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Vacation Visions

Tweaking silicon could make it faster, but for optical capabilities you still need compound semiconductors. While a number of researchers are trying to grow compound semiconductors on silicon, Motorola believes it has a head start in the race to commercialize the technology-thanks to both Ramdani and the company’s well-established manufacturing and marketing infrastructure.

The history of Ramdani’s breakthrough actually begins at least a year before his fateful Spanish vacation. Ramdani was part of a Motorola research group trying to make silicon faster when he made an accidental discovery that would lead to the gallium-arsenide-on-silicon project. At that time, he and his colleagues were focusing on the thin, glasslike layer of silicon dioxide that forms on top of silicon when it is exposed to oxygen during chip processing. This layer, known as a dielectric, is a vital chip component because it enables one transistor to control the electrical state of another while preventing electrons from leaking between them.

But as transistors get smaller and this layer gets thinner, it becomes more prone to leaking electrons. To solve this problem, Ramdani and his colleagues Ravi Droopad and Jimmy Yu were experimenting with an alternative to silicon dioxide-strontium titanate-that could improve the performance of silicon-based chips. Yet as the Motorola researchers deposited a bee’s breath’s worth of strontium titanate on a silicon surface, an intervening layer of silicon dioxide formed. It was like inadvertently covering a window with a layer of black paint when all you wanted to do was slightly tint it.

And then, Ramdani visited that Spanish beach. While relaxing on the sand, he realized that the silicon dioxide layer, along with the strontium titanate, might serve a far larger purpose than he had originally imagined: intermediate layers that, when sandwiched between silicon and gallium arsenide, could reconcile the crystalline mismatch between the two semiconductors. That’s because the distances between the atoms in strontium titanate, when on top of the silicon dioxide layer that forms underneath it, are longer than those in silicon but shorter than those in gallium arsenide. In fact, it’s the silicon dioxide that causes the atoms in strontium titanate to relax completely and assume a configuration more in line with that of the gallium arsenide atoms above. Within days of returning from his vacation, Ramdani and his team of engineers succeeded in growing gallium arsenide on silicon using these intervening layers (see “Superchip Ingredients”).

Compounding Interest

As the Motorola researchers refine their technology over the coming years and learn to grow other compound semiconductors on top of silicon, the potential applications of the material ought to continue expanding. As Ramdani and his colleagues see it, the same type of inner layer that they use to marry gallium arsenide to silicon could be used to grow indium phosphide or any number of other high-performance compound semiconductors on the same inexpensive silicon substrate. Each of these compound semiconductors has its own personality-its own speed and light-emitting properties.

Such technology could also lead to new kinds of devices or applications that previously have not been cost effective. Cheap sources of high-performance chips, for example, could make it easier for designers to add wireless communications to household appliances and connect them to the Internet. Visions of washing machines directly communicating with service centers when they go on the fritz or refrigerators that call in food orders to the supermarket could become cheaper to realize, if no more desirable. More affordable light-emitting and light-detecting chips could change the economics of fiber-optic links for directly connecting home computers, video cameras and other domestic gadgetry to the Internet.

Beyond that, chip manufacturers like Motorola and AmberWave Systems share the same longer-term techno-dream-an all-in-one wafer. In this vision, compound semiconductors are not just layered on top of a silicon substrate, but the different semiconductors are integrated together on the chip. “If we can grow a thin film of gallium arsenide on top of silicon wafers, then maybe we can selectively grow islands of gallium arsenide on silicon,” says Charles Huang, cofounder and chief technical officer of Anadigics, a chip manufacturing firm based in Warren, NJ.

Each island would have its own function-say, sending and receiving messages wirelessly, or transmitting data optically to the outside world. Yet the majority of the silicon would be available to do the actual computing or storing of data. Such multitalented chips would, for instance, be able to shuttle data around a microprocessor optically. In a computer, data currently moves electronically both within chips and between chips-between, say, a microprocessor and a memory chip-through tiny wires that slow everything down. “The wires are the real bottleneck in computers,” says Ramdani. If each silicon chip came with its own onboard lasers made of compound semiconductors for moving data around, such chips would both operate more quickly on their own and be able to trade larger amounts of data with other chips more quickly.

It’s too soon to rule out the possibility that some lurking glitch will send Motorola’s growing investment in its new technology into the vast heap of good tries gone bad. Certainly there are a number of skeptics out there still not convinced that the company’s superchip will ever live up to its hype. Nevertheless, in the three years since Ramdani’s original epiphany, Motorola has increasingly committed itself to making sure the technology fulfills its many promises, throwing its substantial financial and technical weight behind it.

Indeed, Ramdani’s excitement over the breakthrough is far from waning. “The way I see it, this technology is going to revolutionize the semiconductor industry,” he says. “It will allow us to do things that, 20 years ago, we could only dream of doing.”

Pumping Up Silicon

A sample of companies pushing the limits of semiconductor materials

Company Location Technology
Thoughtbeam Austin, TX Compund semiconductors on silicon
AmberWave Systems Salem, NH Strained silicon
IBM Watson Research Center Yorktown Heights, NY Strained silicon
Toshiba Tokyo, Japan Strained silicon

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