May 22, 2002 Santa Barbara, Calif. -- Umesh Mishra, professor of electrical and computer engineering at the University of California at Santa Barbara (UCSB), has teamed up with his old mentor and dissertation advisor at Cornell, Lester Eastman, to write an article on the prospects for the gallium nitride transistor. The article, "The Toughest Transistor Yet," is the cover story of the May issue of IEEE Spectrum.
Both Mishra's and Eastman's research groups have made a Field Effect Transistor (FET) out of gallium nitride. Mishra has also used this most promising of compound semiconductors to make a bipolar transistor. His was the first research group successfully to do so.
Gallium nitride's prominence as the currently most promising compound semiconductor is due in large measure to the work of one man--Mishra's UCSB colleague, Materials Professor Shuji Nakamura, who used gallium nitride to create the first blue, green, ultraviolet, and white Light Emitting Diodes (LEDs), as well as a blue laser.
Nakamura's breakthrough work with the material began in the late 1980s and focused on its optical or light producing properties. Mishra and his UCSB colleague Materials Professor Steven DenBaars were among the group of early researchers working on making electronic devices out of the material.
Research on gallium nitride dates back to the 1930s. Mishra says work in earnest on the material occurred from the 1960s onward. What set Nakamura apart from all the other gallium nitride pioneers? Said Mishra, "Shuji managed to see the forest through the trees. Others kept getting stuck on the many scientific problems. Shuji reached the Holy Grail of the blue LED by solving critical problems, but also by avoiding others. He didn't get stuck! Everybody else got stuck."
Mishra gave a seminar May 10 to Santa Barbara researchers who are participants in the California NanoSystems Institute ([CNSI] one of the California Institutes for Science and Innovation initiated by Gov. Gray Davis to seed the future of the State's economy). Mishra predicted there that the research aimed at gallium nitride use for electronic devices would piggyback on advances made by the optical researchers. The huge market promise of the optical devices--replacement of incandescent lighting and red lasers in storage devices such as CDs and DVDs, for instance--was driving and would drive that research forward faster.
The Spectrum article lays out the promise for gallium nitride electronics. The prospects are not as dazzling as replacing the incandescent light bulb invented by Thomas Edison. In other words, the gallium nitride transistor is not a replacement for the ubiquitous silicon FETs that make up computers. The target for transistor replacement isn't silicon, but gallium arsenide, which has rocketed into prominence in the 1990s with microwave transmission from base station to cell phone and other wireless technologies.
What gallium nitride has going for it is a wide band gap between the valence and conduction bands of the semiconductor. (The valence and conduction bands are the outermost of the energy levels wherein electrons orbit the nucleus of atoms; the conduction band is outermost.) That wide band gap is responsible for the materials' ability to produce high-frequency blue and ultraviolet light.
The bonding between nitrogen and gallium atoms in a unit cell of gallium nitride is very tight. The strength of the bond between the individual atoms determines the width of the gap between the valence and conduction bands of a semiconductor. The stronger the bonds, the larger the separation between the valence and the conduction bands, and the greater the band gap.
The larger the band gap, the more voltage that can be applied, and that is what is key for electronic devices. Power is the product of voltage and current. Gallium nitride enables a transistor that can handle not only high voltage but also high current. So the product--power--goes up between 10 and 100 times over silicon or gallium arsenide.
For a communications device, higher power enables one of two outcomes: greater range of transmission or smaller transmission devices. The value of greater range is immediately evident for military uses--i.e, spotting enemy aircraft 100 miles further away provides a life-over-death advantage.
The smaller the device, the greater is the bandwidth from the device. Explains Mishra, "With the same amount of power but smaller size, a device has lower capacitance--which is basically how much 'tax' [storage of charge] you have to pay to the device before it releases microwaves. The smaller the device, the lower the tax and the faster the energy can move out of it. And broadband means high frequency microwaves.
"People want to do everything on the phone," said Mishra. "Gallium nitride can take wireless communication to the next level--downloading, video for instance, at blazing speed. With gallium nitride transistors will come a whole new set of functions."
Gallium nitride transistors will, according to Mishra, look pretty much like transistors made of other semiconductors, but perform much better. "That is always appealing for adoption of the technology because you don't have to reeducate people on how to use it. In effect, the box will look the same, and the human interface with the box will be the same, but what is inside will perform much better."
Mishra predicts that gallium nitride will do for wireless transmission what it has done for optoelectronics: bring about a revolution.
That prediction is based, of course, on insider knowledge. Mishra gave hints at the May 10 CNSI seminar at UCSB of dramatic developments in the gallium nitride FET that may issue from his lab in, say, 18 months: "a stable device with high performance and that performs according to specifications."
He envisions the market entry of the gallium nitride transistor at the wireless base station, which sends transmission to individual cell phones. The next step would be the use of these transistors in cell phones. The results, he says, are likely to be magical: the all-purpose phone- or wristwatch-sized communication device for conversing, surfing, e-mailing, video viewing. All but the video viewing now take place, of course, but the really operative words Mishra keeps using are "blazing speed," the result of high wireless bandwidth.
Asked about those developments in his lab, to which he alluded in the CNSI seminar, Mishra deploys a baseball metaphor: "I am a believer in grand slams. First we have to get the bases loaded, and then we have to hit the home run. Hitting a homer with the bases loaded is the kind of development that propels technology forward. The bases are loaded. Now we need to hit that home run. That's where we think we are now. If not, we strike out. That's part of the game too. But I am an optimist."
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