Silicon is the material of choice for the vast majority of chips used in microprocessing applications; it’s easy to handle, and manufacturers have learned to carve into it the tiny circuitry that makes possible today’s fast and inexpensive computers. But for all of its celebrity, silicon cannot match the wireless and optical capabilities of more expensive semiconductors like gallium arsenide and indium phosphide.
These materials are called “compound” semiconductors because their crystals-unlike silicon’s-are composed of more than one element. This more complex composition often endows them with desirable physical traits. For instance, because electrons travel faster in many compound semiconductors, the materials can process higher-frequency radio signals and hence larger amounts of data, which is just what you need if you want, say, handheld wireless devices that can receive seamless streams of video.
And unlike silicon, many of these compound semiconductors can emit beams of light when fed just a little bit of electrical current. That makes possible the solid-state lasers that can read the small bits of information densely packed on a CD or DVD. High-speed optical communication networks also rely on compound semiconductors for converting optical information into electronic information, and vice versa, at the thousands of places where optical fibers meet electronic switches and computers.
However, the same complexity that makes compound semiconductors so useful also makes them brittle, hard to synthesize, tough to integrate with other materials-and very expensive. At the moment, a 15-centimeter wafer of gallium arsenide costs about $300, while a 20-centimeter silicon wafer can cost about one-tenth as much. Ramdani’s breakthrough involves a way to deposit a patina of gallium arsenide atop a wafer of standard silicon. The top layer of gallium arsenide provides all the unique capabilities of that material, but putting it on a silicon substrate makes it much easier to handle and cheaper to manufacture.
At first blush, the procedure sounds about as simple as slathering peanut butter on top of a slice of bread. But in practice, it’s much trickier. The fundamental problem, says Fitzgerald, is that the underlying crystalline structures of silicon and gallium arsenide are so different that layering one on top of the other is like stacking grapefruits on a bed of oranges. “You get misfits and extra spaces,” says Fitzgerald. These defects in the crystal tend to snag electrons, disrupting the functions of the semiconducting devices.
So far, the mismatch problem has defeated just about anyone who has ever attempted to make gallium-arsenide-on-silicon wafers. That helps explain why many researchers at companies like IBM and the semiconductor technology startup AmberWave Systems of Salem, NH, are pursuing an alternative approach to more versatile and powerful semiconductors: tweaking silicon so that it behaves more like its fancier cousins. That way, they get the cost benefits of using the 50-year-old infrastructure of silicon manufacturing technology and yet still approach the performance of compound semiconductors.
AmberWave Systems, which was cofounded by MIT’s Fitzgerald, has developed a form of “strained” silicon crystal in which electrons move faster than in regular silicon. The material makes faster transistors possible, and that means, for example, higher-frequency radio signal processors. The researchers grow a layer of a silicon-germanium alloy on top of a silicon wafer and then top the alloy with a thin layer of silicon. Because the distances between atoms in the silicon-germanium crystal are longer than in silicon, the silicon atoms in the top layer must stretch in order to match the spaces between the atoms in the silicon-germanium below. When the silicon atoms are farther apart, electrons move about more freely, hence faster.
In fact, this little bit of crystal engineering has yielded samples in which electrons move up to 80 percent faster than they do in ordinary silicon wafers. Within the next year, AmberWave hopes to see devices made from this material hit the market-microprocessors, for instance, or signal-boosting chips in cell phones.