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Single-Electron Transistors
Texas Instruments
Benefit: Ultrasmall integrated circuits
By Peter Fairley

CEOs at technology firms like to boast that the intuitions of individual researchers and engineers are their companies’ greatest assets. A string of recent patent filings authored by Texas Instruments electronics researcher Christoph Wasshuber shows that in some cases, at least, there’s truth to that claim. By giving the 36-year-old Austrian-born engineer the flexibility to follow his instincts in designing a novel type of single-electron transistor, Texas Instruments has secured a toehold in the development of a technology that could transform semiconductor microchips in the decades to come.

In many ways, a single-electron transistor, which is turned on and off by the addition or subtraction of a lone electron, is the ultimate in semiconductor miniaturization. Not only could it allow the manufacture of powerful, ultrasmall electronic devices, but it could also slash power consumption. While exotic versions of these highly sensitive electronic switches have been around since the late 1980s, research on them has stalled because of severe problems in making them robust enough. The same property that makes them attractive, their ultrasensitivity, also makes it difficult to get them to work effectively in the real world. In particular, single-electron transistors are easily overwhelmed by background noise or signals from neighboring circuits. But Wasshuber and his collaborators at the Swiss Federal Institute of Technology in Lausanne have designed a single-electron transistor that, incorporated into standard silicon circuitry, is immune to interference.

If the innovative design works, suggests Wasshuber, it could result in ultrafast single-electron processors. What’s more, Wasshuber’s transistors should be compatible with standard semiconductor fabrication processes, enabling manufacturers to push beyond conventional microchip technology without abandoning their multibillion-dollar investments in production capacity. The first uses of the single-electron transistors will likely be in memory chips and ultrasensitive electrometers for testing electrical circuits. Konstantin Likharev, a physicist at New York’s Stony Brook University, estimates that a memory chip with single-electron transistors could store a terabit of data in a square centimeter of silicon, a data density about 100 times greater than that of today’s best memory.

But even more important to the future of microelectronics, single-electron transistors could solve one of the gravest problems facing conventional chip technology; as more and more transistors are packed together, heat becomes harder to dissipate. Hundreds of thousands of electrons flow through a conventional transistor, and as a result, switching it on and off usually takes at least one volt. Over the next decade, chips will be jammed with billions of transistors, and the power required to switch them could literally cook the circuits. In contrast, a single-electron transistor, turned on or off by just one electron, runs cool and consumes one-tenth as much power. If you look ahead to the end of the industry’s technology road map, we’re going to have 30 to 50 billion transistors on a chip, says Dennis Buss, Texas Instruments’ vice president of silicon technology development. The thought of operating those at one volt is unthinkable.

Texas Instruments hired Wasshuber in 1998 for his computer modeling expertise and set him to work on a series of near-term projects. But at night the young physicist cranked out designs of single-electron transistors like those he had worked on in graduate school. When a Texas Instruments task force scouting future technologies vital to the semiconductor industry selected single-electron transistors as one idea warranting a closer look, Wasshuber was in luck. Finally, he had the green light to pursue his hobby in the daylight hours at the lab.

Despite single-electron transistors’ broad implications, however, experts who have been working on them for more than a decade caution against overenthusiasm. Likharev calls Wasshuber’s ideas clever but wants to see proof that the new designs will work in real circuits. Then there’s the challenge of manufacturing actual devices based on the designs. A single-electron transistor that operates at room temperature will require features as small as one to two nanometers across. That’s the size of molecules, notes Greg Snider, an electrical engineer at the University of Notre Dame and an expert in single-electron transistors. The semiconductor industry is quite a ways away from doing that controllably.

Wasshuber agrees that plenty of work remains before devices using single-electron transistors show up in cell phones and desktops. And a large-scale research and development effort on the new chip technology is far more than Texas Instruments can justify funding today, given its expectation that it can continue to miniaturize conventional transistors through 2015. For the moment, Texas Instruments is salting away its patents while keeping a close  eye on the emerging field. But whether Wasshuber’s design for single-electron transistors proves practical or not, the company’s opportunity to explore the future of microelectronics is worth the investment in turning his after-work hobby into part of his day job.

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