Technical Insight
The maturing MOSFET
Sales of SiC MOSFETs are rising on the back of falling prices, expanding product portfolios and the entrance of new chipmakers into the market
Electric and hybrid electric vehicles are markets that could lead to surging sales of SiC MOSFETs.
RICHARD STEVENSON REPORTS
Diodes and transistors are two of the key building blocks in many circuits. Diodes are used to control the direction of the current, while transistors act as switches, turning the flow of charge on and off.
Circuits incorporating diodes and transistors may be used to power motors, drive electrical equipment and convert the output from a solar cell into a form that can be fed into the grid. In all these cases, increases in circuit's operating efficiency are highly valued, because they trim carbon footprints.
To increase circuit efficiency, designers are replacing silicon components with those made from SiC. Switching to the wide bandgap alternatives slashes recovery times, which means that the devices cannot only turn on and off more efficiently "“ they can be deployed in circuits operating at far higher frequencies. Going up in frequency allows a trimming of the size of the capacitors and inductors, leading to savings at the system level, thanks to a reduction in the size and weight of the circuit. What's more, SiC devices have a far higher maximum operating temperature than their silicon equivalents, so cooling demands are far, far lower.
Since 2001, circuit designers have been inserting diodes made from SiC. But only more recently have they been able to use them alongside SiC transistors, and start to exploit all the benefits associated with this wide bandgap technology.
Waiting for the MOSFET
Designers had to wait until 2008 for the first SiC transistor to hit the market: a SiC JFET from the now defunct SemiSouth. But this class of transistor, along with several other forms of device, such as the BGT and BJT, has not been that successful. Why? Because they are not drop-in replacements for the silicon IGBT. Instead, they have to be paired with another device to be converted from a normally on to normally off transistor, and this transformation adds to cost and size while impairing efficiency.
One device that doesn't suffer the same fate is the SiC MOSFET. Due to this, market analyst Philippe Roussel from Yole Développement is tipping this particular transistor to lead the way. "To me it's logical and obvious that the MOSFET is the perfect solution," says Roussel.
The first to go to market with this form of SiC transistor was the US chipmakers Cree, which launched a 1200 V MOSFET in May 2011.
"We got lots and lots of interest and shipped a lot of parts," recollects John Palmer, co-founder of the company and chief technology officer for firm's Power and RF business units. "[The SiC MOSFETs] did everything that people thought they would," says Palmer, who points out that the only major downside was their price.
In the intervening years Cree has worked hard to address this weakness. Costs have fallen, partly through increases in yield, and also via a reduction in chip sizes while maintaining current ratings.
Succeeding on these fronts has helped Cree to maintain its pole position in the SiC MOSFET market. Competition initially came from Rohm of Japan, but in the last few years the likes of Microsemi and Mitsubishi have launched rival products.
Figure 1: A Cree fabrication engineer removes a 100 mm SiC wafer from a spin rinse dryer, which is a fabrication tool used to clean the SiC wafer as part of the fabrication process used to make SiC MOSFETs.
Expanding Cree's portfolio
One move that Cree has made to increase the competiveness of its MOSFETs is to broaden its portfolio, by offering different products with different current ratings. This approach, which is one that makers of silicon IGBTs and MOSFETs have taken for many years, means that customers don't have to buy a bigger, more expensive chip when a smaller one with a lower current rating will suffice.
"There is no point in making [customers] pay more "“ that only hurts both of us," argues Palmer. He points out that if prices are too high, customers will not buy these parts, and that is detrimental to the adoption of the company's SiC MOSFETs.
Expansion of the Cree portfolio has not included the launch of a 600 V SiC MOSFET to complement its 600 V SiC Schottky barrier diode. The reason is the competition from silicon: 600 V diodes are bipolar, so inherently slow, whereas 600 V transistors can be unipolar, super-junction MOSFETs. "They are quite fast," admits Palmer. "That does not mean that SiC could outperform it, because our capacitances would be far lower. But we choose to take on bipolar devices at higher voltages."
Going up in voltage makes a lot of sense. The recovery losses for SiC are one-fifth of those for silicon at 1200 V, but just one-tenth at
1.7 kV, and one-thirtieth at 3.3 kV. To allow customers to benefit from this superiority at higher voltages,Cree launched a 1.7 kV device in 2012.
Cutting costs
In 2013, Cree took a tremendous stride in increasing the affordability of its MOSFET line up by launching a range of second-generation devices that roughly halved the cost-per-amp. By reducing resistances, such as trimming the specific on-resistance from 8 mΩ cm-2 to about 5 mΩ cm-2, engineers were able to maintain current ratings while shrinking die size. In turn, this led to an increase in yield, further trimming production costs.
Circuit designers are embracing the new products. "Gen II is now the majority of MOSFET sales," says Palmer. "We've had a very good adoption rate there."
One reason for this is that these second-generation devices can lead to costs savings. "Even though the component cost is higher than silicon, it saves money at the system level," says Palmer.
This is the case in solar inverters, a market where Cree is seeing a lot of activity. The company's 1200 V MOSFET is now being deployed in Delta Energy Systems' 11 kW PV inverter.
"Another area where we have had a lot of success is industrial high-frequency power supplies," says Palmer. These units, which are being deployed in semiconductor processing equipment, enable an increase in the power from a rack-mounted power supply. "You can double the amount of power they get out of the same sized box."
Expansion of the family of Cree's second-generation products continues. "We recently announced a 50 A packaged discrete: That's a lot of juice for a discrete package, and we've had a lot of interest in that," reveals Palmer.
Within the research and development group, efforts are focused on generation III products, which continue the die shrink approach applied during the move from generation I to generation II.
At a recent conference, Palmer presented results for generation III products that can operate at 900 V, 1200 V, 1700 V, 3.3 kV, 6.5 kV, 10 kV and 15 kV. "So we've done it across the board, and the question is what the first product will be," says Palmer. "We'll have to wait and see, but I would not expect it to be that long "“ I would expect that somewhere in 2015 we'd see a gen III product announced."
Although the number of suppliers of SiC MOSFETs is on the rise, Palmer still sees silicon as the main competitor for chip sales. "It's not all done in silicon. There is still room to improve, and it has a thirty year head-start on us."
Figure 2: European power electronics giant ST Microelectronics is launching a 1200V SiC MOSFET this autumn.
European supplier
Right now, the number of SiC MOSFET suppliers is set to increase, with one of the biggest manufacturers of power transistors, ST Microelectronics, launching a 1200 V, 45 A device.
"We are using all of the skills we've assembled "“ manufacturing, design, service, supply chain "“ to become a leader also in wide bandgap technologies," claims Michele Macauda, SiC and GaN Marketing Manager at STMicroelectronics.
According to him, many customers that buy SiC Schottky diodes from them have been asking the company for SiC MOSFETs. ST has taken some time to answer this call by ensuring that the new transistor delivers industry-leading reliability and quality. Guide price for the MOSFET, which is housed in a proprietary package optimised for high thermal performance, is $35 when shipped in quantities of 1000 or more.
"We are working with our key customers and partners to ensure that we are competitive," says Macauda. "As is the history of the semiconductor industry, prices will likely come down in the coming years."
Plans for the company include expanding the MOSFET range by introducing devices with higher and lower current ratings, and higher blocking voltages. "We also plan to offer different package options," adds Macauda, who expects the majority of transistors to be initially deployed in solar invertors. Further ahead, makers of products for hybrid electric and electric vehicles should account for the lion's share of sales.
Competition in Japan
The pioneer of SiC MOSFETs in Japan is Rohm, but in the last few years Mitsubishi Electric has also launched products. In July 2012 Mitsubishi started shipping samples of SiC power modules, and in 2013 it followed this up with the launch of modules for home appliances, industrial equipment and rail traction systems, and this year it added modules for high-frequency switching applications.
Figure 3: Mitsubishi's recently launched traction inverter system, which features SiC diodes and transistors, has a switching loss approximately 55 percent less than its conventional inverter system incorporating IGBTs power modules.
"We have in-house customers in various applications fields, like traction, home electronics and industry," explained a spokesperson on behalf on Mitsubishi. "We believe that we can enhance competitive advantage by providing devices to in-house customers and getting detailed feedback from them."
To increase market share for SiC MOSFETs, those at Mitsubishi believe that SiC substrates must fall in price and their procurement must be more stable.
Meanwhile, Roussel argues that the increased number of suppliers could help all of these firms: "With the JFET there are only two sources, whereas with the MOSFET there are various sources. Multi-sourcing is something that is key for the system integrators."
The French analyst does not calculate a figure for the SiC MOSFET market, but he has determined a value for the SiC power electronics market, which is a mixture of die, discretes and modules "“ and he estimates that two-thirds of this is related to the MOSFET, with the remainder associated with Schottky barrier diodes.
Figure 4: A Cree wafer containing 50 A SiC MOSFET die.
"Today, the overall market [for SiC power devices] will be $115-120 million, and the market size will probably range from $500-600 million in 2020." When looking that far ahead, there are uncertainties, with market growth relying on the deployment of SiC devices in electric and hybrid electric vehicles. Recently, it appears that makers of these types of vehicles may be pushing out deployment of wide bandgap devices until the next decade.
That's not good news for the MOSFET, but even so, shipments of this device will still rise at a healthy rate over the coming years.
