Silicon Carbide (SiC): History and Applications
Contributed By Digi-Key Electronics
The only compound of silicon and carbon is silicon carbide (SiC), or carborundum. SiC does occur naturally as the mineral moissanite, but this is extremely rare. However, it has been mass produced in powder form for use as an abrasive since 1893. As an abrasive, it has been used for over one hundred years in grinding wheels and many other abrasive applications.
With today’s technology, high-quality technical grade ceramics have been developed with SiC which exhibit very advantageous mechanical properties such as:
- Exceptional hardness
- High strength
- Low density
- High elastic modulus
- High thermal shock resistance
- Superior chemical inertness
- High thermal conductivity
- Low thermal expansion
These high strength and very durable ceramics are widely used in applications such as automotive brakes and clutches along with ceramic plates embedded in bulletproof vests. Silicon carbide is also used in semiconductor electronic devices operating at high temperatures and/or high voltages such as flame igniters, resistance heating, and harsh environment electronic components.
Automotive uses of SiC
One of the primary uses of silicon carbide is high performance "ceramic" brake discs. The silicon combines with the graphite in the composite to become carbon-fiber-reinforced silicon carbide (C/SiC). These brake discs are used on some sports cars, supercars, and other performance vehicles.
Another automotive use of SiC is as an oil additive. In this application, SiC reduces friction, emissions, and harmonics.
Early uses of SiC
Electroluminescence was first discovered in 1907 using silicon carbide light emitting diodes (LEDs). Shortly thereafter, the first commercial LEDs produced were SiC-based. The Soviet Union manufactured yellow SiC LEDs in the 1970s while blue ones were manufactured in the 1980s worldwide. Then, with the introduction of gallium nitride (GaN) LEDs, which can produce ten to one hundred times brighter emissions, SiC LED production was all but halted. Nevertheless, SiC is still a popular substrate for GaN devices and it is also utilized as a high-power LED heat spreader.
SiC has a high resistance until a threshold voltage (VT) is reached, at which point its resistance drops to a much lower value until the applied voltage drops below VT. One of the earliest SiC electrical applications that took advantage of this property was lightning arresters in electric power distribution systems (Figure 1).
Figure 1: SiC lightning arrester application (Image courtesy of ArresterWorks).
With its voltage-dependent resistance, SiC pellet columns were connected between high-voltage power lines and the earth ground. If lightning strikes the power line, the line voltage rises over the VT of the SiC arrester causing the strike current to conduct to ground and pass harmlessly to the earth rather than along the power line. However, these SiC arresters were determined to conduct significantly at normal operating power-line voltages. This required them to be placed in series with a spark gap. When a lightning strike raises the voltage of the power line conductor, the spark gap ionizes and conducts, effectively connecting the SiC arrester to the power conductor and earth ground. Later it was found that spark gaps used in lightning arresters were unreliable. They either failed to strike an arc when needed or the arc failed to quench when the lightning event ended due to material failure or dust or salt contamination. SiC lightning arresters were originally intended to eliminate the need for the spark gap, but due to their unreliability, the gapped SiC arrester has largely been replaced by no-gap varistors that use zinc oxide pellets.
SiC in power electronics
Several semiconductor devices are produced using SiC including Schottky diodes (also called Schottky barrier diodes, or SBDs), junction-gate FETs (or JFETs), and MOSFETs utilized in high-power switching applications. The first commercial 1200 V JFETs were introduced in 2008 by SemiSouth Laboratories (which closed its doors in 2013), followed in 2011 by the first commercial 1200 V MOSFETs being manufactured by Cree. In this same timeframe, some companies started implementation of bare SiC Schottky diode chips into their power electronic modules. In fact, SiC SBDs are widely used in IGBT power modules and power factor correction (PFC) circuits.
Figure 2: Representation of SiC components: Schottky Diode, JFET, and MOSFET.
SiC pros and cons
What makes SiC-based power electronics components so attractive is the fact that for a given blocking voltage, the doping density can be almost one hundred times higher than in silicon-based devices. The result of this is a high blocking voltage can be obtained with a low on-resistance. A low on-resistance is crucial for high-power applications since less heat will be generated as the on-resistance decreases, reducing system thermal load and increasing overall efficiency.
However, there are a few difficulties inherent in manufacturing SiC-based electronic components, with the elimination of defects being the foremost problem. These defects result in poor reverse blocking performance in components made of SiC crystals. In addition to this crystal quality problem, difficulties in the interface of silicon dioxide with SiC have hindered the advance of both SiC-based power MOSFETs and insulated-gate bipolar transistors. Fortunately, using a nitridation process during production has resulted in significantly decreasing the defects causing these interface problems.
SiC abrasive films
Silicon carbide is still used as an abrasive in many industrial applications. In the electronics industry, the main use is in lapping films which are used for polishing the ends of fiber-optic strands prior to splicing. These films produce the high surface finishes required for fiber-optic splices to function in the most efficient manner.
Silicon carbide has been manufactured for over one hundred years. However, it has only been recently that SiC has seen use in the power electronics industry. Its physical and electrical properties make this material particularly useful in high voltage and high temperature applications.
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