Addressing GaN Semiconductor’s Thermal Paradox

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Thermal management has always been an important factor in semiconductors because continuously shrinking chip designs manufactured on cutting-edge nodes generate more heat within the same size footprint. As a result, thermal management is intertwined with the size, weight and power consumption of electronic designs.

While thermal management is no longer an afterthought in the semiconductor industry, it’s considered a major stumbling block for gallium nitride (GaN) semiconductors deployed in mobile chargers, data centers, and solar and electric vehicle (EV) designs. This design premise holds truth in the backdrop of the fact that GaN transistors are considered thermally more efficient than their silicon counterparts.

Here, it’s worth considering if GaN devices are thought to be thermally more efficient, they should inevitably require smaller thermal management effort for cooling. So, why do industry luminaries like Alex Lidow, founder and CEO of Efficient Power Conversion (EPC) and a prominent evangelist of GaN technology, place so much emphasis on thermal management? Lidow is quick to point out that GaN requires more attention to disciplined power and thermal management skills.

Lidow, former CEO of International Rectifier, the power semiconductors firm that Infineon later acquired in 2016, founded EPC in 2007 with a sole focus on GaN semiconductors. For a start, he said, thermal dimensions of GaN designs are different, and they are still evolving. Therefore, design engineers must acquire a new set of fundamental skills in the thermal management of GaN devices.

“Thermal mechanics is far more mysterious for GaN-centric designs,” Lidow said.

Thermal management in GaN semiconductors is also crucial because of its increasing use in automotive applications like EV propulsion systems. While automotive designs operate in high-temperature environments, EV designs also introduce the high-voltage factor. At CES 2023, GaN Systems showcased GaN-powered EV solutions ranging from onboard chargers (OBCs) to DC-DC converters bridging high-voltage battery packs with low-power auxiliary circuits.

Figure 1: GaN semiconductors are increasingly used in EV design building blocks like DC-DC converters and OBCs. (Source: GaN Systems)

Thermal management solutions

In GaN transistors, two parameters are important in relation to device temperature: RDS(on), which relates to operating losses and transconductance, which then relates to switching losses. Such issues exacerbate thermal management imperatives with a boost in power density that wide-bandgap (WBG) technologies like GaN offer.

GAN Systems CEO Jim Witham acknowledged the crucial importance of thermal management in GaN devices. He said thermal is always important because if you can get the heat out of a GaN device, you end up spending less on the system. “So, we continue to do those things,” he added. “However, what’s more critical is how we use GaN transistors in a better and better way.”

Moreover, in a power subsystem, it’s also critical how design engineers put all the devices together and co-package them. “You make things small, have tight gate loops and ensure good heat transfer,” Witham said. In other words, thermal management of GaN devices will increasingly entail a holistic approach.

Stephen Oliver, VP of corporate marketing at Navitas Semiconductor, also recognized the vital significance of thermal management techniques and elaborated on the solutions that his company is adopting for GaN semiconductors. He said the good thing about GaN is that resistance is much lower than silicon, allowing design engineers to develop a very small chip. “The bad thing is that smaller chips have a smaller surface area, so thermal resistance to the outside world is reduced.”

Consequently, designers must ensure that the GaN device doesn’t create much heat. Oliver said that what Navitas has done with its initial GaN platforms is use the standard PQFN package. Such package solutions have been critical in enabling high-density power supplies because a good thermal design positively impacts the variation in power density.

Figure 2: A simplified schematics of a 6 x 8-mm PQFN package that employs the cooling pad option for thermal management. (Source: Navitas Semiconductor)

GaN’s thermal paradox

According to Anthony Schiro, VP for quality and sustainability at Navitas, a GaN IC saves 4 kg of CO2. That amounts to a lot of energy savings for the environment. Because GaN is a relatively new semiconductor technology, however, means significant work is still needed to streamline thermal performance.

As mentioned earlier, GaN devices offer much better thermal conductivity, enabling power systems to move from active to passive cooling systems. Furthermore, a boost in switching frequency lowers the overall system size and the number of magnets and capacitors.

At the same time, however, GaN is a new technology, and GaN devices aren’t fully commoditized yet. That means there are complex thermal management challenges waiting for design engineers’ attention in the GaN realm. Robust thermal management is also crucial to ensure a reliable system design under all operating conditions, as in automotive designs, where GaN devices are increasingly being deployed in EV applications.

Thermal management can make or break a high-power design. It’s been commonly said for silicon devices, but it’s equally valid for high-voltage designs built around GaN semiconductors.

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