Revving Up SiC And GaN

Silicon carbide (SiC) and gallium nitride (GaN) are becoming more popular for power electronics, particularly in automotive applications, driving down costs as volumes scale up and increasing the demand for better tools to design, verify, and test these wide-bandgap devices.

Both SiC and GaN are proving essential in areas such as battery management in electric vehicles. They can handle much higher voltages than silicon, and their proven usefulness in safety-critical applications is attracting attention in other areas, as well.

“SiC, in particular, offers higher-speed switching, which makes battery management its killer application,” said Lee Harrison, automotive test solutions manager for Siemens EDA. “The capability to block high voltages makes it perfect for voltage regulators in EV vehicles.”

Researchers have spent decades working on and industrializing both SiC and GaN, with chipmakers such as STMicroelectronics and Infineon improving yield and lower defectivity to the point where these technologies now can be used commercially.

“These wide bandgap semiconductors have been getting more attention lately because we, as an industry, have gotten better at industrializing and manufacturing them,” said Gianfranco Di Marco, marketer for the Power Transistor Division at STMicroelectronics, pointing to the electric field strength and thermal properties of these wide bandgap compound semiconductor materials. “Progress has been recognized by the successful adoption of products in a range of several important applications, such as traction inverters, OBC (on-board chargers), DC-DC, charging stations for electric vehicles, as well as the charging infrastructure for SiC, and the power converters in consumer electronics for GaN. As both SiC and GaN offer power efficiency benefits over traditional silicon in several applications, they are attracting more attention.”

GaN also is gaining traction in RF-enabled commercial systems, such as 5G, and radar used in advanced driver-assistance systems (ADAS). “Depending on the power requirements called for by these high-frequency systems, GaN is becoming a dominant semiconductor due to its performance, which includes power, linearity, and power-added efficiency,” said David Vye, senior product marketing manager at Cadence. “Currently, GaN MMICs and discretes are widely used in RF power amplifiers and low-noise amplifiers.”

SiC MOSFETs, meanwhile, lend themselves to be used in charging stations, which will be the backbone of global infrastructure for BEVs and plug-in hybrids. “The deployment of silicon carbide in automotive applications is also helping to address other application areas in the industrial domain, while simultaneously helping designers to conceive future generations of SiC and GaN products for space and avionic applications,” Di Marco said. “SiC MOSFETs and GaN HEMTs are largely complementary, as each of them addresses different applications. Electric vehicles are now benefiting from large-scale adoption of both, with SiC MOSFETs and their ability to operate at voltages between 650V and 1,700V being ideal for traction inverters, DC-DC converters, and on-board chargers.”

GaN, on the other hand, operates at voltages from 900V down to 100V. Eventually, as it matures and it becomes cost-effective, GaN also may be a valuable technology for the latter two applications due to its higher frequency capability.

SiC and GaN technical benefits
As wide-bandgap technologies, both SiC and GaN can operate at higher voltages without sacrificing performance.

“They can handle far higher temperatures more safely and can work at higher frequencies,” Vye said. “Their physical and electrical characteristics make it possible to reach unrivaled levels of miniaturization, reliability, and power density. These are all necessary features in demanding applications such as electrical vehicles, inverters and chargers, data center converters, and industrial drives, to name a few. The two materials also can contribute to addressing the environmental concerns that are so widely debated, and which are under the spotlight to drive governments’ policies over the future of energy.”

GaN and SiC also can withstand higher electric fields than silicon and III-V devices, which means they can handle higher power densities and operating temperatures than competitive technologies, Vye noted. “Additionally, GaN offers many technical advantages, such as higher output impedance. That results in easier impedance-matching for power amplifiers and power combining, leading to broader frequency coverage and greater adaptability in many RF power applications.”

As a result, power amplifiers based on GaN microwave monolithic ICs (MMICs) have been developed for a wide range of systems, such as infrastructure equipment, missile defense, and radar.

At the same time, wide bandgap devices typically have 10X the electric breakdown field strength and 3X the band gap, enabling them to work at much higher temperatures than regular silicon technology, which is what makes them perfect for power regulation and management, Harrison said.

Design and manufacturing challenges of SiC and GaN
Despite these benefits, technical hurdles remain — which is often the case with new technologies.

“ST takes the long view in technology and is willing to invest in overcoming these hurdles for technologies where we see big potential,” Di Marco said. “For SiC, while there have been too many challenges to name in our 25-year journey. One is that SiC requires much higher processing temperatures in dedicated equipment, and we had to develop the processes to be able to manufacture at these temperatures.”

For GaN, the biggest issues relate to the maturity of the technology and fully understanding the failure mechanisms to allow for effective screening to isolate defective devices. “We’ve focused heavily on GaN’s industrialization processes and effective screening methodologies to ensure the highest levels of reliability,” DiMarco said. “In conjunction with the discrete offering, STPOWER GaN, we’ve also identified greater design challenges for the engineers adopting GaN in their circuits. Those challenges have driven us to develop our MASTERGAN and STi2GAN offerings, where we combine driver, GaN power device, and (optionally) BIPOLAR/CMOS/DMOS logic control in a single die or package.

Thermal issues
Because they are high-power devices, both GaN and SiC devices dissipate a considerable amount of thermal energy, which raises their operating temperature.

“Higher operating temperatures impact RF performance and threaten an amplifier’s reliability, since a semiconductor device’s mean time to failure (MTTF) is directly linked to channel temperature,” Vye said. “Increasingly, the RF designer needs access to the likely operating temperature in order to make key design decisions and ensure proper heat-sinking strategies. Traditionally, thermal analysis may have been performed by a mechanical engineer with data provided by the RF designer, or the device would have been manufactured and its operating temperature measured in the lab using equipment such as an infrared (IR) sensor.”

New thermal analysis tools can be used, some of them directly within the RF circuit design environment. The Cadence flow, for example, uses the geometric and material data defining the MMIC structure and the power dissipation data from the non-linear circuit simulation to calculate the resulting thermal dissipation using a finite element analysis (FEA) solver. This allows an RF designer to address thermal design issues concurrently with RF performance optimization.

Design considerations for wide-bandgap devices
Still, from the design side, there needs to be a recognition that SiC and GaN devices have different characteristics than those made with silicon.

“The way you control these devices, i.e., the gate drive, is different,” said George Liang, director of system application engineering for switching power and battery applications at Infineon Technologies. “When you try to implement a solution for a specific application, you need to look at how the device is working during the transition. There is a significant difference between wide-bandgap devices versus conventional silicon technology in that the switching loss is quite different. Switching can be a bit different. To maximize the benefit for using a wide-bandgap device, you really need to learn how to control the switch on this device.”

Cost also needs to be factored in. “If you look at today’s cost for designing and manufacturing wide-bandgap devices, they are still much higher than silicon MOSFETs,” Liang said. “It’s getting lower gradually, but it takes time. If you have a design you believe you need to do right away and think the product window isn’t going to last for long, the engineering team needs to consider when the product is going to launch. But then, longer term, they need to consider what industry trends are driving the need in order to determine if it makes sense to design it in now, or wait for a couple of years until the device technology matures and is available from multiple vendors.”

An additional consideration is whether there will even be a benefit in switching to a wide-bandgap device. “Will silicon MOSFET be sufficient? Is it a better choice because there’s so many vendors available for the given application? For example, if size is not the number one concern, then why don’t you want to consider a gallium nitride device? Can you use newer switch infrastructure and still get high efficiency? If I need multiple sources, then the silicon MOSFET is a better choice because they are much more available in the market. So it depends on the specific application use case,” Liang said.

Fig. 1: Automotive SiC MOSFET. Source: Infineon

Then, from the design tool perspective, a number of updates are needed for SiC and GaN devices.

“Since silicon MOSFET technology has existed for a very long time, there are more simulation models available,” he noted. “Also, application guidelines for how to use the device are needed for the design engineers to design them in. Most companies working on a new wide-bandgap device use a reference design and emulation board, so that’s getting better. The modeling side is the issue. There’s still a long way to go. People need to understand the switching behavior for wide-bandgap devices under all conditions. To capture this in the model working under any conditions requires some time, but this will happen. I expect in a few years these materials are going to be easier to use.”

In comparison, nearly all of this has been worked out repeatedly for standard silicon devices. “A silicon carbide device naturally is controlled very similarly to conventional MOSFET, so how to drive the silicon carbide can be very similar,” Liang said. “But still you need to control the voltage. From the user’s perspective, as time goes on, I expect the semiconductor manufacturers to provide better design tools, reference designs, and simulation models designed to interpret the reference design. It will happen gradually.”

Test challenges
Another issue involves test. Defect density is still higher for GaN and SiC, which makes high coverage essential — especially when these are used in automotive or other safety-critical applications.

“Everyone is familiar with the silicon lifecycle diagram for these devices to ensure that we do not see the early life failures, which would result in early customer returns due to latent faults,” Harrison said. “Manufacturers of these devices run stress testing to ensure any latent faults in the device are uncovered before they end up in a vehicle. For this technology, that stress testing period can be quite extensive — many times longer than regular silicon. One of the challenges with SiC and GaN technology is low-temperature operation, which can impact the lifespan of the device. A lot of the risk can be removed by extensive testing. Differences in testing are more due to the conditions under which the test program is applied rather than the type of tests that are run.”

Conclusion
The future of SiC and GaN is promising in many application areas, but most notably in automotive for battery management because these materials can handle high voltages. Once device characterization and modeling support are improved, the costs will drop even further and both wide-bandgap materials are expected to find their way into many more applications.

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