Scanning tunnelling microscope image of a silicon surface lithographically prepared for two electrodes and a single transistor atom in the center. Credit: ARC Centre for Quantum Computation and Communication, at UNSW

The shift from fragile, bulky vacuum tubes to solid-state transistors paved the way for the information age. And the steady downsizing of transistors has made the devices of the information age ubiquitous, thanks to processors that become smaller, cheaper and faster with each passing year. Now a group of physicists has demonstrated how far that downsizing can proceed by shrinking transistors down to the atomic scale.

In a report published online February 19 in Nature Nanotechnology, researchers in Australia, the U.S. and South Korea announced that they built an operational transistor based on a single phosphorus atom. (Scientific American is part of Nature Publishing Group.)

Transistors are key processor components because they control the flow of electronic signals through a device; they can be used as switches or to amplify those signals. In the new demonstration, the researchers used a phosphorus atom embedded in a silicon crystal as the transistor, mediating the flow of electric charge between two electrodes, each of which was about 10 nanometers from the phosphorus atom. A second set of electrodes, positioned about 50 nanometers on either side of the phosphorus atom, set the state of the transistor. The voltage between those more distant electrodes determined how much current the atomic transistor allowed to pass.

Another group in 2002 reported using single atoms of cobalt as transistors, but those atoms were contained within larger, specially designed molecules. The new approach produces a smaller transistor by placing a lone phosphorus atom onto the silicon wafer using atomic-scale lithography techniques. "This is the first time anyone has shown control of a single atom in a substrate with this level of precise accuracy," Michelle Simmons, a University of New South Wales physicist and study co-author, said in a prepared statement.