April 2003

Computing on Glass

A new form of silicon enables see-through circuitry.

By Bruce Gain

Silicon is a substance that demands compromises. If you want fast electronics for your PC, you need the good stuff: single-crystal silicon. If you can do with somewhat slower electronics but need them to be thin and transparent for the screen of your laptop, you use slower amorphous silicon. A few years ago, attempting to achieve the best of both worlds, Sharp and Semiconductor Energy Laboratories in Japan collaborated to develop continuous-grain silicon, a new form of the element that's both transparent and relatively fast at carrying electrons. Now, applying the technology to assemble a rudimentary processor on the back of the pane of glass used for a liquid-crystal display (LCD), they have created a prototype "sheet computer."

Continuous-grain silicon's slower cousin, amorphous silicon, can also be deposited in a see-through film onto glass and plastics. That makes it good for the thin-film transistors already used to control pixels in a laptop's active-matrix LCD; the material transports electrons fast enough to switch pixels on and off at the required 60 to 160 times per second. But the Sharp researchers found that continuous-grain silicon, composed of many small silicon crystals linked by atomic bonds, is a far more fluid medium for electrons: transistors made from the material can handle thousands or millions of switching operations per second-enough to run useful programs.

"Continuous-grain siliconoffers atomic-level continuity, which enables electrons to travel smoothly and with high mobility," explains Shigeo Misaka, the Sharp executive vice president in Japan who heads his company's research efforts.

In demonstrations in Japan last October, the 13,000-transistor prototype's clock speed hit 2.6 megahertz-not fast enough to run such programs as Microsoft Windows XP, which requires at least 233 megahertz of processor speed, but enough to manage such personal digital assistant (PDA) functions as datebooks, address books, and simple two-dimensional games. Because of continuous-grain silicon's 600-fold speed advantage over amorphous silicon, Sharp's prototype could be the precursor to a full-fledged sheet computer: a device with processor, memory, and LCD elements all bonded to a single layer of glass or plastic only a few millimeters thick. Such a computer need be no thicker than its screen. Within two years, Sharp expects to be using the technology to mass-produce fully functional PDAs the size and thickness of credit cards.

Continuous-grain silicon, however, offers only medium-speed-computing capabilities, says Philip J. Bos, a Kent State University physics professor who specializes in flat-panel, liquid-crystal media. "So you couldn't build a Pentium on this substrate, but it could likely serve as an electronic touch pad, wall bulletin, or PDA for the typical LCD flat-panel price of $200," he says.

Eventually, the new form of silicon could be used in products such as tablet computers, even flatter flat-screen monitors, and television screens thin enough to blend into walls. Not a bad compromise.