Machine Design

Single electron toggles transistor on and off

A new type of transistor takes only one electron to switch between on and off, a breakthrough that could lead to lowpower chips with higher computing densities.

A micrograph of the single-electron transistor (SET) superimposed on experimental circuitry shows the source, S, along with the applied ac signal, Vac, superimposed on a dc bias voltage, V. The circuit detects the net current, ID, at the drain, D, with a current amplifier. The third electrode, G, is floating.

The device, dubbed the single-electron transistor (SET) by its developers Robert Blick at the University of Wisconsin-Madison and Dominik Scheibe at Ludwig Maximilian University, Munich, moves electrons using a vibrating arm less than 1 millionth of an inch long topped with a gold tip. The tip, or island, sits between two electrodes, the drain and the source. Applying a voltage to the source sets the arm vibrating at 350 to 400 GHz between the electrodes. Each time the arm touches the source, a single electron hops on the tip, where its presence is detected, signifying a "1" state. The arm then carries the electron to the drain.

Conventional transistors require hundreds of thousands of electrons to flow for the transistor to flip from "1" to "0," explains Blick. So when you use 100,00 electrons to switch a single bit of information in a computer containing millions of transistors of information, it generates a lot of heat. This limits the number of conventional transistors that can squeeze on a chip.

The SET has a few other advantages as well. Because a mechanical action transfers charge carriers between the drain and source, the device withstands much more radiation than transistors that depend on charge transfer. This makes SET better suited for satellites and other devices that routinely see high radiation levels. SET also has a high signal-to-noise ratio. That's because conventional transistors in the "off" state always allow a trickle of electrons to leak through, generating a background signal. The arm on SET, however, has no contact with the two electrodes when it is inactive. SETs are also constructed of silicon and operate over a wide temperature range (333 to 258°F), making them easy to manufacture and integrate into existing, silicon-based circuits. And because the structures are so small and made of ultra-pure silicon, there have extremely small defect densities, and thus are relatively insensitive to wear and breakage.

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