industry SiC electronics
Exploiting the high temperature promise of SiC
Replace silicon diodes and transistors with those made from SiC and the operating temperature of power electronics can soar to such an extent that bulky thermal management systems are no longer
needed.The upshot: Squeezing grid-scale renewable energy inverters, downhole electronics and aerospace engines and actuators into far smaller spaces, says Ranbir Singh of GeneSiC.
E
ngineers searching for ever-increasing stealth in fighter jets, greater fuel-efficiency in airliners and vehicles, and compactness in grid scale solar inverters are united in one ambition: To throw out liquid- cooling loops, because this trims the size, weight, volume and cost of the electronic systems.
In all these applications, consumption and generation of electrical power occurs at different voltages and currents. For example, solar panels generate a low- voltage DC output, but this must be transformed into a high-voltage AC output before it is fed into the grid. This conversion process takes place in electronics circuits built from power semiconductors and passive components, such as inductors and capacitors. Electrical conversion is not 100 percent efficient, with losses converted into heat, which must be managed to ensure that these systems function optimally. Often, the thermal management apparatus is bigger than the circuits – and this difference is even more pronounced when the entire circuit operates in a high temperature environment. This means that the temperature tolerance of these components plays a critical role in determining the size and weight of these systems.
When power semiconductors operate at high temperatures, increases in leakage currents at high operating voltages tend to limit system performance. A high leakage current can result in unacceptable levels of power loss, or device failures by thermal runaway, a vicious spiral of over-heating and higher leakage currents. In high- voltage-blocking pn junctions, these leakage currents are directly proportional to the intrinsic carrier
March 2012
www.compoundsemiconductor.net 33
concentration in the semiconductor. This concentration is several orders of magnitude lower in GaN and SiC than it is in silicon, which is why these wide bandgap materials hold tremendous promise for making high- temperature devices with incredibly low leakage currents.
Ideally, GaN power devices would be built on a native substrate. However, GaN substrates are very pricey, so foreign substrates such as sapphire, SiC and silicon must be used instead. Although this trims costs, hetero- epitaxial growth creates high levels of crystal defects, which are to blame for the high leakage currents in devices biased to high voltages. As operating temperatures increase, this leakage current grows exponentially, preventing GaN devices from operating at reasonably high junction temperatures.
SiC does not suffer from the same fate, because affordable native substrates are widely available. This has spurred many companies to develop SiC
Power electronics is needed in many different regions of a more-electric aircraft
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