Built for speed: novel transistor design spurns limits.

By restructuring a key part of a transistor, engineers in Illinois say that they have captured the world record for the speed at which such devices operate. More important, the new approach opens the door to much greater speeds, perhaps more than l0 times the rate of transistors in today's home computers, say the new transistor's developers.

The technological payoffs of future transistors that operate at a trillion cycles per second, or terahertz (THz), could include improved surveillance systems for detecting concealed weapons, more jam-resistant battlefield communications, and medical-scanning devices that provide more information valuable for diagnoses.

Terahertz transistors"could change some of the fundamental elements in the way we monitor, sense, and communicate," says Russell D. Dupuis of the Georgia Institute of Technology in Atlanta.

Under test conditions, typical silicon-based transistors such as those in Pentium microprocessors run at about 50 billion to 100 billion cycles per second, or gigahertz (GHz). Many products, such as cell phones, include much faster transistors made of more-exotic semiconducting compounds such as gallium arsenide or indium phosphide.

The speed of the new Illinois recordholder is 604 GHz. That bests a Japanese team's 562 GHz transistor that had been in the top slot. Walid W. Hafez and Milton Feng, both of the University of Illinois at Urbana-Champaign, unveil their new design in the April 11 Applied Physics Letters.

To make transistors operate at super speeds, designers typically increase operating voltages to drive current more rapidly through the devices. However, that tactic also heats up the transistors, which wastes power and can destroy chips. What's more, Hafez and Feng have calculated that terahertz operation of previous specialty transistors would require temperatures beyond those at the surface of the sun.

Hafez and Feng specialize in extremely fast versions of devices known as bipolar transistors (SN: 11/20/04, p. 324). In those components, a small current traversing a base layer of material controls the magnitudes of a much larger current flowing between layers known as the emitter and the collector.

Until now, the proportions of ingredients in a collector layer were the same throughout the structure. In the new transistor, however, Hafez and Feng thinned the collector and varied the relative quantities of indium and gallium throughout that layer by a process that enabled them to sequentially deposit atomic layers of specified compositions.

The result: a transistor that electrons zip through more quickly and with less heat-inducing electrical resistance. The new transistor runs nearly 30[degrees]C cooler than does one of the team's earlier, 550-GHz transistors, which operated at 176[degrees]C. The researchers plan to further enrich indium in a yet-thinner collector to push toward terahertz speeds.

Akira Endoh of Fujitsu Laboratories in Atsugi, Japan, rates the advance as an important step toward the realization of the terahertz transistor. He and his colleagues used a different architecture, known as a field-effect transistor, to make their now-dethroned 562-GHz device. With the new materials, that architecture is also likely to ultimately attain terahertz operation, he says.