In recent years, some important, high-growth applications have experienced an increase in the use of silicon carbide (SiC) and gallium nitride (GaN) power devices. GaN is being utilized in charging systems and chargers for mobile devices. GaN-based chargers have been chosen by manufacturers like Apple, Samsung, and Xiaomi because they offer high power densities while preserving, or even reducing, the weight of these components. These chargers make use of power GaN HEMT chips made available by businesses like EPC, GaN Systems, and Navitas Semiconductor.
On the other hand, SiC technology has mainly been applied to electric transportation. Manufacturers of electric vehicles, like Tesla, opted to employ SiC-based motor controllers in 2017, which increased the effectiveness of their systems. Leading device companies including STMicroelectronics, Infineon Technologies, Wolfspeed, and Rohm Semiconductor are currently meeting the growing need for e-mobility power conversion through high-volume production of SiC power devices.
What makes these new semiconductor materials so unique, and why are they being considered as silicon substitutes?
As explained by Victor Veliadis, executive director and CTO of PowerAmerica, in his July 28, 2022, PSMA webinar, “SiC Power Technology Status and Barriers to Overcome”: “SiC and GaN materials have a critical electric field that is about 10× higher than that of silicon, with a bandgap that is 3× higher. In a semiconductor system, the drift layer is what holds its rated voltage, which makes the thickness and doping levels of this layer determine the voltage capability of the device.”
As a result of their thinner, more highly doped drift layers for a given specific on-state resistance and breakdown-voltage specification, SiC devices are smaller in size than silicon, which decreases their capacitances. These devices can therefore efficiently switch at frequencies much higher than what is possible with silicon. Due to the higher switching frequency, the size of passive components and magnetic devices like inductors also decreases. This leads to a significant reduction in the overall size of the system, which increases its power density. Furthermore, the high thermal conductivity allows for high-temperature operation with simplified cooling management, further decreasing system weight and volume.
Silicon is still a strong contender in devices rated from 15 V to 650 V while being much cheaper and more reliable, whereas GaN has been gaining popularity in low-power applications like mobile chargers and similar charging systems. As previously mentioned, GaN is the only viable wide-bandgap alternative to silicon in low-power applications, as relative advantages of SiC over Si decrease at voltages below 650 V, especially when the higher cost of SiC is taken into consideration.
GaN & SiC
The zero-reverse–recovery charge in the unipolar GaN HEMT device enables a power-factor–correction (PFC) technology known as “totem-pole bridgeless PFC topology.” These topologies can have advantages of high efficiency at high switching rates.
Stephen Russell, subject matter expert for power devices at Tech Insights, said during a company webinar, “Gallium nitride has truly found its killer app in replacing silicon and USB-C chargers for mobile devices. 2021 [was] a watershed year in market acceptance, and we only expect this momentum to continue. Gallium nitride’s real advantage, however, is its switching: It is the only viable wide-bandgap replacement for silicon at voltages less than 600 V.”
All of these devices compete heavily at the 650-V capacity, which is important, as these devices are used in the 400-V capacity bus for EVs.
SiC is suitable for higher-power applications than what is possible using GaN and is available in voltages ranging from 650 V to 3.3 kV, with higher-voltage devices being developed. It is expected to have an edge in the EV sector, as more and more manufacturers are moving toward 800-V EV systems due to its efficient high-voltage operational capability. Manufacturers like Porsche, Audi, BYD and Hyundai are already working on 800-V battery systems, while Lucid has a 900-V system under development. As Veliadis said, “Moving to 800 V while keeping the current the same doubles the power, with smaller losses. This reduces heavy copper cables, bringing lighter weight and space-saving advantages.”
Challenges in widespread adoption of SiC and GaN power devices
Currently, SiC devices can cost almost 2× to 3× as much as silicon.
Apart from the high cost, manufacturing SiC has its own set of challenges, such as the presence of defects and slower fabrication times compared with silicon. Most SiC manufacturing is on 6-inch manufacturing lines, though 200-mm pilot line efforts have started at some companies like Wolfspeed. Efforts are also being put in at process improvements and better screening capability to improve yield and reliability.
Due to their high-voltage potential, SiC devices are excellent candidates for deployment in power applications like HVDC transmission and renewable-energy systems. For example, in the case of photovoltaic applications, although the SiC device cost is 3× higher than that of silicon, the overall system cost is lower due to the reduction in the size of the passive elements.
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