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Raising a crop of transistors.

Raising a crop of transistors

The latest development in theworld of miscroelectronics is almost as magical as pulling a rabbit out of a hat and potentially a lot more useful. A team of researchers at GTE Laboratories in Waltham, Mass., has succeeded in "growing' the basic components of a transistor. In effect, they let nature do the work of creating a silicon structure that can easily be turned into an electronic device. The technique circumvents much of the costly, delicate, fault-prone processing normally used to construct integrated circuits on the surfaces of silicon wafers. And, as a bonus, it produces transistors that can survive large electrical currents.

The GTE process, developed by Brian M. Ditchek, starts witha mixture of molten silicon and tantalum metal. In the liquid state, both ingredients mix completely, with tantalum spread evenly throughout the silicon. However, when the mixture is cooled and starts to solidify, it separates into two components. Each component seeks its own kind, and patches of the compound tantalum disilicide form within a silicon matrix.

To capture this structure in a useful crystal form, theresearchers lower a silicon "seed' rod into the molten mixture just before it begins to solidify. When this rod is slowly drawn out of the liquid, solidification occurs at the interface between the rod and the liquid. Tantalum disilicide appears as numerous microscopic threads that run the length of the resulting crystal. The rest of the material has the orderly structure of a single crystal of silicon. The tantalum threads, about 1 micron in diameter, are, on the average, 6 microns apart.

The cylindrical crystal is then sliced into wafers, about 25millimeters in diameter and 1 millimeter thick. Each wafer is converted into an array of transistors by laying down a set of target-shaped electrical contacts on the wafer's surface (see photograph). Each target consists of three conducting rings. Current normally flows from the bull's-eye (the current source) through the intervening silicon layer to the outer ring (the drain). That flow can be regulated by changing the charge on the inner ring (the gate), located between the source and the drain.

This particular transistor structure has the advantage ofextending through the full thickness of the wafer. As a result, the GTE transistor can handle high power levels. In contrast, transistors laid down as part of integrated circuits are etched in thin films on a silicon surface. Such delicate features usually can't withstand large currents.

The GTE technique works best when tantalum makes up 2percent by volume of the original mixture. The process fails if the initial composition is slightly different. Why this is true isn't clear yet. The whole process is somewhat mysterious, says Ditchek. Meanwhile, GTE researchers are studying combinations of silicon with other metals to see if a similar structure can be created. They are also manipulating the crystal-growth environment to find the best possible conditions for producing suitable crystals. They have discovered, for instance, that the tantalum threads are farther apart when the crystal is pulled out of its liquid bath more quickly. In laboratory tests, prototype devices have turned out to be particularly efficient in detecting light.

"Although still early in our research, we are obviouslyexcited that we have found a significantly simpler and, we believe, cheaper way to produce an electronic device,' says GTE's C. David Decker. "However, what is truly remarkable is that we've created an entirely new electronic material and device form, which may open up a spectrum of uses that can't even be imagined at this time.'
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Publication:Science News
Date:Jul 11, 1987
Words:586
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