Efficient power conversion has a major role to play in saving energy and reducing greenhouse gas emissions, while preserving or increasing living standards worldwide. At the same time, consumer desires are driving demand for electric vehicles that can travel further and charge faster, affordable solar or wind micro-generators that can produce more electricity per joule of renewable energy, home appliances with even better energy ratings, more efficient industrial drives, and tougher and more reliable aerospace systems.
Wide band-gap (WBG) semiconductor technologies can help meet both goals by enabling device manufacturers to create diodes and power transistors that are more energy efficient and robust than standard silicon alternatives in roles such as power-factor correction (PFC), DC-DC converters, rectifiers and inverters for applications such as industrial motor drives, hybrid/electric vehicles (H/EVs), renewable-energy feed-in and uninterruptible power supplies (UPS).
The two leading WBG technologies are silicon carbide (SiC) and gallium nitride (GaN). While a wide variety of SiC devices are already on the market, GaN could offer some advantages, although these devices have been slower to reach the market and costs remain high. Understanding WBG device properties, and assessing performance in a variety of scenarios, can help you understand the potential advantages of WBG devices, and the suitability of the technology for new designs.
WBG Physical Principles
Semiconductors have “bound” electrons, which occupy distinct levels around an atomic nucleus – valence and conduction bands. Electrons can move up to the conduction band and be available for current flow, but require energy to do so. In WBG devices this energy requirement is much greater than with silicon. For example, SiC requires 3.2 electron-volts (eV), whereas 1.1eV is sufficient for conventional silicon. The increased energy required to move electrons in WBG devices into the conduction band translates to higher voltage breakdown performance compared with silicon of the same scale. For the same reason, SiC can withstand higher temperatures (thermal energy) before failure and also, as a material, has about 3.5 times better thermal conductivity than silicon. In practice these attributes promise high-temperature operation at high voltage and power levels. This ability to withstand high temperatures can allow designers to make savings in other aspects of a system: for example, by specifying a smaller or less expensive heatsink for a power module, or trimming additional weight from an EV by reducing the cooling-system capacity.
Since the first simple SiC diodes were successfully produced, the material technology has advanced to allow fabrication of JFETs, MOSFETs and bipolar transistors. Figure 1 shows a SiC JFET cell designed with vertical “trench” construction, which gives very low ON-resistance compared to the GaN HEMT cell with lateral construction, shown alongside.
SiC JFET structure gives lower ON-resistance than GaN built using lateral construction.
Although the SiC JFET is normally “ON” with zero gate voltage, a “cascode” arrangement combining a co-packaged silicon MOSFET with the JFET gives a hybrid device that delivers the advantages of a WBG device yet can be controlled with gate-drive voltages used for ordinary silicon MOSFETs (Figure 2).
Co-packaged cascode structure combines SiC performance advantages with simple gate control.
Performance in Practical Circuits
The high temperature capability of WBG devices, combined with potentially fast switching and low losses, makes them ideal for military and industrial applications where performance is key. Bridge circuits are an obvious candidate used at high power for inverters, welding, class D audio amplifiers, motor drives and more. The “bridgeless totem-pole PFC” circuit shown in Figure 3 is a typical application that can be significantly improved by changing to WBG semiconductors. The slow performance of conventional silicon MOSFET body diodes forces designers to operate the circuit in critical conduction mode, which in turn produces high peak currents and high EMI. With cascode SiC JFETs, the circuit can operate in continuous conduction mode, which allows for a smaller inductor while also raising efficiency and easing EMI challenges. A 1.5kW test circuit powered from a 230VAC line has achieved efficiency of 99.4%.
Bridgeless totem-pole PFC circuit.
Resistant to Electrical Hazards
Devices suitable for high-power applications must be able to withstand transient short-circuits and over-voltages. Typical cascode SiC JFETs not only allow high junction temperatures, but also benefit from inherent protection against the effects of excessive current or voltage.
High currents cause a “pinch-off” effect, which limits current to a saturation level. Additionally, the heating effect produced by the current decreases the channel conductivity, giving a self-limiting characteristic.
In the event of excessive voltage, the SiC JFET gate-drain diode conducts, causing current flow in the gate-drive circuit and turning the JFET channel ON to clamp the over-voltage. Again, the inherent high temperature rating of the SiC die gives a good margin of safety for significant avalanche energy levels even though typical die size is small compared to ordinary silicon devices.
Another factor to consider is that SiC can withstand voltage avalanche conditions. This makes it inherently well suited to use with inductive loads such as industrial motors, home appliances or H/EV motors. Manufacturers such as UnitedSiC have demonstrated robustness by qualifying SiC parts throughout 1000 hours of operation, biased into avalanche at 150o C. As an additional confidence measure, all parts are subjected to 100% avalanche at final test.
Current Markets, Future Expectations
Compared to SiC, GaN displays higher electron mobility that should allow devices to support higher switching speeds. On the other hand, GaN’s inherently lower thermal conductivity could restrict power density in applications where miniaturization is key, such as industrial drives or H/EV inverters.
GaN devices in the market today are typically more expensive than SiC counterparts at comparable voltage ratings, because the technology is less mature. Also, the highest voltage rating for commercial GaN devices is 650V, with ON-resistance of about 25mΩ. This is equivalent to many SiC parts, although some GaN products at lower voltage ratings like 100V have ON-resistance no better than traditional MOSFETs.
In contrast, available SiC device ratings extend up to 1200V or more, and about 85A. Cascode devices have ON-resistances down to 30mΩ, and “super cascode” series-connected JFETs are also available with voltage rating greater than 3.5kV. Single devices up to 1700V at around 70A and 45mΩ are available, but as MOSFETs rather than JFET cascodes.
As GaN technology and processes mature, devices may become more price-competitive against existing SiC products. Manufacturers are expected to target low-voltage/low-power applications such as data-center power conversion and UPS, EV/HEV inverters and on-board chargers, and photovoltaics. However, SiC devices are readily available for these types of applications, and are proving successful in circuits such as bi-directional DC-DC converters and 650 V SiC cascodes in a bridgeless totem pole circuit suit EV on-board chargers perfectly PFC circuits, as described earlier.
To assist adoption as drop-in replacements for ordinary silicon MOSFETs, and so enable existing products to be made more efficient and robust quickly and easily, SiC devices are offered in industry-standard power packages such as TO-220 and TO-247. GaN device manufacturers have adopted a different strategy by leveraging surface-mount chip-scale packages with low internal inductance to maximize GaN’s theoretical speed advantage. However, the packages are usually single-source and therefore only suited to new designs. Moreover, equipment designers are often unable to utilize the full speed potential of fast-switching semiconductors, and must insert a gate resistor to control dV/dt and meet EMI specifications.
High temperature capability, small size and greater efficiency are major advantages of WBG power semiconductors, compared to conventional silicon devices. Of the two leading WBG technologies, GaN can offer faster switching speeds although ON-resistance is only as good as silicon in some cases.
On the other hand, SiC technology is more mature, costs are lower, and performance and robustness are already proven. SiC diodes, MOSFETs and JFETs including cascode and super-cascode components are available now, covering a wide range of ratings up to 85A, and 1200V or higher. Devices compatible with ordinary silicon gate voltages, in industry-standard power packages, allow easy, drop-in replacement to bring next-generation performance gains in a wide variety of industrial and consumer equipment.