They won’t replace silicon, but GaN and SiC are becoming much more attractive as prices drop.
Several vendors are rolling out the next wave of power semiconductors based on gallium nitride (GaN) and silicon carbide (SiC), setting the stage for a showdown against traditional silicon-based devices in the market.
Power semiconductors are specialized transistors that incorporate different and competitive technologies like GaN, SiC and silicon. Power semis operate as a switch in high-voltage applications such as automotive, power supplies, solar and trains. The devices allow the electricity to flow in the “on” state and stop it in the “off” state. They boost the efficiencies and minimize the energy losses in systems.
For years, the power semi market has been dominated by silicon-based devices, namely power MOSFETs and insulated-gate bipolar transistors (IGBTs). Both are mature and inexpensive, but they are also reaching their theoretical limits in several respects.
That’s where GaN and SiC fit in. In the market for years, GaN and SiC devices compete against IGBTs and MOSFETs in various segments. Both GaN and SiC are wide-bandgap technologies, meaning they are faster and more efficient than silicon-based devices.
Fig. 1: How power switches are categorized. Source: Infineon
Still, GaN and SiC devices have relatively low adoption rates and won’t displace their silicon rivals anytime soon. Today, silicon-based devices have more than 90% market share in the overall power semi market, according to Yole Développement. Generally, GaN and SiC devices are expensive technologies with various challenges.
That’s beginning to change because new GaN and SiC devices are making a bigger dent in the market. SiC, for example, is growing at a double-digit pace, compared to single digits for silicon-based devices, according to Yole. “The silicon carbide power device market is expected to grow very fast, driven mainly by the automotive market,” said Hong Lin, an analyst at Yole. “The market potential is huge and is attracting a lot of players. We expect the competition to be very strong in the coming years.”
GaN is a tiny market today. “When we calculate the growth rate in five years, GaN could be really much bigger than silicon carbide and IGBTs. The market could grow very fast,” Lin said.
As it turns out, though, there is no one power semi that fits all needs. There is a place for all technologies. “The total market is growing very quickly. Every device can still have their place, at least for the foreseeable future,” Lin said.
Nonetheless, there are a number of events taking place in this market. Among them:
In total, the power device market is expected to grow from $17.5 billion in 2018 to more than $21 billion by 2024, according to Yole. Of that, the SiC device market will grow from $420 million in 2018 to $564 million in 2019, according to Yole. In 2018, the GaN device market was less than a $10 million business, according to the firm.
The end of silicon?
In total, the world generates 12 billion kilowatts of power every hour, according to North Carolina State University. More than 80% of the world’s electricity is transported through a power electronic system, according to the school.
Power electronics make use of various devices that control and convert electric power in systems, such as cars, motor drives, power supplies, solar and wind turbines.
Generally, power is wasted during the conversion process in systems. In one example, the power wasted in desktop PCs sold in 1 year is equivalent to 17 500MW power plants, according to NC State.
Therefore, the industry requires more efficient devices, such as power semis and other chips. Each power semi is denoted by a numerical figure with a “V” or voltage. “The ‘V’ as in VDSS is the maximum allowed operating voltage, or drain-source voltage specification,” explained Alex Lidow, chief executive of EPC. “The terminology ‘DSS’ means drain-to-source with the gate shorted.”
Nonetheless, there are several power semis to choose from. On the silicon front, the choices include power MOSFETs, super-junction power MOSFETs and IGBTs.
Considered the least expensive and most popular device, power MOSFETs are used in adapters, power supplies and other products. They are used in lower-voltage 10- to 500-volt applications.
Super-junction power MOSFETs, which are souped-up MOSFETs, are used in 500- to 900-volt systems. Meanwhile, the leading midrange power semiconductor device is the IGBT, which is used for 1,200-volt to 6.6-kilovolt applications.
MOSFETs compete against GaN devices in the lower voltage segments, while IGBTs and SiC go head-to-head at the high end. All devices compete with one other in the 600- to 900-volt range.
Regardless, IGBTs and power MOSFETs will remain the mainstream technologies for the foreseeable future. “Silicon is a very mature technology, leading to better cost positions in many aspects, including from a supply chain and internal production processes to existing designs and processes at the customer side,” said Gerald Deboy, a senior principal at Infineon. “This is why for many applications silicon-based power switches will continue as the preferred technology for many years.”
Silicon-based devices, however, have several limitations, such as high conduction losses and low switching frequencies. Conduction loss is due to the resistance in the device.
That’s why OEMs are interested in two wide-bandgap technologies—GaN and SiC. Silicon has a bandgap of 1.1 eV. In comparison, SiC has a bandgap of 3.3 eV, while GaN is 3.4 eV.
“GaN and SiC are wide-bandgap materials, which means higher bonding energy of the atoms in the crystal,” said Steven Liu, vice president of corporate marketing at UMC. “SiC and GaN are promising components for the power semiconductor market due to the higher efficiency and smaller form-factor characteristics, compared to their silicon-based peers. The devices can be made much smaller in size for the same relative voltage and current handling capability.”
GaN and SiC power semis have been shipping for some time, but they are still expensive. “The manufacturing cost is the main obstacle to market growth, since today both are mainly still using 6-inch and below wafers for production. Once the cost can be improved to a certain threshold, the market size could explode,” Liu said.
Still, there is a place for all power semi types, according to Infineon. Infineon has a unique perspective, given that it sells IGBTs, MOSFETs, GaN and SiC.
“Criteria for selection of a wide-bandgap device instead of traditional silicon depends on balancing system cost and performance requirements for particular applications,” Infineon’s Deboy said. “There are various applications where a tipping point in cost and performance goals has been reached for systems based on wide bandgap material. Depending on the specific application, GaN or SiC devices have a better cost position at the system level, even while the GaN or SiC device itself is more expensive than the silicon alternative.”
What is SiC?
The SiC market is heating up. Suppliers of SiC devices include Cree/Wolfspeed, Infineon, Mitsubishi, On Semiconductor, STMicroelectronics, Rohm and Toshiba.
SiC is a compound semiconductor material based on silicon and carbon. It has 10 times the breakdown field strength and 3 times better thermal conductivity than silicon.
There are two SiC device types—SiC power MOSFETs and diodes. SiC power MOSFETs are power switching transistors. A diode passes electricity in one direction and blocks it in the opposite direction.
SiC devices are produced in 150mm fabs today, although 200mm is in R&D. In the production flow, a SiC substrate is developed. An epitaxial layer is grown on the substrate. It is then processed into a device.
Making the substrate is the biggest challenge. “This wider bandgap gives the materials interesting qualities such as faster switching and higher power density,” said Llewellyn Vaughan-Edmunds, director of strategic marketing at Applied Materials, in a blog. “A major challenge is substrate defects. Basal plane dislocations and screw dislocations can create ‘killer defects’ that must be reduced for SiC devices to achieve the high yields required for commercial success.”
The issues with the SiC substrate translates into higher costs and potential supply constraints during boom cycles. “Cost is a challenge versus silicon or even GaN-on-silicon approaches. There are two main reasons—the cost of the SiC substrate and the material yield. Given the tight supply conditions, those prices are unlikely to start dropping soon, but the situation will improve,” said Kevin Crofton, president of SPTS Technologies and senior vice president at KLA.
To address the cost issues, some vendors are working on 200mm SiC fabs. This will increase the die per wafer, thereby reducing the cost.
Meanwhile, SiC MOSFETs are based on two structure types—planar and trench. Planar incorporates a traditional source-gate-drain structure. Trench forms a “U shape” vertical gate channel.
“At the device level, first and second silicon carbide MOSFET generations based upon planar technologies are now well established, but scaling of these technologies to reduce cost and improve performance is technically challenging,” said David Haynes, senior director of strategic marketing at Lam Research. “Trench-based silicon carbide MOSFETs offer the potential to overcome this scaling barrier for key applications in electric vehicles and hybrid electric vehicles, but the performance and reliability of these devices still must improve to address a broader range of automotive applications.”
In both cases, suppliers are striving to make good parts with a low on-resistance. This involves unwanted resistance between the source and drain. “My goal is to make every generation with a lower specific on-resistance,” said John Palmour, CTO of power and RF at Cree. “It also has to be reliable.”
SiC-based devices, meanwhile, are used in 600-volt to 10-kilovolt applications. At the high end, some sell 3.3- to 10-kilovolt devices, which are used for power grids, trains and wind power.
The big market for SiC falls in the 600- to 1,200-volt range. For this, battery-electric cars are the biggest market, followed by power supplies and solar.
For years, electric-vehicle OEMs used IGBTs and MOSFETs in many parts of the vehicle. Then, instead of using IGBTs, Tesla began using STMicroelectronics’ SiC power devices for the traction inverter within its Model 3 car. The traction inverter provides traction to the motor to propel a vehicle. SiC devices also are used for the DC-to-DC converter and on-board charger in electric cars.
Other OEMs are also evaluating or using SiC. “The performance of silicon carbide provides higher efficiencies. That allows you to go further on a battery charge or reduce the battery, which is the most expensive part of the vehicle,” Palmour said.
Each electric car vendor has different requirements. “We refer to it by bus voltage. A standard bus voltage would be 450 volts. If you have a 450-volt bus, you would use a 650-volt power transistor. Some people are also looking at using an 800- or 850-volt bus. There, you would use a 1,200-volt transistor,” Palmour said. “Some people are looking in between. We are having discussions with people about 650, 900 and 1,200 volts.”
Some OEMs will use SiC. Others may stick with IGBTs due to cost. “The cost is still a challenge and competition from proven silicon-based IGBT solutions will be a reality for some time to come,” Lam’s Haynes said. “At the integrated power module (IPM) level, SiC can win out. Indeed, a SiC power module can deliver significant cost benefits versus all silicon-based IPMs or indeed hybrid IPMs in a smaller form factor. However, at the device level, IGBTs are still substantially cheaper than SiC MOSFETs and have proven performance and reliability, as well as being established in the automotive supply chain. As a result, it is likely that IGBT and SiC technologies will coexist for many years.”
One expert summarized the situation with SiC. “If you look at Japan, they are making 3.3-kV SiC MOSFETs. They have a massive rail infrastructure there,” said Victor Veliadis, executive director and CTO at PowerAmerica, a Manufacturing USA institute that is accelerating the development of wide bandgap semiconductor technology.
“1,200 and 900 volts are automotive. That’s the biggest application for SiC,” Veliadis said. “If you look where silicon carbide is going, it started at 1,200 volts, which is far from where silicon is competitive. Now, it’s trying to work it’s way down and trying to get market share in the 900- and 600-volt range.
“If you go to 600 volts, and you look at everything silicon does, SiC will likely do it more efficiently. There are obstacles when you go to a lower voltage. At 600 volts, silicon is still relatively efficient because of the infrastructure. It’s very cheap. When you go to 1,200, it’s very expensive to do it with silicon,” he said.
Going, going GaN
Meanwhile, GaN, a binary III-V material, has 10 times the breakdown field strength with double the electron mobility than silicon.
GaN is used for LEDs, power electronics and RF. The RF version of GaN is used in 5G, radar and other applications. GaN power devices, which are different, are used in power switching apps. EPC, GaN Systems, Navitas, Panasonic, Transphorm and others sell GaN power devices.
These devices are made on 150mm wafers. Many suppliers have their devices produced on a foundry basis by Episil and TSMC.
In EPC’s GaN flow, a thin layer of aluminum nitride (AlN) is deposited on a silicon substrate. A GaN layer is grown on the AlN layer. A source, gate and drain are formed on the GaN layer.
“From a technical perspective, GaN remains less mature than SiC,” Lam’s Haynes said. “If one considers GaN-on-silicon HEMT (high electron mobility transistor) technologies, yield remains a concern because of the quality of GaN MOCVD growth on silicon. There are also still challenges associated with device performance and reliability.”
That’s not the only issue. “Wafer breakage caused by wafer bow is still an issue for this technology,” KLA’s Crofton said. “The plasma processes are also required to be very low energy because of concerns about plasma-induced device damage. Equipment makers are striving to make their reactors sufficiently benign to the device.”
On the device front, meanwhile, GaN semis are targeted for different markets. EPC and others compete in the lower voltage segments from 15 to 200 volts. In these segments, GaN competes against power MOSFETs.
Others compete in the 600-, 650-, and 900-volt markets. These devices compete against IGBTs, MOSFETs and SiC.
One company, EPC, is expanding its efforts in the low-voltage GaN arena. “It was only about a year ago that EPC did what I call a frontal assault on silicon MOSFETs —and we did that at the 48-volt node,” EPC’s Lidow said. “GaN, at least from EPC, is not only higher performance than MOSFETs, but we also priced it right at MOSFET prices.”
Indeed, there are some changes taking place in the 48-volt market in automotive, data centers and other segments.
For example, data centers consist of a multitude of servers, each of which are housed in a cabinet or rack. The power is distributed in the backplane of the rack using rack-mounted power supplies.
In the data center, AC voltage is generated and fed into the server racks. The power is then converted into a lower voltage. At one time, the racks used 12-volt power supplies. Over time it became inefficient to distribute the power at 12 volts because it caused an increase in power consumption and losses for servers in the data center.
So in 2011, Facebook and others formed the Open Compute Project (OCP). As part of its efforts, OCP pushed for a 48-volt power rack distribution spec, which is more efficient and also reduces power.
“In the past, you would have 12 volts coming onto the board. It would go to a point-of-load converter, and that would convert from 12 volts down to the voltage needed by the microprocessor. But it took a lot of real estate, and it wasn’t efficient,” Lidow said. “Now, they’re bringing the 48 volts onto the server board. And in one big step, they are going all the way down to like 5, 4 or 3 volts. They are using GaN to do that.”
In 12-volt backplanes, some OEMs used 40-volt power MOSFETs, according to a blog from GaN Systems. At 48 volts, OEMs are forced to use 100-volt MOSFETs, but these are less efficient, according to the company.
OEMs also have the option to use 48-volt DC-DC converters based on GaN devices. “They are smaller, cheaper and more efficient. And the dominant topology is something called an LLC topology. And these things are tiny and 98% to 99% efficient,” Lidow said.
Besides servers, GaN is targeted for other 48-volt apps. “It’s coming in on automotive. Instead of having a kilowatt of electrical load, they’re going up to 6 and 8 kilowatts. So they’re going to 48-volt systems. And they’re going to GaN,” he said. “Robots are going to 48 volts for similar reasons. So there are different marketplaces feeding off each other in the advances in GaN.”
GaN, meanwhile, is gaining traction at the 600-, 650- and 900-volt markets, especially the consumer adapter market and other systems. “GaN is making inroads into areas where silicon performance is simply not acceptable,” said Primit Parkih, chief operating officer at Transphorm. “As GaN prices decrease, GaN will continue to chip away at the current market.”
For 600 and 650 volts, GaN is targeted for adapters, automotive and power supplies. At 900 volts, GaN is targeted for automotive, battery chargers, power supplies and solar. Like SiC, GaN is trying to get more traction in electric vehicles, especially for on-board chargers and DC-to-DC converters.
Conclusion
Clearly, GaN and SiC are taking off. These devices give engineers some new and different options.
But they won’t displace silicon. It’s difficult to replace a familiar technology in mission-critical products.
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Indeed, SiC is becoming the ‘go-to’ semiconductor substrate material for PE applications for all the reasons mentioned. At the top of the pyramid are companies like ours (GT Advanced Technologies). We’re focused on producing the crystal boules from which the SiC wafers are made. We are at-scale now for 150mm product, with ‘Tier One’ quality. But our focus is really on rapidly boosting supply and lowering cost so global markets (like EV) can take advantage of this marvelous material.