Rediscovering the perfect switch with SiC FETs

The first electronic switches were made from vacuum tubes, which were large, lossy, and fragile. Early transistors were a further step backward, with higher resistance and lower breakdown voltage, but they could switch on and off much more quickly than other mechanical devices. But they were small, so they could handle only tiny currents. In the 75 years since Shockley and his team invented the transistor, many engineers have struggled to get back to Volta’s ideal solution, but the pressure to achieve faster MHz switching speeds, keep the size small, and increase the current rating has been enormous.

The advent of switch-mode power supplies enabled the development of transistors to higher power levels, making it possible to efficiently convert DC-DC power without a motor-generator set. The SMPS was patented in 1959, and the bipolar junction transistor was first commercialized in 1970, in the Tektronix 7000 Series oscilloscope. The BJT was successful in applications, but it was difficult to drive efficiently at higher powers, and the switching losses at tens of kHz were unacceptable. The fast and easy-to-drive MOSFET was patented as early as 1960, but early versions had a fairly high on-resistance and high power dissipation at higher currents due to the squared term in I 2 R. The breakthrough came with the invention of the IGBT, which combines the simple gate drive of the MOSFET with the turn-on characteristics of the BJT, and remains a practical solution for very high power converters. However, a “practical solution” is not an “ideal solution.” To avoid unacceptable dynamic losses in higher power applications, IGBT switching frequencies must be kept below approximately 10KHz and must be used with large, heavy and expensive magnetic components. At the same time, MOSFETs switching at frequencies up to 500kHz have evolved to advanced “super junction” MOSFETs, which today dominate DC-DC and AC-DC conversion applications in the low to mid-power range.

To bridge the gap between IGBT and silicon MOSFET applications, wide bandgap semiconductors have been developed using silicon carbide and gallium nitride. Wide bandgap semiconductors are expected to reduce switching and conduction losses due to their superior electron mobility and higher dielectric tolerance, enabling smaller devices with lower capacitance and shorter conduction channels. Although there are many difficulties in making switches using new materials, such as the mismatch in thermal expansion coefficients between the actual substrate and the GaN HEMT unit, as well as the "lattice defects" and "basal plane dislocations" present in SiC MOSFETs, all of which can reduce the performance and reliability of the switch. However, due to continuous improvements in manufacturing processes, performance has been continuously improved, and such devices (especially SiC MOSFETs) are now mainstream and are beginning to be used in traditional IGBT high-power applications.

To some extent, this regression has already occurred. SiC MOSFET and GaN HEMT cells are not as easy to drive as silicon MOSFETs, the required gate voltage level is critical to achieve optimal performance and reliability, and the threshold of SiC MOSFETs also has large variability and hysteresis. The reliability of SiC MOSFET gate oxides is also questionable, and GaN HEMT cells are not avalanche rated, so significant voltage derating will occur. Another regression is in the performance of the device. When conducting in the reverse direction through "commutation", the automatic commutation of the current caused by the inductive load SiC MOSFET causes the body diode to drop about 4V when forward biased, and there will be significant reverse recovery losses in the subsequent reverse bias. GaN devices conduct through the channel during commutation without any reverse recovery issues, but the voltage drop is still quite high, depending on the gate drive.

Looking back at the early technology, "cascode", may get us going in the right direction. UnitedSiC, a leader in manufacturing and technology, named the "cascode" of silicon MOSFET and SiC JFET "SiC FET" . Compared with SiC MOSFET or GaN HEMT units, SiC FET has a better overall loss quality factor, non-critical gate drive, stable threshold, fast body diode, low recovery loss, and only about 1.5V voltage drop. In addition, these devices have robust avalanche rating and short-circuit rating, and are not affected by gate drive. These devices are available in four types of 650V, 750V, 1200V and 1700V, with on-resistance as low as 7 milliohms in different packages, and most parts have obtained AEC-Q101 certification to eliminate any potential reliability issues.

Additionally, the challenges of high-speed switching with these devices can be addressed with the help of a simple RC snubber circuit, which can help manage turn-off overshoot and ringing, achieving optimal performance from SiC FETs.

Is the perfect switch here? Designers are still working on it, but one thing is for sure: SiC FETs are getting us closer to the ideal switch.