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Racing transistors.

A semiconductor switch that turns on and off in only 5.8 picoseconds has been built by scientists at AT&T Bell LAboratories in Murray Hill, N.J. It now takes the lead as the world's fastest electronic device, at least when cooled to 77 kelvin, the temperature of liquid nitrogen. In such a short time, light travels less than 2 millimeters.

Device, made up of transistors arranged in a simple circuit, is essentially a stack of gallium arsenide and aluminum gallium arsenide layers. Some layers are heavily doped with impurities (in this case, silicon atoms), while the others are as pure as possible. To further speed up the device, the active area (gate) within each transistor is as short as possible, much less than a micron across.

"We use silicon impurities to provide the electrons," says Bell Labs' Nitin J. Shah. In conventional gallium arsenide materials, the electrons sit in the same volume as the impurity atoms. As the electrons move, they often strike the impurities and scatter. "Whereas, for our structures, simply because of the way the layers are built up," says Shah, "you separate the electrons from the impurities." The electrons, once released, move about in an adjacent, undoped layer. With much less scattering, especially at low temperatures, the electrons travel faster.

"I'm sure there are ways to increase the speed further," says Shah. "But I think that what we have is about as far as we can go with the present technology. We're very close to the physical limits." He adds, "It's not just a freaky, odd research device." This work is part of a long-term project to develop faster circuits for computers, telecommunications and other applications.
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Title Annotation:world's fastest semiconductor switch
Publication:Science News
Date:Feb 1, 1986
Words:281
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