Power Electronics

SiC: A Rugged Power Semiconductor Compound To Be Reckoned With

Smaller packages and higher performance will be realized from Silicon Carbide in applications where conventional silicon compounds fall short.

After years of research and development in the lab, Silicon Carbide (SiC) is poised to make its mark as a commercially available compound semiconductor technology for energy efficient power control applications. A few companies claim they can supply several power device functions, much of it as Schottky barrier diodes. Others claim they have available transistors, thyristors, as well as junction field-effect, bipolar junction, MOS field-effect, and insulated-gate field-effect transistors (JFETs, BJTs, MOSFETs and IGBTs).

As for market availability, separating fact from fiction is difficult. Some companies assign part numbers and data sheets. Others claim impressive performance parameters but have no data sheets. Then there are those that claim they’re working on devices that should soon be ready. What is obvious is that they’re all in the SiC power device business.

Compared to silicon, SiC is more efficient for switching power, has lower losses at higher switching frequencies, and can operate safely at much higher temperatures. In the lab, it has demonstrated operation at “red hot” temperatures of 650ºC or more, making operating temperatures of 250ºC become the norm. SiC’s higher efficiencies also mean smaller size and less need for heat sinking. It also features higher dielectric breakdown voltages (10x higher), lower EMI emissions, faster recovery times and lower forward-voltage drops as diodes than silicon.

What kinds of applications require such high power levels? Surprisingly, quite a few, like automotive, military/aerospace, motor drives, solar inverters, the smart grid, high-end audio, geophysical exploration, rail transportation, power factor correction (PFC) for power supplies and uninterruptable power supplies (UPSs), HVAC, hybrid electric vehicles, and some high-performance functions silicon can never handle.

To put things in perspective, another compound semiconductor technology like gallium-nitride (GaN) is also making its own advances. However, SiC holds the most promise for as a future wide-band-gap material that can handle voltages of 1200 V or more, as it has three times the bandgap than other compound semiconductors.

Suppliers

A number of companies are said to supply SiC Schottky barrier diodes. These include Cree Inc., United Silicon Carbide Inc., Rohm Semiconductor Inc., GE, SemiSouth Laboratories Inc., Fairchild Semiconductor Inc. (formerly Transic), GeneSiC Semiconductor Inc., and Infineon Technologies Inc. Some of them also say they offer other SiC device functions like transistors, thyristors, JFETs, BJTs, MOSFETs and IGBTs.

A large supplier of SiC devices is Cree. It makes available Z-REC and Z-FET 600-V, 1200-V and 1700-V Schottky barrier diodes and MOSFETs in TO-220, TO-247, D-PAK and D2PAK packages. Most recently, it has provided these devices in qualified “bare die” or chip form for 1200-V diodes and MOSFETs to be integrated in modular power packages (Fig. 1).

“Manufacturers of power modules can now realize the performance advantages of 1200-V SiC devices—better high-temperature operation, higher switching frequencies and lower switching losses—without the limitations imposed by conventional plastic packaging of discrete devices,” explains Cengiz Balkas, Cree’s vice president and general manager of power and RF products.

GeneSiC Semiconductor is one company that offers a wide variety of SiC device functions that include Schottky barrier diodes, super-junction transistors (SJTs), thyristors and modules (Copaks) that include silicon IBGTs and SiC rectifiers. Its 1200-V 220-mΩ SJTs deliver performance unparalleled by other semiconductor technologies (see the table).

This device features high-temperature (>300ºC) operation, <15-ns switching time, extremely low losses, and short-circuit withstanding time of 22 µs (see “SiC ‘Super Junction’ Transistors Deliver High Temp Performance,” Power Electronics Technology, Nov. 2011, p. 21).

GeneSiC Semiconductor also makes available the 1200-V/20-A GBL10SLT12 SiC Schottky barrier diode, and the 1200-V/35-A GA35XCP12 silicon IGBT and SiC diode combo, both in TO-247 packages. It also offers the 1200-V/100-A GAX100 CP12 silicon IGBT and SiC diode combo Co-pack in a TO-227 package (Fig. 2).

Infineon Technologies offers the 1200-V thinQ diode in a TO-247 package. The thinQ term refers to the 6.3-mm creepage distance between the TO-247’s pins (versus 2.7 mm for others’ products) to boost the safety margin between pins for short circuits. The company is also working on a SiC JFET.

SemiSouth Laboratories Inc. makes available the SJDP120 1200-V JFETs in TO-247 packages. And Union Silicon Carbide claims that it is ready to ship diodes, JFETs and BJTs but does not provide any part model numbers or other data.

Fairchild Semiconductor is said to be developing compact high-voltage transistors for the oil and gas industry, where down-hole temperatures of 250ºC are common. Fairchild recently acquired TranSiC, a leading developer of bipolar SiC power transistors.

Japanese suppliers

Japan’s-based Rohm Semiconductor USA is one supplier with the 600-V SCS family of Schottky diodes in TO-47 and TO-220 packages. It also offers the 1200-V SCH2090KE n-channel MOSFET in a TO-247 package.

Japanese companies are keen on developing SiC device technology for a number of applications, including automotive and solar inverter uses. In fact, as far back as 2005, Japan’s Denso Silicon employed SiC devices in power control units (PCUs) for the Lexus LS 600h and the Lexus LS600hL hybrid EVs.

These PCUs can produce higher power with a smaller size than PCUs using silicon devices. A 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 unique cooling structure that significantly improves cooling performance for the power devices. As a result, the SiC-equipped PCU can produce output power that is approximately 60% higher per unit volume, compared to Denso’s conventional technology, improving hybrid system performance. When the PCU is designed to produce the same output as conventional technology, it can be reduced approximately 30% in size and approximately 20% in volume.

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

Toshiba is growing hexagonal crystalline SiC (4H-SiC) to produce a floating-junction Schottky barrier diode that can operate up to 2700 V. The floating junction allows it to have an on-state resistance of a mere 2.57 mΩ/cm2. This device was designed for power sources and inverter drives.

Most recently, Nippon Steel Corp. successfully developed at its Advanced Technology Research Laboratories 6-in.-diameter SiC single-crystal wafers, a key for mass production and the spread of high-performance, high-quality and low-cost future power semiconductor SiC ICs.

Large-diameter, high-quality

The issue of developing large-diameter high-quality SiC wafers is a crucial one. Presently, most SiC devices are made on 2-in. and 3-in. diameter wafers, but the industry is rapidly transitioning to using 4-in.-diameter wafers. Besides their smaller diameters compared to silicon which translates into lower costs (for silicon), SiC wafers do not have the defect density levels of silicon to be practical.

Whereas commercial silicon crystals have defect densities of less than 1 dislocation/cm2, SiC wafers have defect densities of more than 1000 dislocations/cm2. This number represents a vast improvement over SiC defect densities about a decade ago, but needs further improvement.

Researchers at the National Aeronautics and Space Administration (NASA) are addressing this challenge. They have patented a SiC growth technique they hope will meet the high defect-density problem.

Given the growth in the use of solar energy, SiC devices for solar inverter applications are predicted to be a growing market, according to market research firm Yole Développment. The company has identified the present move to 4-in. SiC wafers as a significant development for photovoltaic inverters where it sees the wafer market for 6-in. wafers in the future taking a bigger chunk of the total wafer market of both 4- and 6-in. wafers (Fig. 3). It also forecasts a rapid increase in the market for SiC diodes and transistors for photovoltaic inverters in the next few years as they take on a bigger chunk of the total silicon plus SiC market for photovoltaic inverters (Fig. 4).

Large firms like General Electric (GE) are also active in the SiC arena for aerospace and military applications. GE has partnered with GE Global Research and announced a new line of SiC-based power-conversion devices for air, land and sea-based platforms. This is part of GE’s broader and long-term research strategy to pursue advanced energy conversion technologies to address the increasing demand for electric power in aviation and other industries. “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 Vic Bonneau, president of Electrical Power for GE Aviation Systems.

At the British University of Warwick, SiC and power electronics are a major research target of study at the school’s special laboratory for materials physics and fabrication technology on SiC devices. The lab is supported by funding from Advantage West Midland and the European Regional Development fund as part of Birmingham Science City Research Collaboration (SCRA).

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.

Design Tools Available

The design of efficient SiC-based power systems requires a thorough knowledge of many aspects of the technology. It requires a thorough understanding of the physics of SiC devices, their thermal behavior, and the use of calibrated compact models that can be included in industry standard simulation tools such as SPICE and MATLAB Simulink. Unique aspects of 4H-SiC based devices such as interface trap states, incomplete ionization, transition region, surface roughness and Coulomb scattering mobility need to be understood and incorporated in the device models to carry out accurate modeling and simulation.

This is what the Virtual Design Platform from CoolCAD LLC offers (Fig. 5). It enables modeling and simulation of SiC devices and circuits using a multi-tiered approach. It was developed by CoolCAD LLC in partnership with iMPower Systems Inc. Cree Inc., General Atomics Inc., and the U.S. Army Research Laboratory are assisting in the design of this software tool, and will also act as test sites for the beta version. The beta version release is in testing, and the software will be ready for eventual release this year.

The platform provides designers with physics-based TCAD models to create behavioral models for SiC devices. The models can be used in circuit and system simulations.

The platform is said to provide a crucial, economically indispensable link between SiC power device manufacturers (CREE, SemiSouth, Infineon, etc.) and commercial (GM, Ford, etc.) and military (General Atomics, General Dynamics, Raytheon, etc.) power electronics system manufacturers.


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