Markets Look to WBG Devices to Boost Power Efficiency

Article By : Maurizio Di Paolo Emilio

As the market continues its race toward more power, the industry is moving away from silicon in favor of wide-bandgap (WBG) semiconductor materials, which are more suitable for power applications...

Efficiency is a driving force in all industrial sectors, including the consumer sector. In electronic systems, compromised efficiency can limit system performance and shorten service life. The pursuit of higher efficiency pushes the industry toward higher power density with the possibility of having smaller, lighter, and more reliable products that overcome performance limits and provide higher power levels in data centers and automotive systems.

As the number of connected devices increases daily, more efficient power conversion can reduce the overall financial costs of powering products that number in the billions. And because of the huge numbers involved, it is equally important to improve the devices’ efficiency and thereby reduce their environmental impact.

Losses are endemic to power conversion, and efforts to limit them can, in turn, limit the efficiency and increase the heat dissipation of the system. Heat is a particular enemy of semiconductor devices, and removing unwanted heat from electronic equipment adds unwelcome cost and is harmful to the environment. This has fueled the research and development of more efficient semiconductor devices in order to increase power conversion efficiency, improve power density, and reduce the overall financial and environmental impact of energy management.

Power semiconductors have historically been based on a silicon substrate. While silicon is an excellent general-purpose semiconductor, its limitations when dealing with high voltages are well-documented. As the market continues its race toward more power, the industry is moving away from silicon in favor of wide-bandgap (WBG) semiconductor materials, which are more suitable for power applications. WBG semiconductors are able to operate at higher switching frequencies than crystalline silicon while keeping losses at a manageable level.

Transistors are the building blocks of the microelectronics industry. The substrates on which they are built dictate their behavior and therefore define their properties and functioning. The transistor is essentially a voltage-controlled switch, so the greater the power required, the larger the size. This is mitigated by the use of WBG materials. Transistors made with the WBG materials silicon carbide (SiC) or gallium nitride (GaN) are currently in demand to build power systems for the automotive industry and alternative energy systems.

The supply chain for WBG wafers is still being optimized, however, and is not as mature as the huge infrastructure and supply chain already in place for silicon wafers. Mass production imposes challenges that require robust and well-thought-out manufacturing processes. This includes wafer testing, which requires the test of smaller devices that work at higher current and voltage ranges.

To negotiate around this obstacle to greater efficiency, the industry is turning to substrates that continue to exploit silicon’s economies of scale while leveraging the capabilities of WBG semiconductors.

Gallium nitride, in the form of high-electron–mobility transistor (HEMT) epitaxial transistors on various substrate materials, is the most recent and promising semiconductor technology for high-performance devices in RF, microwave, and millimeter-wave (mmWave) applications. Two variants of GaN technology are GaN-on-silicon (GaN-on-Si) and GaN-on-SiC. GaN-on-SiC has contributed a lot to space and military radar applications, and RF engineers are looking to apply it to new applications and solutions. GaN-on-Si, meanwhile, enables the growth of low-cost, large-diameter substrates and allows the use of high-volume Si fabs for device production.

GaN-on-Si power transistors are better able to provide high efficiency than traditional silicon power transistors, in part because of the converter topology and technology needs. Peak current is associated with reverse-recovery charge, which can be so large in silicon power transistors that they cannot be used in conversion topologies that have a repetitive reverse recovery, including half-bridge topologies. With zero reverse-recovery charge, GaN can be used in topologies that were not previously considered for power supplies. The combination of recently enabled topologies with the new GaN technology helps designers achieve new levels of power performance.

GaN-on-Si substrates support cost reduction by offering high integration. The ability to integrate the low-noise amplifier (LNA), switch, and power amplifier (PA) on a single substrate has played a big role in driving the interest in GaN-on-Si. Integration has its advantages, but high-power, high-connectivity terminal connectivity that requires application is needed first. In these cases, GaN-on-SiC demonstrates its effectiveness (Figure 1).

Figure 1: GaN market evolution (Image: Yole Développement)

While silicon semiconductors will remain a mainstream solution for many years, there are certain applications in which customers can leverage WBG semiconductor characteristics, including improved bandgap (eV), breakdown field (MV/cm), thermal conductivity (W/cm-K), electron mobility (cm2/V-s), and electron drift velocity. Without getting into the semiconductor-physics details, suffice it to say that these improved parameters make WBG semiconductors suitable for high-voltage, high-switching–frequency applications while improving power density and heat dissipation.

Major advantages of WBG semiconductor power switches include high current density, faster switching, and lower drain-source on-resistance (RDS(on)). These device performance improvements lead to significant system-level benefits from an end-customer perspective. In real-life applications, customers can achieve high-temperature operation, along with overall system size and weight reductions.

Consumer market trend
As for GaN’s expansion in real-life applications, wireless charging is one of the hottest areas. As wireless charging becomes an increasingly common trend for mobile phones, GaN enables industrial customers to leverage the technology’s advantages as well. GaN exhibits its most evident advantages over silicon at high frequencies. Silicon is used in lower-power applications, but efficiency becomes more important as application requirements extend into the tens of watts or even kilowatts. Higher switching frequencies not only enable increased efficiency but provide further advantages from which customers can benefit.

A current trend in the consumer market is strong demand for new and higher-performing functionalities for the mobile phone. High-speed data transfer, larger and higher-quality screens, face-sensing features, and the next 5G specifications will soon require new power management solutions. All of these features need higher-performing or larger batteries. Larger batteries mean longer charging times. In addition to identifying the correct size for the portable device, new charging solutions are emerging on the market that can quickly recharge mobile phones.

Current battery units require at least two hours to charge fully. The need to lower this time has fueled efforts to bring new power solutions to the market, from 15 W to 100 W. GaN has been a factor here, allowing designs that achieve higher power and therefore recharge the device in less time than other solutions.

“In 2019, we saw the first example from OPPO, a Chinese OEM that used gallium nitride-based devices in its fast chargers,” said Ezgi Dogmus, technology and market analyst at Yole Développement (Lyon, France). “From that moment, we have seen more and more traction in this market. Xiaomi is an influential OEM in China and recently introduced GaN-based fast chargers.

“Regarding the major OEMs, Samsung has already incorporated GaN devices in its accessory fast chargers and, in our understanding, could soon integrate [GaN] into its in-box chargers,” she added. “There are also some rumors about GaN adoption in Apple’s and Huawei’s next-generation fast chargers. We think that we will have a clearer idea in the next [few] quarters, with the official announcements, but we can say there’s really big traction in gallium nitride.”

Silicon remains very active, especially below 30 W. Silicon solutions still address the application area between 30 and 100 W as well, but gallium nitride is increasingly competitive, offering high efficiency and fast charging time with good thermal management and good design.

“OEMs want a small, fast charger; they don’t want a big charger with their phone, because it’s not good for the aesthetics of the device,” said Dogmus. “With gallium nitride, they can do this.”

Yole expects strong adoption not just by Chinese OEMs but by Samsung and Apple as well. “To our knowledge, one of the GaN manufacturers is Power Integrations, [which] has publicly announced that they are providing IC solutions for Samsung’s upcoming chargers,” said Dogmus. The smartphone market is much larger than any other consumer market, so GaN’s suitability for smartphone chargers will drive up volumes when prices drop.

Figure 2: GaN fast-charging trends (Image: Yole Développement)

Figure 3: Yole began to see increased traction for GaN-based high-power fast chargers in 2019. (Image: Yole Développement)

“All of the OEMs are looking for market acceptance and further cost reduction of these GaN products,” said Dogmus. “In this context, 2020 and 2021 will be critical years for GaN-based power devices.” Integration is key to minimizing delays and eliminating the parasitic inductances that have restricted the switching speed of Si and earlier discrete GaN circuits. With propagation delays down to 5 ns, and robust dV/dt up to 200 V/ns, traditional 65- to 100-kHz converter designs can be accelerated to megahertz and beyond. These integrated circuits extend the capabilities of traditional topologies such as flyback, half-bridge, resonant, and others at frequencies that are on the order of megahertz, allowing the commercial introduction of revolutionary projects (Figures 2–5).

One of the most interesting and fastest-growing applications is electric-vehicle (EV) off-board charging, which includes the markets for fast chargers and charging stations. SiC can really add value to this application. Two of the most lucrative applications for SiC and GaN are electric vehicles and hybrid electric vehicles (HEVs) because the devices operate at higher voltages and temperatures, are more rugged, have longer lifetimes, and switch much faster than conventional semiconductor devices. SiC is being adopted in several applications, particularly e-mobility, to meet the energy and cost challenges in the development of high-efficiency and high-power devices.

Figure 4: Power versus frequency (Image: Yole Développement)

Figure 5: Power GaN market (Image: Yole Développement)

Strong traction for SiC is found in Tesla inverters and, therefore, for all high-voltage solutions in EVs. Yole notes that there is competition in this market between SiC and GaN solutions for on-board charger applications; the best solution will depend on the individual OEM’s cost/performance strategy.

“Almost all OEMs are looking at silicon carbide and also gallium nitride; nobody is really excluding either,” said Dogmus. “It’s a matter of costs and also of qualifications. So perhaps it can be said that silicon carbide is a bit more advanced because it has already been qualified for the automotive sector and has already started to be used in main inverter and on-board charger applications in several models by OEMs such as Tesla and BYD.”

GaN vendors such as Efficient Power Conversion (EPC) and Transphorm, meanwhile, have qualified products for the automotive sector, including low-voltage and high-voltage technologies for EV/HEV applications. Others, including Nexperia, are investing in the market with new solutions.

“I think that in the coming year, we will see more and more qualification of gallium nitride, and it could be able to compete with silicon carbide in terms of cost and performance,” said Dogmus.

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