Out of the frying pan, into the fire? No problem for silicon carbide ICs

Engineers familiar with silicon-based semiconductors sometimes have a hard time wrapping their heads around silicon carbide electronics. The reason: It is difficult to conceive of a semiconductor device able to work just fine while glowing red hot. That high thermal conductivity, of course, is one of the big attractions of SiC devices.

After years of research and development in the lab, the compound semiconductor material SiC is taking a bigger role in handling electrical power. Though not quite yet a main-stream technology, it is carving out a niche in applications that demand an ability to work at high-voltages and temperatures while demonstrating high efficiency. Several suppliers now provide Schottky barrier diodes based on the technology, with about 40 times lower reverse leakage current than that of silicon Schottky diodes. Also available are a host of junction field effect, bipolar junction, MOS field effect and insulated-gate field-effect transistors (JFETs, BJTs, MOSFETs and IGBTs). Big names in the field include Cree, United Silicon Carbide, Rohm, GE, SemiSouth, Fairchild (formerly Transic) and Infineon Technologies. Mitsubishi, Toshiba and Denso are known to have developed SiC devices for motor drivers and automotive uses.

To be sure, other compound semiconductor materials like gallium nitride (GaN), as well as ordinary silicon, are also improving. However SiC holds the most promise for future wide-band-gap materials as silicon approaches its theoretical limits for handling power. It is safe to say that future power applications demanding voltages upwards of 1,200 V or more will be largely filled by SiC devices, which have bandgaps 3x greater than other compound semiconductor ICs.

This is not surprising given the many useful qualities of SiC materials, not the least of which is ruggedness. This makes them prime candidates for providing the long lifetimes needed for electric vehicles and solar/photovoltaic inverters. In the lab, SiC devices have worked at red hot temperatures of 650°C or more. In actual field applications, operating temperatures of 250°C are becoming the norm. Higher efficiencies also mean smaller size and less need for heat sinking. Compared with ordinary silicon semiconductors, SiC semiconductors feature higher dielectric breakdown voltages (by 10x), lower EMI emissions, faster recovery times and lower forward-voltage drops than diodes.

In the beginning

The first commercially available SiC devices surfaced about a decade ago. The renewed recent interest in them has been driven by advances that include lower production costs, a push for greener energy, and more efficient power conversion. IMS Research senior analyst Richard Eden thinks the global market for SiC devices will reach $100 million this year.

But for now, the technology will remain a niche player. Eden says SiC is unlikely to take over from silicon and gallium-nitride power devices because SiC devices are considerably more costly than both of them. Furthermore, the latest silicon super-junction MOSFETs are pushing toward 600-V operation. Silicon IGBTs are already widely used for 900-V applications and above. And GaN shows similar energy efficiency improvements at voltages of around 200 V. But Eden does see a brighter future for SiC as lower wafer costs push down device pricing.

There's a large market for SiC devices in solar inverters. The market research firm Yole Développement has called a recent move to four-inch SiC wafers significant for these inverters and sees more fabs moving to six-inch wafers. It also sees more SiC diodes and transistors for photovoltaic inverters in the next few years.

In 2005, Japan's Denso Silicon employed SiC devices in power control units (PCUs) for the Lexus LS 600h and the Lexus LS600hL hybrid EVs. The SiC components let these PCUs handle higher power using a smaller package than PCUs using ordinary silicon devices. The Denso PCU consists of a boost converter that raises the main battery voltage (288 V) to the maximum system voltage (650 V), and two inverters that convert dc into ac to drive the main traction motors.

For the PCU, Denso developed a special cooling structure for the power devices. As a result, the SiC-equipped PCU can output about 60% more power per unit volume than that of Denso's conventional technology. A SiC PCU designed to produce as much output as conventional technology can be just 30% of the size and about 20% of the volume of ordinary PCUs.

Large firms like General Electric are also active in SiC production. This year GE Global Research developed a new line of SiC-based power-conversion devices for air, land and sea-going craft. “With SiC technology, we have the potential to reduce the weight on an aircraft by more than 200 lbs. while also delivering higher performance and freeing up precious cargo space,” explains GE Aviation Systems' president of Electrical Power Vic Bonneau.

SiC technology also has its share of academic research efforts. One such program is at the University of Warwick in the UK which houses a special laboratory for materials physics and fabrication technology on SiC devices. The lab gets funding from a regional development agency called Advantage West Midland and an EU funding entity called the European Regional Development fund.

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At Wayne State University's Smart Sensors and Integrated Microsystems (SSIM) laboratory, scientists under the direction of Greg Auner are developing a wide range of high-temperature and high-voltage semiconductor materials that go way beyond the capabilities of silicon. One of those materials is SiC, which is being investigated for use in a SiC-based hydrogen sensor.

At the North Carolina State University, Alex Huang, profesor of electrical engineering, heads up an effort to reinvent utility-scale transformers by making them smarter, smaller and more friendly to the electric power grid (“Modern Metering, Old Grid,” EE&T May/June, 2011). He is developing power semiconductors based on SiC technology that can handle thousands of volts. His work is part of a smart grid program called the Future Renewable Electric Energy Delivery and Management Systems (FREEDMS), headquartered at NC State.Diodes and transistors

A number of companies now make SiC Schottky barrier diodes, transistors and other power devices, some with record performance levels. Toshiba, for example, has grown hexagonal crystalline SiC (4H-SiC) to produce a special floating-junction Schottky barrier diode that can operate at up to 2,700 V and has an on-state resistance of a mere 2.57 Ω/cm², thanks to the presence of the floating junction.

It is designed for power sources and inverter drives. The device came out of research enabled by Japan's Ministry of Economy, Trade and Industry and its New Energy and Industrial Technology Development Organization (NEDO).

Mitsubishi Electric is also active in high-power SiC devices. It is developing SiC MOSFETs and Schottky barrier diodes that can be used in 11-kW inverters.

In the U.S., Fairchild Semiconductor recently acquired TranSiC, a leading developer of bipolar SiC power transistors. It is developing compact high-voltage SiC power transistors that target the oil and gas industry, where down-hole temperatures of 250°C are common.

Infineon Technologies AG offers the SiC thinQ Schottky barrier diode for 1,200-V operation in TO-247HC high-creepage packages, so called because they have a higher creepage distance that boosts the safety margin for short circuits, especially arcing, which might be triggered by the presence of dust or dirt in the wrong spot. This package design reduces the need for additional chemical (silicone gel or cream) or mechanical means (sheaths or foils) of avoiding pollution between the package leads.

Specifically, the new package complies with the standard TO-247 package but adds the extra creeping distance between the device pins (6.3 mm versus 2.7 mm). Infineon also produces SiC JFETs. It has set up a joint venture called SiCED with Siemens to produce high-power SiC devices.

Rohm Semiconductor USA recently came out with the 600-V SCSIxxAGC family of SiC Schottky barrier diodes. Besides the low-forward-voltage and fast-recovery features that come with Schottky barrier construction, they reduce switching losses by two-thirds compared with conventional fast-recovery silicon diodes. They are also stable for use as blocking diodes in active boost converters for switch-mode power supplies as a means of meeting European and U.S. requirements for EMI and efficiency. The family features devices with maximum forward voltage drops of 1.7 V, continuous forward current as much as 20 A, reverse currents as low as 120 µA, and switching times as short as 15 nsec.

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Rohm is also producing SiC MOSFETs for EV drives. Its 1,200-V SCH2090KE n-channel MOSFET in a TO247 package has a has a continuous drive current, I_D, of 26 A. The 600-V SCH2200AX n-channel MOSFET in a TO-220FM package has a V_dss of 10 A.

Long-time SiC device maker Cree Inc. also makes available SiC Schottky barrier diodes and MOSFETs. Its 1,200-V Z-Rec is said to be an economical device for power-conversion applications. The devices are designed for use as boost and anti-parallel diodes in solar inverter and motor drives, as well as for power factor correction (PFC) in power supplies and uninterruptable power supplies (UPSs). ¡°This new family of diodes allows a higher current density and more avalanche capability over previous generation Schottky barrier diodes with no penalty in performance,¡± explains Cree co-founder and chief technical officer John Palmour.

When used with Cree's recently introduced 1,200-V SiC MOSFETs (the CMF20120D), the diodes enable the implementation of all-SiC power circuits able to operate at up to four-times-higher switching frequencies when compared with conventional silicon and IGBT devices. Cree claims the CMF20120D SiC MOSFET delivers 1,200-V blocking, with the lowest switching losses in the industry. Its on-state resistance is just 80 m§Ù at 25¡ÆC.

SemiSouth Laboratories Inc. offers the SJDP12-0R045 SiC 1200-V JFET with a record low RDSon of 45 m §Ù (0.045 §Ù). The fast-switching normally-on device features no ¡°tail¡± current during turn-off and four- times faster switching frequency than competitive 1,200-V devices.

SiC transistors aren't just for driving motors and super-efficient power supplies. SemiSouth also recently developed inexpensive high-linearity and low-distortion SiC JFETs for high-end audio uses. The SJEP120R100A/ SJEP120R063As in TO-247 packages are said to cost 15% less than conventional SiC devices for high-end audio applications and are compatible with standard gate driver ICs. The positive temperature coefficient of the SiC devices minimizes the chance of thermal runaway in circuits that parallel the transistors. Other advantages of the SiC parts in high-end audio include a maximum operating temperature of 150¡ÆC, and a low R_DS on of 0.100 and 0.063§Ù, respectively. ¡°In push-pull topologies, these parts exhibit a 50 to 70% improvement in distortion. In single-ended circuits, this improvement has been nearly ten-fold,¡± says Nelson Pass, founder of leading audio amplifier maker Nelson Pass Inc.

Also offering a wide range of SiC devices is United Silicon Carbide Inc. It develops JFETs, BJTs and Schottky barrier diodes used in EVs, hybrid EVs (HEVs), industrial motor control and high-performance military and aerospace applications.

Needed: larger wafers

Engineers familiar with the platter-sized wafers that give rise to mainstream computer chips may be surprised to find that many commercially available SiC devices are still made from 3-in.-dia. (sometimes even 2-in.-dia.) wafers. The industry is slowing transitioning to 4-in.-dia. wafers for SiC. But industry observers see an eventual move to 6-in. SiC wafers, which should help narrow the cost differential with less expensive compound semiconductor devices like GaNs.

The smaller wafers are clearly problematic. Larger wafers let a semiconductor manufacturing fab of a given size exhibit greater throughput, thus spreading fixed production costs over more product. For example, consider that a 4-in. wafer has 78% more surface area than a 3-in. wafer. So even small increases in wafer size have favorable economics.

Then there's the issue of SiC crystal purity. ¡°Present crystals grown for SiC power ICs have defect densities greater than 1,000 defect dislocations/cm². For comparison, commercial silicon crystals have less than 1 dislocation/cm². This needs improvement before SiC applications can take off,¡± explains Phil Neudeck, a researcher at the NASA Glenn Research Center's Sensors and Electronics branch.

His group has a patented crystal growth technique that they hope will meet this challenge. ¡°Although the crystal growth process for SiC on 4-in. wafers is commercially quite advanced compared with where it was 10 years ago, it is not where it needs to be compared with silicon,¡± he adds.

Elsewhere, scientists at Caracal Inc. are actively working on SiC growth techniques that will let wafer diameters grow rapidly from 3 in. to 4 in. and beyond, while simultaneously eliminating the vast majority of defects. The company specializes in studying crystal growth and epitaxial growth of SiC materials.

Japan's Denso has also claimed it is trying to reduce SiC wafer defect densities and has gotten the figure down to hundreds of dislocations/cm². Other Japanese, European and U.S. companies and research centers are also pursuing this effort.

Finally, SiC isn't just a semiconductor. Silicon carbide powder has been mass-produced since the 1890s for use in abrasives. A form of SiC has also been used as high-performance brake discs, and SiC (as well as other ceramics) have gone into bulletproof vests. One of the more recent applications that uses the material's physical qualities can be found at the NASA Goddard Space Center, where researchers have come up with reaction-bonded SiC Etalon mounts for Fabry-Perot interferometers.

Heretofore, tunable Fabry-Perot interferometers, including mounting hardware, have been made from the low-thermal-expansion material Invar, a nickel/iron alloy. Problem is, the steel is relatively heavy. The switch to SiC mounts has significantly reduced the interferometer's mass.

Resources

Proceedings of the International Conference on Silicon Carbide and Related Materials (ICSCRM-2011), held in Cleveland, OH, Sept. 11-16, 2011 and organized by the NASA Glenn Research Center, www.icscrm20011.org.

¡°Silicon Carbide Power Devices¡± by Jayant Baliga, World Scientific Publishing Co. Inc.

¡°Ultralow-Loss SiC Floating Junction Schottky Barrier Diodes (Super-SBDs),¡± Nishio, J. Ota, C. Hatakeyama, T. Shinohe, T. Kojima, K. Nishizawa, S. Ohashi, H., Corp. R&D Center, Toshiba Corp., Kawasaki, http://tinyurl.com/65ts8p2

IMS Research, www.imsresearch.com

Cree, Inc., Durham, NC, 919-313-5300, www.cree.com

United Silicon Carbide Inc., Monmouth Junction, NJ, 732-355-0550, www.unitedsic.com/

Rohm Co. Ltd., Japan, www.rohm.com

GE Aviation, Oshkosh, WI, http://tinyurl.com/62fhtnb

SemiSouth Laboratories Inc., Starkville, MS, 662-324-7607, www.semisouth.com

Fairchild Semiconductor (formerly Transic), Sweden, www.transic.com

Infineon Technologies AG, Milpitas, CA, 866-951-9519, www.infineon.com

Mitsubishi Electric, Tokyo, www.mitsubishielectric.com

Denso Silicon, www.denso-europe.com

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