially available 6-inch (2010) zero-micropipe 4H-SiC substrates. Meanwhile, the manufacturing costs of SiC power devices are also very high, with equipment and technology controlled by a few foreign companies. This high price typically limits their application to high-temperature, irradiated, and other areas where Si devices cannot operate. The smaller market size and high costs constrain the development of SiC power devices.
Currently, the price of SiC power devices of the same specifications is 5-6 times that of Si devices. When this figure drops to 2-3 times, SiC power devices will be widely used in electric vehicles, locomotives, and EMU converters, driving the rapid development of traction systems.
3.2 Numerous material defects result in low current per chip.
Although significant progress has been made in SiC device research, its performance is still far from the limits of the SiC material itself. In recent years, remarkable advancements have been achieved in SiC crystals grown using physical vapor transport (PVT) and SiC thin films grown using chemical vapor deposition (CVD). Techniques such as buffer layers, step-controlled epitaxy, and position competition have greatly improved the quality of SiC thin film crystals and enabled controllable doping. However, the crystals still contain numerous defects such as micropipes , dislocations , and stacking faults , which severely limit the yield and high-current requirements of SiC chips.
For SiC power electronic devices to be applied in traction applications, the area of a single chip must be greater than 1.2 cm² to ensure a current carrying capacity of over 100A and reduce parasitic parameters caused by multiple chips connected in parallel. Therefore, SiC materials must overcome the aforementioned shortcomings before SiC devices can be mass-produced and used in traction applications.
3.3 Device packaging materials and technologies need improvement.
Currently, SiC power device packaging processes and methods typically draw on Si IGBT packaging technology. However, there are still some issues in areas such as DBC layout, chip bonding, high-temperature solder, silicon gel filling, and sealing materials, which prevent SiC materials from fully leveraging their advantages in high-temperature and high-frequency applications.
To address the unique packaging requirements of SiC devices, companies such as Mitsubishi, Semikron, and Fuji have proposed new approaches to packaging materials and structures. Examples include Mitsubishi's copper pin wiring technology, Semikron's low-temperature nano-silver sintering technology, and Fuji's low-inductance and optimized DBC layout design. With international manufacturers placing greater emphasis on SiC packaging technology, continuous development of packaging materials, and optimization of packaging structures, packaging will no longer be a bottleneck limiting the performance of SiC devices, and the advantages of SiC materials will be fully realized.
4. Conclusion
Compared to the widely used Si power electronic devices, SiC devices can operate at higher switching frequencies, enabling miniaturization of energy storage and filtering components such as capacitors and inductors; they also offer higher chip power density, reducing the size of devices and power modules; and they have lower losses and higher operating junction temperatures, reducing the size of cooling devices. These superior characteristics collectively drive the development of traction converters towards miniaturization, lightweighting, and high efficiency. Currently, switching devices composed of SiC SBDs and SiC MOSFETs have begun to be used in locomotive traction applications, demonstrating superior performance.
The main factors currently restricting the application of SiC power electronic devices in the traction field include: high substrate and epitaxial costs, resulting in high chip prices; numerous material defects limiting chip yield and single-chip current; and bottlenecks in packaging technology preventing the full realization of SiC material performance. However, it is foreseeable that with the continuous development of SiC material technology and the increased attention paid to SiC devices by major manufacturers, SiC power electronic devices are expected to see significant improvements in yield, reliability, price, and packaging technology in the coming years. They will be widely used in the traction field, gradually demonstrating their performance advantages and reducing the cost of converter systems, thus continuously driving the development and transformation of traction converters.
