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SiC’s bandgap, the energy needed to excite electrons from the material’s valence band into the conduction band. [...] need about three times as much energy to reach the conduction band, a property that lets SiC-based devices withstand far higher voltages and temperatures than their silicon counterparts.
[...]
Silicon devices can’t withstand electric fields in excess of about 300 kilovolts per centimeter. [...] SiC can withstand much stronger electric fields, up to about 10 times the maximum for silicon. [...] a SiC device can be less than a tenth the thickness of a silicon device but carry the same voltage rating [...] These thinner devices are faster and boast less resistance [...] To sustain voltages beyond about 200 volts, a silicon transistor has to be quite thick. This added thickness boosts resistance, which in turn demands impractically large devices in order to maximize current-carrying capacity. To mitigate this problem, high-voltage silicon switches tend to be bipolar transistors: They use both holes and electrons. The design carries more current, but it takes time for all the charge carriers to fully exit the device. When the transistor is being switched from its “on,” current-carrying state to its “off,” voltage-blocking state, there is a period of overlap where the remaining charge carriers are exposed to high voltage and dragged through the device, dissipating heat. Using silicon carbide instead of silicon in high-voltage devices will let manufacturers replace slow silicon bipolar transistors with single-carrier, or unipolar, devices such as metal-oxide-semiconductor field-effect transistors, or MOSFETs. Very few charge carriers are left behind in such devices, so the transistors can be switched quickly and far more efficiently.
[...]
For all its fine qualities, silicon carbide has been a difficult material to master. One of the biggest hurdles to its widespread use in power electronics has been in wafer manufacturing. When engineers first started working with the material in the 1970s, they struggled to grow large single crystals of the stuff—the silicon and carbon atoms had a habit of combining with one another to form a hodgepodge of different crystalline structures.

Over the years, researchers succeeded in creating larger and larger single-crystal wafers. And in 1991, a few years after the company was founded, Cree released the first commercially available SiC wafers. They were just an inch across and used mostly for research, but it was a start. Since then Cree and other manufacturers, including Dow Corning, SiCrystal, TankeBlue, and II-VI, have made steady progress in boosting the size of the wafers; these days 4-inch SiC wafers are common, and 6-inch wafers are on the horizon.
[...]
In less than 10 years, SiC Schottky diodes have all but replaced the silicon p-n diodes in switched-mode power supplies for computers, particularly those in large data centers. Manufacturers now offer SiC Schottky diodes that can withstand voltages as high as 1700 V, more than five times the maximum voltage of comparable silicon devices.
[...]
It wasn’t until 2008 that the first SiC transistors—junction field-effect transistors (JFETs) manufactured by Mississippi-based SemiSouth Laboratories—finally hit the marketplace. The number of transistor offerings has since boomed. SiC transistors with a range of architectures are now offered by the likes of Cree, Infineon, Rohm, and TranSiC.
[...]
for all-electric and hybrid electric vehicles. After the battery, electronics are the key added cost to these vehicles. Electronics are needed to convert wall power into battery power, to recharge the battery from the engine or from the brakes, and most important, to operate the traction drive, which transforms battery power into electricity that can run the motors that propel the vehicle. Of all the electronics in an electric vehicle, the traction drive draws the most power.

The drive has two main parts: a boost converter that increases the DC voltage from the battery and an inverter that converts this electricity into the three-phase AC needed by the motor. The three-phase inverter in turn consists of six diodes and six transistors. In computer and laboratory simulations at Oak Ridge, we’ve shown that simply swapping silicon diodes with SiC Schottky diodes cuts the inverter’s energy loss by 33 percent, consistent with other estimates. The reduction doubles if you also replace the silicon transistors with SiC transistors. This boost in efficiency results mainly from SiC’s lower resistance—which means it loses less power to heat—and from faster, more efficient switching.
[...]
DOE’s Advanced Research Projects Agency–Energy [...] Two grants went to teams led by Cree and GeneSiC Semiconductor that are exploring ways to make SiC devices that can operate at more than 10 000 V, up to 15 000 V [...] if the research succeeds, it will pave the way for new devices that can connect distribution lines to higher-voltage transmission lines. At present, that job is performed by massive, multiton transformers, which dominate power substations. Someday, though, utility companies could replace these behemoths with far more efficient solid-state transformers, each the size of a suitcase.
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http://spectrum.ieee.org/semiconductors/materials/silicon-carbide-smaller-faster-tougher
http://spectrum.ieee.org/semiconductors/materials/silicon-carbide-smaller-faster-tougher

Labels: Oleg Zabluda

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