What Is SiC?
Silicon carbide is a crystalline semiconductor material with the chemical formula SiC. Its structure is hexagonal (4H-SiC), has an energy band-gap of 3.26eV, electron mobility of 900cm2/VS , a thermal conductivity of 4.9W/cm2, and breakdown field of 3 x 106 V / cm.
SiC has actually been used in electronics for more than 100 years since being used in lighting arresters in mains distribution. When its junction capabilities were discovered, however, SiC quickly became used in radio equipment as the diode element (i.e., the carborundum).
A silicon carbide wafer. Image courtesy of STMicroelectronics.
SiC was the semiconductor used to produce the first LEDs due to its electroluminescent properties but gallium nitride (GaN) quickly replaced SiC after it was discovered that GaN was up to 1000 times more efficient due to its direct band-gap favoring photon emission.
The Ever-Expanding SiC Landscape
While SiC devices aren't a silver bullet solution for every application, many corporations are investing in continuous research into SiC, including alternative packing methods and ways to improve the performance of extent SiC devices.
Here's a quick look at some of the companies currently offering SiC MOSFETs, diodes, and modules.
Wolfspeed is one company in particular that strongly focuses on SiC MOSFETs for power applications including factories, vehicles, and even IT power supplies. Their SiC device portfolio includes SiC MOSFETs that have blocking voltages of 1000V, SiC Schottky diodes for blocking voltages up to 1700V, and even half-bridge modules that can handle currents of up to 120A at 1200V.
The CAS120M12BM2 1200V 120A half-bridge. Image used courtesy of Wolfspeed
ROHM Semiconductor is another company that produces SiC-based MOSFETs which have a stated loss reduction of 73% when compared to IGBTs. Their range of MOSFETs can handle up to 1700V with an on resistance ranging from 45mΩ to 1150mΩ. They are offered in TO-247N, TO-3PFM, TO-268-L, and TO-220 IC packages.
ROHM also produces SiC Schottky barrier diodes which have short recovery times, high-speed switching capabilities, low-temperature dependence, and a low forward voltage with their diodes handling up to 1700V and currents between 10A to 100A.
A few packages that ROHM uses for their SiC devices. Image used courtesy of ROHM
Infineon offers its own portfolio of SiC components, including MOSFETs and Schottky diodes for automotive applications under the product category name CoolSiC. According to Infineon, SiC is allowing "radical new product designs with best system cost-performance ratio". Infineon poses their CoolSiC MOSFET portfolio as an alternative to silicon-based IGBTs and MOSFETs.
United SiC (as indicated by their name) is one of the companies focused on SiC development, especially SiC FETs that utilize unique cascode configuration.
Let us know which other companies are offering SiC portfolios.
Why Is SiC a Promising Semiconductor?
While silicon has been the choice of semiconductor for most applications for the past 50 years the demands from the industry for high powered, efficient, and small devices is putting too much strain on silicon.
So why are so many large corporations investing in SiC devices?
SiC as a semiconductor has several advantages over silicon including
- Higher voltages: SiC can handle up to 10 times the voltage of silicon
- Higher current: SiC can conduct as much as 5 times the amount of silicon
- Higher temperature: SiC can operate normally up to temperatures of 400°C
- Higher thermal conductivity: SiC can dissipate more heat that silicon (up to 3 times)
- Faster switching: SiC is capable of switching 10 times faster
- Higher band-gap: The SiC band-gap is almost 3 times that of silicon
A semiconductor that, overall, has better characteristics would be a far better choice for many applications ranging from high power radio equipment to processor design whereby small high-speed transistors could easily stay cool, operate efficiently, and continue to improve processing capabilities. So, if SiC is so great why are semiconductor companies not throwing out their old silicon stock and placing orders for SiC?
Challenges to Adopting SiC
SiC is already being used in many industries including abrasives, heating elements, and even cladding for the nuclear industry. It does, however, face some issues as a semiconductor.
One issue with SiC devices is their awkward driving requirements. One of the main goals for SiC is that they could replace IGBTs but the driving requirements for these two devices are very different. While most (if not all) transistors typically have driving requirements that use symmetric rails (such as ±5V) SiC devices require a small negative voltage to ensure that they are fully off and so require rails that are asymmetric (such as -1V to -20V). While such voltages can readily be generated using tapped transformers it can provide a challenge for portable equipment as additional DC-DC drivers will be needed or specialized batteries with three connections (+, 0V, and -).
Another issue is that, while SiC has fantastic thermal properties and is capable of conducting large amounts of thermal energy compared to silicon, packing technology is still yet to catch up. Currently, most SiC parts are housed using packing that is used for silicon, such as die bonding and wire bonding. These methods are cheap, well-tested, and commonly available. While this method for packaging can be used with SiC, it is only practical for low-frequency circuits (tens of kHz). As soon as high frequencies are used, parasitic capacitance and inductance become too great which prevent the SiC-based device from realizing its full potential. This means that widespread use of SiC may require updates to manufacturing facilities, something which cannot be achieved at the drop of a hat.
The Future of SiC
SiC as a semiconductor clearly has many advantages over silicon—but the current high production costs can be limiting for smaller companies. When it comes to power circuitry, silicon as we've known it will no doubt be phased out over time—but that does not mean SiC will take its place. There are, after all, other alternatives to consider.
Gallium nitride, for example, has similar advantages such as high-temperature handling, high voltage, and high current capabilities. SiC MOSFETs also appear to be available in n-channel types only which makes push-pull arrangements more of a challenge and this also impacts the possibility of producing CMOS logic using SiC. P-type devices may be possible but such devices may not have the same electrical capabilities as their N-type counterparts which would make them useful for logic but not for power circuitry.
Regardless of the challenges to come, the industry clearly has a growing interest in SiC devices that can handle greater voltages and currents as well as being able to operate at much higher temperatures than silicon. SiC may or may not be the semiconductor of the far future but it will certainly play a significant role in power electronics for at least the next decade.
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