SiC chip market will usher in a big explosion

Source: This article is translated from "Semiconductor Engineering" by the public account Semiconductor Industry Observer (ID: icbank), thank you.

Electric vehicles are driving the market for SiC power semiconductors, but cost remains an issue.

The silicon carbide (SiC) power semiconductor market is experiencing a sudden surge in demand as electric vehicles, among other systems, grow.

But demand has also led to tight supply of SiC-based devices in the market, prompting some suppliers to increase fab capacity during a tricky wafer size transition. Some SiC device makers are transitioning from 4-inch wafers to 6-inch wafers.

SiC is a composite semiconductor material based on silicon and carbon. In the production process, specialized SiC substrates are developed and then processed in the fab to obtain SiC-based power semiconductors. Many SiC-based power semiconductors and competing technologies are specialized transistors that can switch the current of the device at high voltage. They are used in the field of power electronics to achieve the conversion and control of power in the system.

SiC stands out for its wide bandgap technology. Compared with traditional silicon-based devices, SiC has a breakdown field strength 10 times that of traditional silicon-based devices and a thermal conductivity 3 times that of traditional silicon-based devices, making it very suitable for high-voltage applications such as power supplies, solar inverters, trains and wind turbines. In addition, SiC is also used to manufacture LEDs.

The biggest growth opportunity is in the automotive sector, especially electric vehicles. SiC-based power semiconductors are used in onboard chargers for electric vehicles, and the technology is finding its way into a key part of the system, the traction inverter. The traction inverter provides traction to the electric motor to propel the vehicle forward.

For this application, Tesla uses SiC power devices in some of its models, while other electric vehicle manufacturers are evaluating the technology. "When people discuss SiC power devices, the automotive market is undoubtedly the focus. SiC activities by pioneers such as Toyota and Tesla have brought a lot of excitement and noise to the market. SiC MOSFET has potential in the automotive market. But there are still some challenges, such as cost, long-term reliability and module design," said Hong Lin, an analyst at Yole Développement.

According to Yole, driven by automotive and other markets, the SiC power device business reached $302 million in 2017, up 22% from $248 million in 2016. “We expect a leap in 2018, driven by the automotive industry, due to the ramp-up of Tesla Model 3 production, which uses SiC MOSFET modules,” said Lin.

According to Yole, the SiC power semiconductor market is expected to reach $1.5 billion by 2023. Suppliers of SiC devices include Fuji, Infineon, Littelfuse, Mitsubishi, ON Semiconductor, STMicroelectronics, Rohm, Toshiba and Wolfspeed. Wolfspeed is part of Cree. X-Fab is the only foundry for SiC.

Power electronics plays a key role in the global power infrastructure. This technology is used in industry (motor drives), transportation (cars, trains), computing (power supplies) and renewable energy (solar, wind). Power electronics converts alternating current and direct current (AC & DC) in the system.

For these applications, the industry uses a variety of power semiconductors. Some power semiconductors are specialized transistors that act as switches in the system. They allow power to flow in the "on" state and stop it in the "off" state.

Power semiconductors are manufactured at mature nodes. These devices are designed to improve efficiency and minimize energy losses in the system. Typically, they are rated based on voltage and other specifications rather than process size.

For many years, the mainstream power semiconductor technology has been (and still is) silicon-based, namely power MOSFETs and insulated gate bipolar transistors (IGBTs). Power MOSFETs are considered the cheapest and most popular devices and are used in adapters, power supplies and other products. They are used in applications up to 900 volts.

In a conventional MOSFET device, the source and drain are located on the top of the device. In contrast, a power MOSFET has a vertical structure where the source and drain are located on opposite sides of the device. The vertical structure enables the device to handle higher voltages.

The most prominent mid-range power semiconductor device is the IGBT, which combines the characteristics of a MOSFET and a bipolar transistor. IGBTs are used in applications ranging from 400 V to 10 kilovolts.

The problem is that power MOSFETs and IGBTs are reaching their theoretical limits and there are unnecessary energy losses. Devices experience energy losses due to conduction and switching. Conduction losses are caused by resistance in the device, while switching losses occur during the switching state.

“Silicon MOSFETs have been a good technology from 5 volts up to a few hundred volts,” said Guy Moxey, senior director of power marketing and applications at Wolfspeed. “When you get to 600 volts to 900 volts, the silicon MOSFET is good, but it starts to lose energy. The IGBT is a good weightlifter, but it’s not fast or efficient.”

That’s where SiC comes in. Power semiconductors based on gallium nitride (GaN) are also emerging. Both GaN and SiC are wide-bandgap technologies. Silicon has a bandgap of 1.1 eV. In comparison, SiC has a bandgap of 3.3 eV and GaN has a bandgap of 3.4 eV.

"The electronic bandgap is the energy separation between the top of the valence band and the bottom of the conduction band in a solid material," Mouser Electronics said in a blog post. "It is this bandgap that allows semiconductors to switch electrical current on and off as needed to achieve specific electrical functions."

Wide-bandgap devices offer several advantages. For example, electric vehicles are powered by motor drives, which traditionally use power MOSFETs or IGBTs. “If you replace that motor drive with SiC, you get 80% lower losses in the drive,” said Wolfspeed’s Moxey. “That means you can use a smaller battery for the same range. A smaller battery means lower cost.”

Meanwhile, SiC-based power semiconductors are used in 600-volt to 10-kilovolt applications. Moxey said: "600-1,700 volts are suitable for most SiC applications. When the voltage reaches 3.3-10 kilovolts, it is very suitable. For example, wind power generation and small grids."

In the power supply field, GaN is used in 30-600 volt applications. Moxey said: "GaN and SiC are complementary technologies, not competing technologies."

Both GaN and SiC devices are faster than silicon, but also more expensive. “Currently, the cost per ampere of a SiC MOSFET device is more than five times higher than that of a comparable IGBT,” said Elena Barbarini, director of devices at Yole’s System Plus Consulting division.

The first SiC-based device appeared in 2002 with the introduction of SiC diodes, followed by the SiC power MOSFET in 2011. Similar to power MOSFETs, SiC-based devices are vertical structures.

SiC power MOSFETs are power switching transistors based on SiC. Mitch Van Ochten, applications engineer at Rohm, explained: “A diode is a device that conducts current in one direction and blocks current in the opposite direction.”

Regardless, SiC power semiconductors are growing. “Silicon plays an important role in power devices,” said Mike Rosa, director of strategy and technical marketing at Applied Materials. “But when you talk about higher power and lower weight, manufacturers are focusing on materials like SiC.”

SiC-based devices are produced in fabs as the industry continues to transition to a different wafer size. “SiC can be used on 4-inch or 6-inch wafers,” Rosa said. “The industry is desperately chasing 8-inch wafers.”

In fact, Cree has already completed the transition from 4-inch (100mm) wafers to 6-inch (150mm) wafers. Rohm and others are in the transition phase. SiC on 200mm wafers will not appear for some time.

Typically, when migrating to a new wafer size, the number of dies per wafer increases by a factor of 2.2. Larger wafer sizes can reduce overall production costs.

In the digital CMOS world, chipmakers transitioned from 4-inch to 6-inch wafers several years ago. Making the same transition for SiC sounds simple, but there are some challenges. “While mass production of SiC power devices on 150mm wafers has been proven for nearly five years, the availability and cost of high-performance, low-defect-density SiC substrates at 150mm remain barriers to adoption,” said David Haynes, senior director of strategic marketing at Lam Research.

“That said, as the transition to 150mm volume production is achieved, the associated cost savings will help drive commercial viability in an increasing number of applications,” said Haynes. “Another example is the roadmap for SiC MOSFET technology. Planar SiC MOSFETs have been proven in commercial applications for some time, but today there is a major push for the development and commercialization of trench SiC MOSFETs, which can offer significantly lower on-resistance compared to planar structures.”

Meanwhile, in the fab, SiC-based power devices generally follow the same process flow as silicon-based chips. But there are also some differences, such as the development of SiC substrates.

For silicon-based chips, the first step in the process is to develop a raw silicon wafer. To do this, a silicon seed crystal is placed in a crucible and heated. The resulting body is called a silicon ingot, which is pulled and cut into silicon wafers of 300mm and smaller sizes.

However, for SiC, the process is that SiC bulk crystals are placed in a crucible, heated, and the resulting ingot is pulled out and sliced ​​into thin sheets.

For many years, SiC bulk crystals have been plagued by defects called micropipes, which are micrometer-sized holes running through the crystal. "Micropipe defects and other defects that can destroy device operation are now almost eliminated. Material suppliers are now offering zero-micropipe products," said Peter Gammon, associate professor at the University of Warwick.

Once the SiC wafer is developed, the next step is to form the SiC substrate. The bare wafer is inserted into a deposition system, and a SiC epitaxial layer is grown on the wafer to form the SiC substrate. The SiC substrate is then processed in the fab and inspected for defects using inspection systems. SiC devices are prone to defects, especially as suppliers move to larger wafer sizes.

“SiC has a lot of defects,” said Lena Nicolades, vice president and general manager of KLA-Tencor’s LS-SWIFT division. “For SiC, our inspection system uses a shorter wavelength. It finds the discontinuities in the substrate.”

At the same time, the automotive industry is the fastest growing sector in the entire semiconductor industry. Walter Ng, vice president of business development at UMC, said: "More and more customers are redefining their product portfolios to adapt to the Internet of Things and automotive markets. This year, our automotive-related revenue has grown significantly. We expect that automotive-related revenue will continue to grow in the foreseeable future."

SiC is also seeing growth in the automotive sector, especially in electric vehicles. Electric vehicles, both pure electric and hybrid, account for about 1% of global car sales today. Driven by China and other countries, the electric vehicle market will grow from 1.6 million units in 2018 to 2 million units in 2019, according to Frost & Sullivan. By 2025, the market is expected to reach 25 million units.

“The adoption of electric and hybrid vehicles is definitely happening,” said Lam’s Haynes. “However, the timing and rate of adoption varies widely around the world and is closely tied to government policy and consumer access to appropriately priced products and charging infrastructure. There is no doubt that the Chinese market is the main growth engine for electric vehicles.”

In electric vehicles, there are several areas of systems such as entertainment system, on-board charger, traction inverter, etc. The traction inverter converts the energy from the battery to the traction motor, which propels the vehicle forward.

SiC is making inroads into on-board chargers, DC-DC converters, and traction inverters. The on-board charger charges the vehicle from the grid.

Figure 1: Power electronics in electric vehicles (Source: STMicroelectronics)

A DC-DC converter takes the battery voltage and steps it down to a lower voltage that is used to control windows, heaters, and other functions.

A battle is taking place between device manufacturers in the field of traction inverters, especially for pure battery electric vehicles. Generally speaking, hybrid vehicles are moving towards 48-volt batteries. For electric inventors, SiC is generally too expensive for hybrid vehicles, although there are exceptions.

Like hybrids, pure battery electric vehicles consist of a traction inverter. A high-voltage bus connects the inverter to the battery and the motor. The battery provides energy to the car. The electric motor that propels the car forward has three wires.

These three wires extend to the traction inverter and are then networked to six switches within the inverter module.

Each switch is actually a power semiconductor that acts as an electrical switch in the system. For the switches, the existing technology is IGBT. So a traction inverter can consist of six IGBTs with a rated voltage of 1200 volts.

“Effectively, they are electrical switches,” said Rohm’s Van Ochten. “We can choose the technology for these electrical switches, and they enable and disable the various motor windings and effectively spin the motor. The most popular electronic semiconductor switch used for this function is called an IGBT. More than 90% of automakers use them. They are the cheapest way to convert battery current to the motor as needed.”

However, there are some trade-offs with using IGBTs. “IGBTs are probably a third of the price of the latest technology, but they are slow,” Van Ochten said.

That’s why the industry is eyeing SiC MOSFETs, which offer faster switching speeds than IGBTs. “SiC MOSFETs also reduce switching losses while reducing conduction losses at low and medium power levels,” said Maurizio Ferrara, director of STMicroelectronics’ Wide Bandgap and Power RF business unit. “They operate at four times the frequency of IGBTs. This reduces weight, size and cost due to smaller passive components and fewer external components. As a result, SiC MOSFETs can increase efficiency by 90% compared to silicon-based solutions.”

So, for traction inverters, it makes sense to move from IGBTs to SiC MOSFETs. But it’s not that simple, as cost plays a big role in the equation.

However, Tesla has already taken the plunge. According to Yole, Tesla is using SiC MOSFETs produced by STMicroelectronics in the Model 3. Yole also added that Tesla also uses products from other suppliers. Other automakers are also exploring the technology, although most OEMs have not joined the ranks due to cost considerations.

However, there are several ways to make the transition from IGBTs to SiC MOSFETs. According to Rohm, there are several options:

• Leave the IGBTs in the system but replace the silicon diodes with SiC diodes.

• Replace all IGBTs and silicon diodes with SiC MOSFETs and SiC diodes.

In the inverter, there are six IGBTs, each with a separate silicon diode. The diodes are used for several reasons. “IGBTs cannot withstand reverse EMF and excessive voltage,” said Rohm’s Van Ochten. “So you need a diode on each IGBT to prevent it from being destroyed when you turn off the switch.”

One way to make the system more efficient is to replace the silicon diodes. Van Ochten said: "The first step to improve the efficiency of the traction inverter is to leave the IGBTs in. Then replace the normal silicon diodes with SiC diodes. SiC diodes have better performance. This can improve efficiency."

The ultimate solution is to replace IGBTs and silicon diodes with SiC diodes and SiC MOSFETs. “SiC is more expensive than silicon because of the price of the materials,” said Wolfspeed’s Moxey. “But if you can switch four or five times faster, you can reduce the cost of magnetics and capacitors.”

Where will all this go? “When we look at different applications, we expect that charging stations and on-board chargers will be among the first to adopt SiC technology,” said Shawn Slusser, vice president of Infineon’s automotive division.

“As for automotive applications, we expect IGBTs to dominate the market over the next decade,” said Slusser. “SiC has the advantages of high efficiency and high power density, but at a higher cost. This means that the advantages of reduced size and battery capacity need to compensate for the higher cost. This is why we believe that SiC will be used first in on-board chargers, as SiC efficiency at higher switching frequencies and smaller passive components can compensate for the high cost of SiC devices. As long as the battery cost savings outweigh the added cost of SiC devices, SiC will be widely used in the main inverter application of large battery electric vehicles. For electric vehicles with 800-volt systems, there are other advantages, such as shorter charging time, higher inverter efficiency and lower cable costs.”

To be sure, SiC is heating up, and so are electric vehicles. If suppliers can drive down costs, SiC power semiconductors appear poised to become the dominant player. But that’s easier said than done.

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