INDUSTRY POWER ELECTRONICS


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


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 Palmour, 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 Palmour, 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.


Electric and hybrid electric vehicles are markets that could lead to surging sales of SiC MOSFETs.


34 www.compoundsemiconductor.net Issue VI 2014 Copyright Compound Semiconductor

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