The secret of SiC's sudden success

Silicon carbide has been touted as a wonder material for power semiconductors, but it has only recently become a commercial success. What was the impetus for SiC's success, and what are the future prospects?

Silicon carbide (SiC) has existed since recorded history (it occurs naturally as a product of cosmic phenomena such as supernovae), but it was first synthesized in 1891. After discovering that the lustrous hexagonal crystals were nearly as hard as diamond and could be produced industrially in large quantities as chips and powders, Aschen found SiC as an incredibly effective abrasive and had his first career. It gave me an opportunity to build.

Of course, for today's electronics engineers, SiC serves a completely different role. SiC power semiconductors can increase energy conversion efficiency, withstand higher voltages and currents, and withstand higher operating temperatures than traditional silicon-based devices, making them ideal for power supplies in data centers, wind and solar power generation. It provides important benefits for equipment such as power conditioning modules, traction inverters in electric vehicles.

But why now? Since the early 20th century, there have been researchers who have studied the electrical properties of SiC. However, these properties were marginal and SiC was quickly replaced by compounds such as gallium arsenide and gallium nitride. With a 10- to 100-fold increase in output, these compounds became the first LEDs, and SiC remained in the lab as a synthetic semiconductor material still looking for applications.

All of this was 40 years before the transistor was discovered, but SiC's properties offer compelling theoretical advantages as semiconductor technology advances. The thermal conductivity of SiC is 3.5 times that of silicon. It can be heavily doped to achieve high conductivity, while maintaining high electric field strengths without failure. Not only does it operate to high temperatures, it is mechanically very stable and has a low coefficient of thermal expansion. So how did silicon emerge at the dawn of the transistor revolution?

The simple answer to this question is economics. Historically, SiC's Achilles heel has been relatively poor in mass productivity, making it difficult to produce high-quality SiC crystals. A wide variety of defects such as edge dislocations, various screw dislocations, triangular defects, and basal plane dislocations occurred in large numbers even on small-sized wafers. In addition, the reverse blocking performance of transistors and diodes is degraded, and the devices do not work effectively. There was also a problem with the interface between SiC and silicon dioxide (SiO2), which is necessary for manufacturing MOSFETs and IGBTs.

Silicon paved the way

While these challenges have prevented chipmakers from realizing the full gains in performance, power density and reliability possible with SiC, silicon semiconductors have proven easier to manufacture at commercial yield levels and have dominated the world of power electronics. However, silicon technology currently cannot deliver the continued improvements required in areas such as datacenter power, automotive and renewable energy.

Fortunately, researchers' efforts to overcome traditional barriers to commercializing SiC are beginning to bear fruit. Improved purity of SiC wafers has increased yields, allowing manufacturers to move from 4-inch to 6-inch wafers, which are believed to reduce device costs by 20-50%. 6-inch wafers began production around 2012. Also, developments in processes such as nitridation (annealing with nitrogen dioxide or nitric oxide) have made it possible to grow silicon dioxide films on SiC substrates, which are required for the manufacture of high-performance power MOSFETs and IGBTs.

While inventive designers continue to squeeze every last drop of performance out of silicon-based devices that have more or less reached their performance limits, the latest SiC technology is advancing even more rapidly: device architecture and dimensional optimization, improved parasitic diode behavior, and new packaging structures are delivering significant benefits that extend the inherent performance advantages of SiC over silicon as high-performance, high-efficiency, ultra-rugged successors in today's most demanding applications.

Many projects do not have the luxury of starting from scratch, and some SiC MOSFETs are not easily drop-in upgrades to existing silicon devices, requiring circuit re-optimization, rethinking gate drive voltages and/or increasing switching frequencies to maximize performance gains.

ON Semiconductor's SiC cascode JFETs can provide the bridge from the silicon past to the SiC future that some system vendors require. Co-packaging low voltage silicon MOSFETs with SiC JFETs to drive high power enables a usable drop-in upgrade, minimizing the commitment required to take advantage of the efficiency, robustness, and power density benefits of SiC. Companies like Sweden's Micropower Group, a manufacturer of industrial battery backup systems, have successfully replaced their outdated silicon MOSFETs with ON Semiconductor's SiC cascode JFETs and have instantly achieved a 10% efficiency improvement at light loads and a 1% efficiency improvement at typical loads.

After more than 100 years and many career changes, the time has come for SiC to play an active role as a valuable power semiconductor technology.

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