STANFORD, Calif., July 8 (AScribe Newswire) -- Silicon computer chips depend on metal wiring that is fast becoming a source of delay and wasted power. Many engineers would rather use light to transmit data over the longer distances on chips, much in the same way that the light in fiber optic cables efficiently carries virtually all long-haul telecommunications traffic. One of the key problems in realizing the goal, creating an intense light source that is compatible with silicon, now seems much closer to being solved based on advances reported by engineers including a team at Stanford University.
"Electrical metal interconnects are consuming almost 50 percent of the total power of an active chip and that is going to increase," says Yoshio Nishi, a Stanford professor of electrical engineering who helped lead the new research. "That's the reason why optical interconnects have been looked at very carefully and very aggressively."
The Stanford research, in which a team built a light-emitting diode on a silicon chip using a ringed structure of the element germanium, appeared in the June 8 version of the journal Optics Express. Only two months earlier, researchers at MIT announced a similar result in Optics Letters. Each paper suggests that light on a chip - an advance sought for decades - finally seems feasible.
Over the years researchers have tried various techniques to prompt either silicon or germanium, or some combination of the two, to produce light, but none had succeeded in producing an intense enough light at the right wavelength, 1.5 micrometers, and at room temperature.
Nishi said the turning point came when researchers focused on filling a gap -quite literally- in germanium's light emitting potential.
The band gap trap
Like many materials, germanium atoms have certain "band gaps," where electrons jump when they are excited by incoming energy. It is not hard to make the electrons jump to an "indirect band gap" where light emission is unlikely. It is much more difficult to make them jump past that gap to the "direct band gap" where light emission right at the 1.5 micrometer wavelength is a much more probable result. The indirect band gap is almost like a deviously placed sand trap on a golf course, where the green is the direct band gap and the ball is the electron.
Using that simile, the trick employed independently by the Stanford and MIT teams was to fill in the sand trap with so many golf balls, that each new ball hit would just bounce right off the pile and roll onto the green. The Stanford means of doing this was to fill the germanium with so much negative charge (a process called doping), that electrons filled the indirect band gap and cleared the way for successively excited electrons to leap to the direct band gap.
The Stanford team found in experiments that the higher the temperature and the higher the voltage applied to the diode, the more intense the light emission.
The next step, Nishi says, is to improve the intensity by making a laser from the new light source. From there, other advances are still necessary before engineers can start integrating light into computer chips. These include on-chip light detectors, and optical conduits, Nishi says, as well as new tools for chip architects to make use of the technology in their designs. These advances will still take more than five years, he predicts.
Once implemented, however, optical signals could make chips both faster and more efficient, two goals that in the current regime of all-electrical chips seem to be directly at odds. For years, performance increases have come with increased energy inefficiency in the form of excessive heat.
The lead author of the Stanford paper was materials science and engineering doctoral student Szu-Lin Cheng. Other authors included electrical engineering doctoral students Jesse Lu, Gary Shambat, and Hyun-Yong Yu. In addition to Nishi, the senior authors were electrical engineering Professor Krishna Saraswat and associate professor Jelena Vuckovic. The research was supported by funding from Toshiba, the semiconductor industry, the U.S. Air Force and the National Science Foundation.
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CONTACT: David Orenstein, Stanford School of Engineering communications, 650-736-2245, davidjo@stanford.edu
Media Contact: See above.
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