Single electron transistor created with tiny mechanical arm
The infinitesimal size and low power requirements of a single electron transistor (SET), created by Electrical and Computer Engineering Associate Professor Robert Blick and physicist Dominik Scheible of Ludwig-Maximilians University in Munich, could eventually lead to advances such as much tinier semiconductor chips; more powerful yet less power-hungry cell phones; and long-lived remote sensors for monitoring everything from airborne toxins to forest fires. And unlike earlier SETs, the device is fabricated in silicon by a simple, two-step process and operates at room — rather than super-low — temperatures, allowing its easy integration into existing, silicon-based circuits, says Blick.
Blick. and Scheible describe the device in the June 2004 issue of Applied Physics Letters and have applied for a patent through the Wisconsin Alumni Research Foundation, the patenting and licensing organization of the UW-Madison.
Transistors are best known as the workhorses of the computing world; a computer's microprocessor chip contains millions of these tiny, voltage-controlled switches. The off and on position of each transistor corresponds to the 0's and 1's, or bits of information, a computer uses to calculate.
In a conventional transistor, thousands of electrons must flow for the transistor to toggle between 0 and 1. "When you use 100,000 electrons to switch a single bit of information inside a computer containing megabytes (8.4 million bits) or gigabytes (8.6 billion bits) of information, a lot of heat is dissipated," says Blick. This heat — the result of the electrons' energy — limits the number of conventional transistors that can be squeezed together on a single chip. In Blick's device, on the other hand, the "on" or "1" state is represented by just a single electron, drastically reducing the number of electrons needed for switching. A device that uses fewer electrons will, in turn, generate much less heat and require less power to move the electrons around — a feature very important in battery-powered mobile devices, such as cell phones.
Blick's and Scheible's transistor consists of miniscule vibrating arm topped by a gold tip, or island, that nestles between two electrodes, known as the source and drain. When the researchers apply a certain voltage, the arm, or nano-pillar, begins vibrating at a frequency of 350 to 400 million cycles per second between the electrodes. Each time the arm swings into contact with the source, a single electron hops onto the island, where its presence is detected. The arm then shuttles the electron to the drain.
The incorporation of a mechanical component into the transistor confers several advantages, says Blick. For example, his transistor withstands radiation much better than traditional transistors that work purely through electronic means, making it suitable for satellite electronics or other devices bombarded by high radiation levels.
The SET also exhibits higher signal-to-noise ratios for signal processing operations. Because they are solid devices, standard transistors in the off position always allow a small amount of current, or electrons, to leak through, says Blick, resulting in a background signal. But in his device, the arm in its inactive, non-oscillating, state has absolutely no contact with the two electrodes, making the flow of background current impossible.
Blick and Scheible originally designed a more elaborate SET whose manufacture required a half dozen processing steps. But when a batch of the devices — made in Germany and brought to Madison for study — were accidentally destroyed in an X-ray machine, the scientists began asking how they could quickly and easily produce more. It was then they conceived of the nano-pillar design and the simplified fabrication process, involving only a lithography step followed by dry etching. "It was the beauty and simplicity of the new design that convinced us of the merit of the mechanical approach," says Blick. "And then, of course, we were sort of mad at ourselves that we didn't think of it before."