Silicon carbide (SiC) is an exceptionally hard crystalline material composed of silicon and carbon. It was discovered in 1891 by Edward G. Acheson after he heated clay and powdered elemental carbon in an iron bowl in an attempt to synthesize diamonds.
Processing
Typically for industrial applications, silicon carbide is synthetically produced using the Acheson process which involves melting silica sand and elemental carbon in a graphite resistance furnace at temperatures around 2,500°C. The resulting aggregated product is screened and ground for different applications. As a semiconductor, it is synthesized through chemical vapor deposition which occurs in a vacuum environment. Volatile compounds containing carbon and silicon are made to react at high temperatures in the presence of a special blend of gases. The resulting large SiC crystals deposit onto a substrate. Many polymorphs exist for silicon carbide, with 4H-SiC being the most ideal for power devices due to its superior electronic properties. SiC can also be doped to create n-type and p-type semiconductors.
Structure and Properties
With an abundant supply of raw materials, silicon carbide is used across a range of industries. It is commonly used as an abrasive due to its physical hardness and wear resistance. Silicon carbide has high thermal conductivity, high-temperature strength, and a low thermal expansion coefficient. It is able to quickly dissipate heat and resist thermal shock, making it an excellent refractory and heating element material.
Used in metal oxide semiconductor field-effect transistors (MOSFETs) and Schottky diodes, silicon carbide is poised as a viable successor to silicon for next-generation power device applications. Power semiconductors operate as a switch allowing power to flow and stop depending on the state (on or off). The best way to understand SiC’s performance as a power semiconductor is to compare it to conventional silicon semiconductors known as insulated gate bipolar transistors (IGBTs). Understanding the structural differences between these two materials helps explain why silicon carbide performs better than silicon especially in high power applications.
Silicon carbide’s bandgap ranges between 2.3 to 3.3 eV, 3x higher than that of silicon. While this makes it harder for the electrons in silicon carbide to reach the conduction band, it allows SiC to withstand electric fields 10 times higher than silicon. Because of this, silicon carbide can accommodate higher voltages before breaking down. Alternatively, this means that a device with the same voltage difference can be reduced 10 times in size.
Smaller devices that can maintain the same voltage difference have higher switching speeds and lower on-state resistance. This results in smaller control circuitry, less energy loss, and overall greater system efficiency. Additionally, silicon carbide’s higher thermal conductivity allows it to keep operating at higher temperatures than silicon, further increasing its applicability.
Huge capital costs associated with the manufacture of SiC, however, is limiting its widespread use. Currently, silicon carbide wafers can be grown up to 6 inches only compared to silicon’s 8-12 inches. It is also challenging to fabricate high-quality SiC wafers with little defects. Inspection of these defects is further hindered by SiC’s high refractive index.
Real-World Applications
The unprecedented increase in energy efficiency brought about by SiC is particularly appealing to a wide range of energy-based industries such as electric vehicles and solar energy systems. SiC is able to answer the huge voltage demands of these systems and accommodate the resulting high voltages and high temperatures. In electric vehicles, silicon carbide can increase the entire inverter system’s efficiency by nearly 80% because it can handle design requirements at a much-reduced size. It also shows promise in optimizing the fast-charging process. With its high switching frequency, the use of SiC leads to smaller circuit magnetics in solar inverters and ensures the stability of the solar systems over longer periods of time.
Fabrication technologies are ramping up in order to keep up with the increasing demand for SiC. Beyond the current set of challenges associated with its production, the widespread use of SiC can ultimately spell a massive reductions in our carbon footprint towards a more energy-efficient and sustainable future.