Over the past decade, one of the biggest stories in semiconductors has been a surprise eclipsing of traditional silicon—in the field of power electronics, where silicon carbide (SiC) and gallium nitride (GaN) have raced past silicon to capture multibilllion-dollar segments of the market. And as major applications fell to these upstarts, with their superior attributes, a question naturally arose. What would be the next new power semiconductor—the one whose superior capabilities would grab major market share from SiC and GaN?
Attention has focused on three candidates: gallium oxide, diamond, and aluminum nitride (AlN). All of them have remarkable attributes, as well as fundamental weaknesses that have so far precluded commercial success. Now, however, AlN's prospects have improved enormously, thanks to several recent breakthroughs, including a technological advance at Nagoya University reported at the most recent IEEE International Electron Devices Meeting, held this past December in San Francisco.
The IEDM paper describes the fabrication of a diode based on alloys of aluminum nitride capable of withstanding an electric field of 7.3 megavolts per centimeter—about twice as high as what's possible with silicon carbide or gallium nitride. Notably, the device also had very low resistance when conducting current. "This is a spectacular result," says IEEE Senior Member W. Alan Doolittle, a professor of electrical and computer engineering at Georgia Tech. "Particularly the on-resistance of this thing, which is ridiculously good." The Nagoya paper has seven coauthors, including IEEE Member Hiroshi Amano, who won a Nobel Prize in 2014 for his role in inventing the blue LED.
[...] "This is a new concept in semiconductor devices," says Jena, of the Nagoya device. The next step, he adds, is fabricating a diode that has a layer of pure AlN at the junction, rather than 95 percent AlN. A layer of AlN just 2 micrometers thick would suffice to block 3 kilovolts, according to his calculations. "This is exactly where this will go in the very near future," he says.
At Georgia Tech, Doolittle agrees that there is still room for enormous improvement by incorporating higher levels of pure AlN in future devices. For example, the breakdown electric field of the Nagoya diode, 7.3 MV/cm, is impressive, but the theoretical maximum for an AlN device is about 15. Thermal conductivity, too, would be greatly improved with more AlN. The ability to conduct heat is vitally important for a power device, and the thermal conductivity of the AlGaN alloy is mediocre—below 50 watts per meter-kelvin. Pure aluminum nitride, on the other hand, is very respectable at 320, in between GaN, at 250, and SiC, at 490.