GaN Power Semi Biz Heats Up

The market for devices based on gallium nitride (GaN) technology is heating up amid the push for faster and more power efficient systems.

Today, GaN is widely used in the production of LEDs. In addition, it is gaining steam in the radio-frequency (RF) market. And the GaN-based power semiconductor market finally appears ready to take off, after several false starts and disappointing results in the arena.

In 2010, vendors announced the first wave of GaN-based power semiconductors. But until recently, product availability was scarce, prices were high, and the technology was in search of an application.

Now, though, GaN-based power semis are making inroads in the power supply market. And over time, the devices are expected to move into electric vehicles, fast-charging adapters for mobile devices, wireless charging and other systems.

“Demand for GaN is sprouting up everywhere,” said Alex Lidow, chief executive of Efficient Power Conversion (EPC), one of the pioneers in the GaN-based power semi market. “The biggest apps are LiDAR (light distancing and ranging), envelope tracking for 4G/5G LTE base stations, and DC-DC converters for servers and telecom equipment. Wireless charging is still small, but I anticipate it will be one of our top three applications by 2020.”

GaN-based power devices and other types of power semiconductors are used in the field of power electronics. Basically, power electronics utilize various solid-state electronic components, which control and convert electric power more efficiently in everything from chargers for smartphones to large power plants. In these solid-state components, the chips handle the switching and power conversion functions.

For these applications, GaN is ideal. Based on gallium and nitride, GaN is a III-V, wide-bandgap technology, meaning that it is faster and provides higher breakdown voltages than traditional silicon-based devices.

The problem is that, in general, silicon-based power semis provide enough performance for many applications and are still cheaper than GaN. In addition, GaN-based power semis are produced using a GaN-on-silicon process, which is a relativity expensive and complex technology.

Fig. 1: GaN vs. Si and SiC. Source: Transphorm.

“GaN semiconductors have started to make a dent in the overall power semiconductor market, but it is still very small so far,” said Richard Eden, a senior analyst at IHS Markit Technology, a market research firm. “The biggest challenges are price, acceptance of the technology and (the need for more) education/support.”

In total, the market for power modules and power discrete devices is expected to grow from $11.9 billion in 2015 to $12.7 billion in 2016, according to Yole Developpement. Of that figure, the GaN-based power semi market is projected to be a mere $10 million business in 2016, according to Yole, but that business is projected to grow at an annual rate of 86% from 2016 to 2021.

The projected growth for GaN-based power semis has prompted a number of companies to enter the market. EPC, GaN Systems, Infineon, Panasonic and Transphorm have shipped these devices, according to Yole. In addition, Dialog, NXP, On Semiconductor, TI and others are developing them.

Fig. 2: GaN vs. Si MOS transistor. Source: Panasonic.

In addition, two foundries, TSMC and Win Semiconductors, provide GaN processes. And now, Wavetek, a business unit of Taiwan’s UMC, plans to enter the GaN foundry business.

What are power semis?
Meanwhile, power semis are designed to boost the efficiency and minimize the energy loss in systems. But in reality, a device may experience energy losses for two reasons—conduction and switching. Conduction loss is due to the resistance in the device, while switching losses occur during the on and off states, according to Panasonic.

So, the idea is to find a transistor technology that provides faster switching speeds with a lower resistance. OEMs also look at several other factors, such as voltage, current, load, temperature, die size and cost.

Today, there are several power semi technologies to choose from. The entry-level market is served by traditional power MOSFETs, which are used in 10- to 500-volt applications. Developed in the 1970s, power MOSFETs are based on a vertical transistor architecture.

The big battle among power semi vendors is taking place in two mid-range voltage segments—600 and 1,200 volts. The main applications for these segments include adapters, automotive, switching power supplies and solar inverters.

For this, OEMs have a choice between two silicon-based solutions and two wide-bandgap technologies. The two silicon solutions, super-junction power MOSFETs and IGBTs, are the incumbent technologies that dominate the landscape.

Based on a lateral device structure, super-junction power MOSFETs are used in 500- to 900-volt applications. The IGBT, meanwhile, combines the characteristics of MOSFETs and bipolar transistors. IGBTs are used for 400-volt to 10-kilovolt applications.

Super-junction MOSFETs and IGBTs don’t require leading-edge processes. They can be produced using 300mm wafers, which makes them relatively inexpensive.

The problem is that super-junction hits the ceiling at around 900 volts. And IGBTs are plagued by slow switching speeds. “For the last three decades, silicon MOSFETs have been the power device of choice for the majority of power electronics applications,” said Domingo Huang, senior manager of sales and marketing at Wavetek. “However, next-generation and emerging applications are continuing to demand further substantial improvements in power conversion performance, as silicon-based power FETs are approaching physical performance plateaus.”

That’s why the industry is interested in two wide-bandgap solutions—GaN and silicon carbide (SiC). Wide-bandgap refers to the amount of energy required for an electron to break free from its orbit. It’s also a parameter that determines the mass of freely moving electrons.

GaN has a bandgap of 3.4 electronvolts (eV). SiC has a bandgap of 3.3 eV. In comparison, silicon has a bandgap of 1.1 eV.

Generally, in the power arena, GaN-based power semis are used in 30- to 650-volt applications. SiC FETs are targeted for 600-volt to 10-kilovolt systems.

So what’s the best technology for the 600- and 1,200-volt segments? It depends on the requirements and cost. “We don’t really see that GaN and super-junction power MOSFETs are necessarily competitive,” said Eric Persson, GaN applications manager at Infineon, the world’s largest power semi vendor. Infineon sells chips based on all of the technologies—MOSFETs, IGBTs, GaN and SiC.

“If you need to use super-junction to meet your requirements, then do that. The only reason that you go to GaN is if it’s cheaper or if you need density or efficiency, meaning higher frequency without sacrificing efficiency,” Persson said. “We believe GaN will remain a 600-volt technology predominately. At 1,200 volts, we believe that silicon carbide MOSFETs will be the future.”

Over time, the technologies will begin to overlap. “In the overlapping regions, that’s where it gets down to specific criteria like cost. It also comes down to whether it’s a cost-driven design or it’s a performance- and density-driven design. That gray area will shift as time goes on and as the production matures,” he said.

What is GaN?
To be sure, though, GaN-based power semis won’t dominate the landscape amid some manufacturing and product challenges. In the process flow, a GaN device starts with a silicon substrate. Then, a layer of aluminum nitride (AlN) is grown on the substrate. Following that step, GaN is grown on the AIN layer using metal organic chemical vapor deposition (MOCVD). The AIN layer acts as a buffer between the substrate and GaN.

“The challenge with these (wide-bandgap) substrates is they’re extremely expensive and difficult to manufacture,” said Ben Lee, director of technical marketing and product strategy at Applied Materials, in a blog. “Compared to (silicon) substrates, GaN substrates are generally six inches or smaller. Some are eight inches, but supply is quite limited. SiC substrates are just entering six inches now.”

There are other issues. “For GaN, AlN-type buffer layers are required due to the lattice mismatch between GaN and Si,” Lee said. “These buffer layers are non-trivial and require tuning to help minimize charge traps.”

So, the industry must continue to address those issues. “(The goal is) to address the inherent challenges, such as lattice mismatch, differences in thermal expansion coefficients, and thicker buffer layers to accommodate vertical voltage drops,” Wavetek’s Huang said.

On the device side, meanwhile, traditional depletion-mode GaN chips are “normally on” in operation. A negative bias must be applied first. If not, the system will short circuit, making them undesirable for many apps, according to EPC.

As a result, suppliers have moved from depletion- to enhancement-mode devices. These devices, which are more desirable for OEMs, are normally off until a voltage is applied to the gate.

Still, the question is clear—Will customers buy these parts? GaN is promising, but the industry prefers to stick with more familiar technologies like power MOSFETs. “There still needs to be a wide-ranging and effective education campaign to explain why and how customers can make the most of GaN technology,” said IHS Markit’s Eden.

Recently, though, GaN device suppliers are addressing some of the other issues. “There is still a misperception that GaN devices are more expensive than MOSFETs,” EPC’s Lidow said. “It takes time to reverse this misperception, but it is happening. EPC started matching or going under MOSFET prices in volume applications in May 2015.”

The apps
This, in turn, is making GaN more attractive for several applications, including high-end power supplies for data centers and telecom. Indeed, data center operators face a plethora of challenges, namely to reduce the power consumption in these giant facilities.

There are several ways to address the problem. One company, Google, utilizes a 48-volt rack architecture in its data centers, which is 30% more energy efficient than current 12-volt technology.

Generally, a rack consists of servers and power supplies. “They have maybe 50 or 60 kilowatts of power in these racks. They want to push it up to 80 or 90. It’s an enormous amount of power, but they want to do it in the same space,” Infineon’s Persson said. “What’s driving them is that they want the highest efficiency, something on the order of 98% end to end.”

For this, the power supply is critical. A power supply may consist of a converter and power factor correction (PFC). PFC ensures that a system can operate at its maximum efficiency.

One common type of power supply topology makes use of a dual-boost continuous conduction mode (CCM) PFC circuit. In some cases, this circuit is powered by a super-junction power MOSFET. “It gets the job done,” Persson said. “You can get 99% efficiency, but it comes at a cost. You also can’t operate it at high frequencies.”

One alternative is using a totem-pole CCM-PFC device, based on 600-volt GaN devices. This solution is more expensive, but there are some benefits. “The totem-pole is a simpler topology,” he said. “I can operate the same topology with GaN at much higher frequencies. You can get above 99% efficiency on the PFC over a broad range of power levels. It saves money and operating costs.”

Meanwhile, there are other emerging applications for GaN. For example, Dialog Semiconductor recently entered the GaN market with plans to field the technology for fast-charging applications.

When a smartphone or other mobile device is low or runs out of power, the system must be recharged using a wall adapter or charger. In the United States, the standard voltage is 120 volts.

“When you plug in your phone with the USB adapter, you usually get 5 volts,” said Tomas Moreno, director of business development at Dialog. “What happens is that there is actually a limit to how much power you can pump out. Power is equal to voltage times current. There is only 1.25 amps of current coming out of that cable. It’s limited by the cable. So you can pump out anywhere from 7 to 8 watts into the phone.”

All told, it takes too much time to charge up a smartphone with traditional wall adapters. In response, the industry has developed fast charging technology, which increases the output voltage. “You are pumping more power into the phone, thereby charging the battery faster,” Moreno said. “You can charge your phone to 80% in under 30 minutes.”

For this application, Dialog offers three chips for fast charging applications in a charger—a regulation controller, a synchronous rectification controller, and a communications IC.

Soon, Dialog will add a fourth chip to the solution—a GaN power semi. That device, called a half-bridge, incorporates a 650-volt GaN power switch and other circuits, which reduces power losses by up to 50% with up to 94% efficiency. “You can increase the power density by almost 40%,” he said. “You can pump out more power and charge the battery faster.”

Targeted for smartphone and tablets, Dialog will begin sampling its GaN-based fast-charging solution in 2017. Meanwhile, over time, GaN will be used in automotive, satellites, medical equipment and other systems.

To be sure, GaN is making inroads. But silicon-based MOSFETs are here to stay and won’t go away anytime soon. This, in turn, will keep this industry dynamic, if not in flux.

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