Built for Speed: Novel transistor design spurns limits

Peter Weiss

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 10 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 record-holder 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: http://www.sciencenews.org/articles/20041120/fob3.asp). 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°C cooler than does one of the team's earlier, 550-GHz transistors, which operated at 176°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.

********

References:

Hafez, W., and M. Feng. 2005. Experimental demonstration of pseudomorphic heterojunction bipolar transistors with cutoff frequencies above 600 GHz. Applied Physics Letters 86(April 11):152101. Abstract available at http://dx.doi.org/10.1063/1.1897831.

Further Readings:

Weiss, P. 2004. Lighthearted transistor: Electronic workhorse moonlights as laser. Science News 166(Nov. 20):324. Available at http://www.sciencenews.org/articles/20041120/fob3.asp.

______. 2004. Flashy transistors: Electronic workhorses also shed light. Science News 165(Jan. 10):21. Available to subscribers at http://www.sciencenews.org/articles/20040110/fob5.asp.

Sources:

Russell D. Dupuis
Georgia Institute of Technology
School of Electrical and Computer Engineering
Center for Compound Semiconductors
777 Atlantic Drive, N.W.
Van Leer Building, Code 0250
Atlanta, GA 30332-0250

Akira Endoh
Fujitsu Laboratories Ltd.
Mimura Fellow Group
10-1 Morinosato-Wakamiya
Atsugi 243-0197
Japan

Milton Feng
Department of Electrical and Computer Engineering
University of Illinois, Urbana-Champaign
1406 W. Green Street
Urbana, IL 61801

Walid Hafez
Department of Electrical and Computer Engineering
University of Illinois, Urbana-Champaign
1406 W. Green Street
Urbana, IL 61801

From Science News, Volume 167, No. 16, April 16, 2005, p. 245.