Despite SiC's wide use in the automotive industry, the technology is primed for use in other industries, including aerospace and power supply.
Silicon carbide (SiC) has come to be known as the successor of traditional Si IGBT devices in the automotive industry due to its higher switching frequencies, and higher junction temperatures. Furthermore, the last five years have observed the automotive industry as a public testing ground for the SiC–based inverters. As the fundamental conversion of DC to AC via SiC converters have proved to be smaller, lighter, and more efficient than the silicon (Si) converters, the potential of wide bandgap devices will significantly grow in the automotive industry.
However, the electrification agenda will not begin and end with cars. Wider transport applications will soon come into view, including trucks and buses, marine and shipping, the further electrification of trains, and even airplanes. On the supply side, grid–connected solar power systems and the transport of energy via high–voltage DC (HVDC) links will also be critical to the generation and distribution of low–carbon energy.
A common theme across these applications is the potential role for higher system voltages and, hence, higher–voltage power devices. In electric vehicles (EVs), the benefit of the shift from 400V to 800V is predominantly the faster charging rate possible. In solar inverters, an ongoing shift from 1,000–V to 1,500–V systems is reducing the number of PV strings, inverters, cables, and DC junction boxes — all of which result in efficiency and cost savings. In gigawatt HVDC installations, in which the nominal voltage is several hundred kilovolts, a higher individual device rating reduces the number of devices required in a multilevel stack, reducing maintenance and overall system size.
This singular focus on the automotive prizes on offer is, of course, understandable. The race to capture market share of this industry has led to companies fighting to drive up capacity, adopt 200mm wafers, and boost yields. This leaves scant room for the substantial R&D activities necessary to open up the high–voltage markets, which are relatively small in comparison.
Thankfully, the research sector has been hard at work, and numerous demonstrators of SiC technology at higher voltages have been designed, fabricated, and trialed, giving us a good understanding of the impact that a SiC superjunction (SJ) MOSFET, IGBT, and thyristor might have on these high–voltage applications.
Up in voltage, not down?
It is a fairly safe prediction that 650 V will remain a floor for the SiC MOSFET. Figure 2 shows the unipolar limit graph, which maps today’s commercial SiC devices, with their resistance plotted against their blocking voltage. This reveals the limitations of the technology. As the voltage–blocking drift region is reduced to a thickness of just 5µm at 650V, the resistance of the device has reduced to such a degree that fixed resistances from the SiC channel region and the substrate dominate, preventing any further downscaling of the resistance. While there appears to be considerable margin for improving 650–V MOSFETs in coming generations, it will be hard to lower these fixed resistances sufficiently far to make the case for a commercial 300–V SiC MOSFET.
At these low voltages, devices without a channel, such as Qorvo/UnitedSiC’s cascode JFETs, have an RDS(on) advantage: some wafer thinning is possible, allowing for a very low–resistance SiC FET. In reality, given the practical limitations as to how much further the SiC channel mobility can be improved using an industry–compatible method, the SiC JFET may be the only device that could achieve a voltage rating below 600V.
Scaling up SiC
What is implied in Figure 2, by the dash–dot line representing the current SiC technology limit, is that while SiC is a good technology at 650V and 1,200V, it has the potential to get even better at higher voltages. As the drift region is scaled to 30µm to support devices rated 3.3kV, its resistance eclipses that of the substrate and channel, pushing the devices ever closer to the technology limit. Therefore, in the future, high–voltage SiC MOSFETs honed to the quality of today’s SiC devices would have an even greater advantage over the incumbent Si technologies at voltages up to 10kV.
Furthermore, the door is open to higher voltage device types, such as 15kV IGBTs and 20+ kV thyristors, for grid applications. Sufficient progress has been made in developing these technologies via epitaxial growth on a N+ substrate, before the substrate is removed by grinding and CMP. Furthermore, the prohibitively low carrier lifetime in as–grown SiC has been improved upon with a lifetime enhancement oxidation process, so enabling these bipolar devices, rated to 20+ kV, will have low conduction losses similar to their silicon cousins.
Technologically, there is little preventing the scaling of SiC MOSFET technology. 3.3–kV devices are quite mature in the academic literature and the technology required to make epitaxial layers of a good quality up to about 10kV already exists. Finding R&D time and capability to produce these devices instead of automotive–related products feels like the largest barrier remaining.
This article was originally published on EE Times.