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Going Ballistic

Ballistic deflection transistors are among the ways that researchers hope to speed up processors.

MOORE'S LAW SAYS THAT the number of transistors on a chip doubles roughly every 18 months, translating into a doubling of computing power. Researchers at the University of Rochester in New York are among several groups working to take that prediction to extremes by developing Terahertz (THz) transistors.

“The highest speed commercial processors today are in the vicinity of 4 GHz,” says Marc Feldman, professor of computer engineering. “That is 0.004 THz.”

Faster chips would enable a wider range of applications, including advanced AV ones such as real-time facial recognition for videoconferencing. In this $1.5 million research program, Feldman and his colleagues are focusing on transistors because they're literally the heart of processors, which use hundreds of millions of them, depending on the application. For example, high-end servers increasingly have processors with 1 billion or more transistors.

Finesse, not force

Simply packing in more transistors to achieve more processing power is a design strategy that has worked for years. The catch is that it's starting to run into a few issues, particularly the amount of heat produced. Many engineers also believe that current transistor designs are reaching their limits in terms of being able to achieve faster speeds.

The University of Rochester researchers believe that their ballistic deflection transistor is one solution. Conventional transistors handle multiple electrons at once, like a stream of water, while ballistic transistors handle them one at a time. The “ballistic” part of the name refers to an electrical engineering concept, as well as to the fact that the transistor manipulates each electron in a way that's akin to a ballistic projectile.

The “deflection” part of the name refers to a part of the design that each electron bounces off of. Altering the voltage near the deflector determines the direction in which the electron flies off. These directions in turn determine whether an electron triggers a 1 or a 0, which make up the internal binary language that processors speak as they go about their tasks. One helpful analogy is to think of the deflector and voltage field working together as a traffic cop at an intersection, pointing some cars in one direction and other vehicles in another.

If bouncing electrons off a deflector sounds low-tech, that's because it is. In fact, that approach is both a major advantage and a major departure from previous attempts that also used electrical fields to herd electrons. By creating a structure that the electrons can ricochet off of, less energy is required for the electrical field. That makes the transistor draw less power and produce less heat — two issues that are nearly as important as speed.

To make their architecture practical, the University of Rochester researchers also had to find new materials. Silicon was out of the question because the way that electrons ricochet in that material would have required a deflector so tiny that it would be difficult to manufacture. Eventually they settled on indium-gallium-arsenide and indium-phosphide because those provide more room for the electrons to move, and thus allow the deflector and other components to be sizes that are practical for manufacturing. “Our fabrication is close to the limit of modern nanolithography,” Feldman says.

That's important because to make ballistic deflection transistors commercially viable, it helps to be able to leverage existing manufacturing techniques. Even so, the University of Rochester researchers say they have a lot of work left to do before their designs are ready for prime time.

“It's too early to speculate as to when the technology will be mature enough for a general computing environment,” Feldman says. “There are a lot of factors to this question, and this is still very early in the development of this device.”



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