GaN Application Base Widens, Adoption Grows

Gallium nitride (GaN) is beginning to show up across a broad range of power semiconductor applications due to its wide bandgap, enabling fast-charging, very high speeds, and much smaller form factors than silicon-based chips.

Unlike silicon carbide (SiC), another wide-bandgap technology, GaN is a lateral rather than a vertical device. GaN tops out at about 900 volts, which limits its use in many battery electric vehicle (BEV) applications. While there are still some overlaps, the demarcation line for where each of those technologies works best is relatively clear — at least for now.

The main applications for GaN over the next five years likely will include consumer products such as power supplies, rapid chargers, which provide higher power density than silicon, as well as some automotive applications. These are large and rapidly expanding markets. Yole Développement expects the GaN consumer handset power supply market will top $597 million by 2026, with a 72% CAGR between 2020 and 2026.

“In the EV/HEV market, there are significant global incentives for the electrification of cars,” said Ahmed Ben Slimane, technology and market analyst for compound semiconductors and emerging substrates at Yole. “There is a growing interest in 48V DC/DC conversion in mild hybrid EVs (MHEVs) and on-board chargers (OBCs), where high-switching GaN devices can allow more compact and less bulky systems. In addition, with new regulations on power supply units, GaN technology is expected to penetrate the datacom and telecom power market for systems with less than 3kW.”

Apple’s 140-watt MagSafe charger for its new MacBook Pro was a big win for GaN, marking the first time Apple adopted GaN technology. In a recent report, market researcher TrendForce said fast-charge production of 100-plus watts has entered a period of growth, accelerating the adoption of third-generation semiconductor devices in consumer applications. With GaN power transistor prices dropping to nearly one dollar, and GaN fast charge technologies continuing to mature, TrendForce expects GaN solutions to reach a 52% penetration rate in the fast charge market over the next several years.

GaN also will play significant roles in data centers and in RF power amplifiers for 5G infrastructure.

“With utility and cooling being major cost items for data center operators, GaN power devices offer energy-saving opportunities with their superior efficiency,” said David Uriu, technical director of corporate marketing at UMC. “For similar reasons, GaN power devices are making a strong inroad into the SMP rapid charger market. GaN RF devices, on the other hand, enjoy much better RF performance compared to silicon-based RF LDMOS (laterally diffused metal-oxide semiconductors), and are therefore becoming an integral part of the 5G infrastructure. No matter which application, GaN will be making crucial contributions to a greener world through power saving.”

GaN basics
GaN power devices are commercially available in the 100V, 200V, and 650V operating voltage ranges by several vendors, along with one 900V offering, noted Victor Veliadis, executive director and CTO for the PowerAmerica Manufacturing USA Institute, which was formed by the U.S. Dept. of Energy to accelerate adoption of SiC and GaN power electronics.

“They have a lateral configuration — unlike SiC power devices, which have a vertical configuration,” Veliadis said. “GaN devices are natively depletion mode (normally-on), and a cascode configuration and several ‘mature’ techniques are being used to design GaN power devices enhancement mode (normally-off), which is what power electronics applications engineers prefer. GaN power device fabrication is similar to that of the RF GaN HEMT, which has been available as a product for several years from volume fabs/foundries. Thus, a big advantage of GaN power devices is that they are fully CMOS-compatible, and thus can be fabricated in large quantities in volume silicon fabs/foundries exploiting the silicon manufacturing economies of scale.”

This compatibility also allows for fabrication of GaN ICs, and the lateral configuration simplifies packaging, noted Veliadis, a professor of electrical engineering at North Carolina State University, which manages PowerAmerica. “GaN devices offer the lower weight, volume, high efficiency, and smaller form factor advantages of wide bandgap in a device fabricated in large CMOS fabs. GaN is capable of efficiently operating at frequencies well above those of silicon and beyond those of SiC, and this reduces the size of system passive components, providing unparalleled small form-factor systems.”

Such small form factor advantages also make GaN power devices excellent candidates for mass applications in consumer electronics, motor drives, data center uninterruptible power supplies, and 5G enterprise equipment.

“With respect to EVs, the majority currently utilize a 400V bus architecture, so 650V GaN devices compete with the mature and rugged silicon IGBTs, and SiC in the lucrative traction inverter, DC-DC converter, and on-board charger,” Veliadis said. “This is indeed a mass market, and all three material technologies are highly competitive.”

For higher efficiency — longer range for the same battery, or the same range for a smaller battery — and fast charging, EVs are rapidly moving to an 800V bus architecture. “At this voltage, 1,200V MOSFETs have an advantage as they were commercialized in 2011 and have gone through several generations of optimization,” he noted. “With 1,200V MOSFETs, a 2-level topology is needed, versus a 3-level for 650V GaN. That means the circuit can be implemented with fewer devices in SiC. There are, of course, numerous tradeoffs when selecting the right device for an application, but the 2-level system simplification is an important consideration.”

Meanwhile, GaN is replacing GaAs and silicon in a number of power-related applications for the military, cell towers, health care, and consumer electronics, said Paul Knutrud, director of marketing for optical products Onto Innovation. He listed high power and high switching speeds as the major advantages of GaN.

Others point to similar advantages. Stephen Oliver, corporate vice president for marketing and investor relations at Navitas Semiconductor, cited speed a major key advantage of the technology, as well as the abundance and low cost of materials for GaN.

“The raw materials are really low cost, and when you put it together, the crystals form a new type of semiconductor that obviously conducts half the time and then doesn’t conduct or isolate the other half the time,” Oliver said. “If you had a wire made of gallium nitride and a wire made of silicon, and pass the same amount of current, the gallium nitride would stay cool. It is lower resistance per unit area of silicon, so it runs much cooler.”

Even more important, though, is the switching speed. “The real key to GaN is it’s a racehorse that really likes to run at high speed,” he said. “And when we’re talking about power conversion, high speed means how fast we switch the device. In power conversion, if you imagine a transformer, you have a wire alongside the windings, and then there’s a loop of ferrite material and there’s a second winding on the back end to take it off. If you put energy into that transformer and it goes around the ring and comes out again, if you do that slowly, you need a big bucket of energy. So the transformer is physically large. But if you do it faster and faster, each bucket of energy gets smaller and smaller. So the transformer doesn’t need to hold that much energy. You can literally make it smaller, and when it gets smaller, it gets cheaper. It gets lighter. And we can also use switching methods — different ways of organizing the circuit into topologies, which means they are no longer academic curiosities. We can now do industry-proven power conversion at high speed using gallium nitride. You run fast and shrink, and you can make a phone charger three times smaller than for silicon. So speed shrinks the things down.”

The first commercial use of GaN was in LEDs and in Blu-ray DVDs. High-frequency capabilities later made it attractive in 5G phone transmitter chips for RF GaN, and now it’s increasingly going into power conversion, especially in GaN-based fast chargers.

Application base widens
Efficient Power Conversion (EPC) has logged more than 100 emerging applications for its eGaN FETs and ICs. Alex Lidow, the company’s CEO, said the five fastest-growing applications are lidar systems for robotics, drones, consumer products, driver alertness systems, and autonomous vehicles; DC-DC converters for AI systems, servers, and telecom power systems; motor drives for e-mobility and robotics; satellite systems, including motor drives and DC-DC power supplies that require radiation hardness; and solar power point trackers.

“The rate of adoption in each of these applications has passed the tipping point,” Lidow said. “The early adopters are all in, and the pragmatists are trying to catch up. There still is a learning curve with each new user, but there are many resources available to get first-time users into successful designs. The remaining obstacles are mostly the myths that people hold onto. For example, GaN is not more expensive than silicon. That bridge was crossed about five years ago, but we hear it from customers until they actually see a price quote. Another myth is that GaN is not as reliable as silicon. Again, this myth has been dispelled through numerous reliability reports, and hundreds of billions of successful hours in demanding applications.”

Compared with other transistor technologies, another key benefit of GaN is its very low switching loss, noted George Liang, director of product and system engineering for switching power and battery-powered applications at Infineon Technologies. “Any power electronic application that values lowest power loss (high efficiency) will be a natural fit for GaN. For applications where small size is critical, GaN can enable higher operating frequencies without increasing power loss, thus leading to higher density solutions. For the EV market, GaN is one of the wide-bandgap technologies targeted for very compact on-board chargers. Another big market is compact charger/adapters for everything from cell phones to larger consumer electronics like TVs. Yet another application space is for server power supplies used in telecom and data centers, where the improved efficiency lowers the operating costs.”

Transphorm is producing GaN devices today across the power spectrum, from 30 watts to 10 kilowatts. “Our FETs are used in cross-industry applications such as powers adapters, crypto-mining PSUs, data server PSUs, solar inverters, electric vehicle chargers, and more. to high 10 kilowatts, said Transphorm CEO Primit Parikh, pointing to opportunities in EVs, smart phones, laptops and IoT device chargers. “GaN devices have big implications in the electric vehicle market, which is expected to grow at a CAGR of 21% through 2030. These devices increase power efficiencies (hence reduced electrical losses and less wasted heat), enabling new design possibilities that can lead to smaller cooler systems and more aerodynamic and/or lighter cars with faster charging and extended driving range.”

Fig. 1: Some common uses for GaN semiconductors. Source: Transphorm

Parikh noted that in-box and aftermarket charging devices are high-volume products, while 5G devices are power-hungry, requiring 65 watts for fast charging. In contrast, data centers and crypto-mining are mission-critical applications, both requiring massive amounts of power. “The computers used to solve complex equations and store streaming data also generate massive amounts of heat,” he said. “These two factors contribute to 40% of total operational costs.”

GaN technology allows these computers to perform at the same capacity, while reducing energy consumption and heat generation, he said. “Ultimately, this reduces the operating costs of data centers and crypto-mining rigs. Crypto-mining alone is expected to grow by more than $2 billion dollars by 2024, and any product that can help cut costs and increase efficiencies is going to be a hot commodity.”

Others report similar growth. Liang said GaN is already a mainstream technology in full-scale production at Infineon. “As with any power transistor technology, there is a continuous process of improvement, striving for ever better quality, reliability, and cost,” he said. “For example, we anticipate increasing production capacity and improving cost structure as we qualify and shift from 150mm to 200mm wafer production in the near future.”

Where it works, where it doesn’t
In April, researchers at Imec and AIXTRON, a deposition equipment supplier, announced a successful demonstration of epitaxial growth of GaN buffer layers qualified for 1,200V applications on 200mm QST substrates, with a hard breakdown exceeding 1,800V. The manufacturability of 1,200V-qualified buffer layers opens doors to highest voltage GaN-based power applications, such as electric cars, previously only feasible with SiC -based technology. The question now is when they will be price-competitive — if ever.

Fig. 2: Imec/AIXTRON epitaxial growth on GaN buffer layer. Source: Imec/AIXTRON

All GaN power transistors available today are lateral transistors, Liang said. “Increasing the breakdown voltage requires proportionally larger die area plus thicker epi layers. For a given on-resistance, not only does the die size increase, but also the cost per unit area. Due to this reason, Infineon believes that 1,200V or greater GaN transistors will not be price-competitive compared to SiC. Note that the (Imec) release is just stating that these voltage levels have been demonstrated on GaN. That is a long way from showing it is qualified, ready for production, or makes any economic sense.”

The penetration of GaN in different markets will vary depending on the requirements of each application, according to Taha Ayari, technology and market analyst at Yole. “In general, the remaining challenges for GaN devices are reliability and performance acceptance, price competitiveness, along with the development of high-voltage devices for high power applications. One of the main obstacles remains the epitaxy of GaN layers on silicon substrate, the lattice mismatch, and the thermal coefficient of expansion mismatch between the two materials, which generates killer defects in the GaN layer. So it requires a sophisticated buffer layer and epilayers. The epitaxy is generally related to an in-house process developed by the manufacturer, rendering epitaxy standardization quite tricky. Also, price pressure and higher volume demand are pushing the industry to transition from the traditional six-inch platform to the eight-inch platform, which would require more epitaxy development for uniformity and higher yield purposes.”

Bigger wafers remain a challenge. “It is true that GaN and silicon-based devices are well established in power and RF applications,” said David Haynes, managing director of strategic marketing for the Customer Support Business Group at Lam Research. “But this is largely on wafers that are six inches or smaller, and in the case of many GaN-based devices, on substrates such as sapphire and SiC.

“Today there is a strong shift to 200mm wafer processing in order to increase compatibility of these technologies with mainstream semiconductor processing, and to improve the economics of the technology for more advanced or higher-volume applications,” Haynes said. “SiC is migrating to 200mm, with production set to ramp in the next two to three years as 200mm wafer cost and availability improves.”

In particular, Lam is focusing its efforts on 200mm SiC trench MOSFET applications, Haynes noted. “For GaN, it is the improved performance of GaN on silicon technologies on 200mm, and in the future even 300mm, that is underpinning the evolution of the technology. The processing of 200mm GaN-on-silicon power and RF devices really opens up the possibilities for CMOS integration and compatibility with CMOS foundry processing. At Lam we have developed a range of ultra-low damage etch and deposition processes compatible with 200mm and 300mm GaN-on-silicon production, as well as advanced single wafer clean processes to support CMOS foundry compatibility.”

In EVs, SiC solutions recently had wins with key advantages over GaN due to it being about 10 years more mature and offering better thermal performance, Navitas’ Oliver noted.

“Silicon carbide is good for high-power, high-voltage stuff. The problem with silicon carbide is that it’s actually quite a slow semiconductor. It does the conduction great, but when it comes to switching, converting power, it’s a lot more lossy than gallium nitride,” Oliver said. Some companies, like Enphase Energy, which makes solar micro inverters, and Swiss auto supplier Brusa Electronik, have started evaluating and designing Navitas’ GaN power systems. “GaN is becoming mature, now that we’ve got 34 million units in production, with 120 billion field hours and zero reported field failures.”

GaN is well positioned and its high frequency operation brings strong advantages for 100V, 200V, and 650V operating voltage range applications, said PowerAmerica’s Veliadis. “With respect to voltage range, commercial GaN devices are lateral and thus not available beyond 650V (although Transphorm does offer a 900V solution). So competing at the 1,200V range and above requires higher topology levels compared to 1,200V commercially available SiC devices.”

GaN does not have physical voltage breakdown limits, but it does have practical limits, EPC’s Lidow pointed out. “GaN and SiC have similar abilities to block high electric fields without breaking down. GaN, however, has the unique feature of a two-dimensional electron gas (2DEG) that makes electrons traveling on the surface extremely mobile compared with either silicon or SiC.”

GaN therefore has a distinct advantage over either of these two semiconductor materials when the active devices are lateral. “This means the contact terminals are all on one surface. In a SiC or silicon MOSFET, the device has source and drain terminals on one side of the device and the drain terminal on the other surface,” Lidow said. “Vertical devices are pretty much required when voltages approach 1,000V because the required distance between terminals gets to be larger than the active device if you make a lateral device. Many people then ask, why not vertical GaN? The answer is that vertical GaN has no advantage over vertical SiC. GaN is disadvantaged due to higher thermal resistance compared with SiC. In addition, there are no cost advantages. Taking all this into account, I predict that GaN will be the dominant power technology up to 650V, and SiC will be the dominant technology above 900V. Between 650V and 900V there will be niche applications for GaN on silicon, and SiC.”

Conclusion
Still, GaN is becoming solidly entrenched in places where it makes sense. GaN technologies have been designed in and tested in the field. While some questions remain on manufacturing hurdles and the scalability of higher-voltage GaN-based devices, these issues should be resolved in the relatively near future.

“I do think we are past the tipping point in the largest applications,” Lidow added of GaN power semis. “That means the largest barrier is the design cycle in various applications. Education and integration both reduce the learning cycle time.”

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