Silicon is Slow

But there's a limit to how small these circuits can get. Chips are made by using ultraviolet light to etch circuit patterns onto a photoresistant surface. By using light with progressively shorter wavelengths, chipmakers have been able to make tinier and tinier circuits. Eventually, though, the wavelengths needed to cut the circuit will be so small that lenses and air molecules will absorb the light before it's able to carve a pattern. At that point, in about 10 to 15 years, silicon circuits will stop shrinking. Moreover, the cost of revamping the chip fabrication process to utilize light at ever-shorter wavelengths is increasingly high. Ultimately, science will need to turn to other kinds of computer systems. "We're surrounded by computation," says Neil Gershenfeld, who directs the Center for Bits and Atoms at MIT. Breakthroughs may happen, he says, "if you ask nature how it solves a problem. A computer can be a tube of chloroform."


"Is there something beyond silicon?" asks Tom Theis, director of physical sciences at IBM Research. "We've been on a continuous search for that for decades."


Silicon Substitutes

One tactic is to avoid the size limitation of silicon without abandoning the comfort of classical computer circuitry. Molecular electronics, or moletronics, which involves building circuits from carbon and other elements, mimics traditional computing architecture while potentially speeding it up immeasurably. Theoretically, it will be possible to design machines that capitalize on both silicon and molecular circuits, possibly before the decade is out.


Last August, researchers at IBM's Watson Labs in Yorktown Heights, New York, built the first working logic gate made from a single molecule. Using carbon nanotubes, arrangements of atoms that resemble rolls of chicken wire, scientists created a circuit just 10 atoms wide, or 1/500th the size of a silicon circuit. Then, in October, Bell Labs scientists Hendrik Schon, Zhenan Bao, and Hong Meng designed a molecular transistor even tinier than a nanotube-one that's one-millionth the size of a grain of sand. Schon and colleagues sandwiched a thiol molecule-a mixture of carbon, hydrogen, and sulfur-between two gold electrodes, then used the thiol to control the flow of electricity through it. What's important about this nanocircuit is not merely its size. In a discovery that baffles even its creators, the molecule also acts as a powerful signal amplifier-an essential part of a transistor that boosts the electronic signal (or gain). "We were amazed to be able to (operate) at low voltage and achieve such high gain," says Schon. "It was a very pleasant surprise."


If molecules can do double duty as both transistors and amplifiers, then logic gates-and, by extension, an entire chip-could be made not only smaller but also more cheaply, says Stan Williams, director of quantum science research for Hewlett-Packard: "The results are quite stunning and extremely puzzling. If this proves true, it has the possibility of outperforming the best silicon can do."