SiC projected to have Significant Impact on Industrial Markets
During the Plenary Session of last month's IEEE International Electron Devices Meeting, John Palmour, co-founder of Cree, Inc. and the company's Chief Technology Officer, peered into the future in his presentation on "Silicon Carbide Power Device Development for Industrial Markets." He opened by commenting that SiC power devices have the ability to greatly outperform their silicon counterparts. SiC material quality and cost issues have largely been overcome, allowing SiC to start competing directly with more traditional Si devices. 150 mm substrates and epitaxy are now commercially available.
â€œWith further optimized device design and rapid advancement in SiC substrate and epitaxial material quality in recent years, the next-generation SiC power MOSFETs being reported here show even greater capability with a further reduction in on-state resistance and improved blocking performance, resulting in a further reduction in switching losses and fabrication cost at voltage ratings from 900 V up to 15 kV compared to our commercially released SiC MOSFETs,â€ Palmour continued.
He noted that: When compared to state-of-the-art silicon high-voltage devices, like IGBTs, Thyristors, and PiN diodes, with a blocking voltage close to 8 kV, SiC MOSFETs with voltage ratings at and above 10 kV can offer a significant improvement in system efficiency, switching speed, and power density due to much lower switching losses, a reduced number of components required in series as well as the number of levels utilized to achieve desired blocking voltage, resulting in a much simplified system design with improved overall reliability at the system level. The next-generation SiC power MOSFETs reported at IEDM will allow even further penetration of SiC MOSFETs into energy conversion systems not only at lower voltages, but will also enable entirely new topologies to be achieved at very high voltages.
Palmour presented an evaluation of boost converter efficiency based on the 15 kV SiC MOSFET as a function of the output voltage under 20 kHz and 40 kHz for both soft-switching (ZVS, zero-voltage-switch) and hard-switching (No ZVS) conditions, separately . When switched at about 6 kV and about 5 A, the 15 kV SiC MOSFET exhibits the capability of a very high conversion efficiency of 98.5% during the 20 kHz soft-switching, 98.2% during the 40 kHz soft-switching, and 93.2% during the 40 kHz hard switching. These results are extremely promising for high power and high frequency applications that can significantly impact the system size, weight, and cost of the future advanced power conversion and transmission systems.
He further noted that above 15kV, the on-resistance of unipolar devices increases to the point where it is impractical from a cost and yield standpoint. This is similar to the case of silicon between 600V and 1200V, but at a much higher voltage due to the much higher breakdown electric field of SiC. Therefore, for devices above 15kV it is thought best to focus on bipolar designs, so that one can utilize conductivity modulation in order to obtain a much higher current density than would be possible with a MOSFET.
â€œWe have focused on two main types of devices for this very high voltage range,â€Palmour stated. â€œThe first is SiC GTOs, which offer the highest current density due to current injection from both sides, and the SiC IGBT, which has less current injection, but has the advantage of a voltage-controlled MOS gate, and is capable of higher-frequency operation.â€
â€œSiC technology has developed to the point where it can have a large impact on industrial markets. SiC substrates and epitaxy are now commercially available in 150 mm diameters with excellent crystal quality and uniformity. We have developed and demonstrated next generation SiC power MOSFETs with excellent performance over a wide range of voltage-ratings from 900 V up to 15 kV. By further optimizing device design and fabrication processes, these SiC MOSFETs show not only record low specific on-resistance but also exhibit very high switching frequency performance with extremely low switching losses over conventional Si power devices at the similar voltage ratings.
â€œThe simple planar DMOS structure allows for very high reliability as demonstrated by Accelerated HTRB, and the threshold voltages are stable. At 10 kV and above, entirely new applications can be explored with 10-15 kV MOSFETs with extremely fast switching speeds. For even higher voltages, bipolar devices such as GTOs and IGBTs may be utilized. We have demonstrated 22 kV GTOs with 200 A capability. The highest voltage switching device demonstrated to date is a 27 kV SiC IGBT,â€ Palmour concluded.