A team of researchers at the University of New South Wales, Purdue University and the University of Melbourne have built the smallest transistor ever using a single phosphorous atom.

The single-atom device was described in a paper in the journal Nature Nanotechnology (published Feb. 19).

Michelle Simmons, group leader and director of the ARC Centre for Quantum Computation and Communication at the University of New South Wales, says the development is less about improving current technology than building future tech.

"This is a beautiful demonstration of controlling matter at the atomic scale to make a real device," Simmons says. "Fifty years ago when the first transistor was developed, no one could have predicted the role that computers would play in our society today. As we transition to atomic-scale devices, we are now entering a new paradigm where quantum mechanics promises a similar technological disruption. It is the promise of this future technology that makes this present development so exciting."

Caption: A controllable transistor engineered from a single phosphorus atom has been developed by researchers at the University of New South Wales, Purdue University and the University of Melbourne. The atom, shown here in the center of an image from a computer model, sits in a channel in a silicon crystal. The atomic-sized transistor and wires might allow researchers to control gated qubits of information in future quantum computers (Purdue University image).

The same research team announced in January that it had developed a wire of phosphorus and silicon - just one atom tall and four atoms wide - that behaved like copper wire.

Simulations of the atomic transistor to model its behavior were conducted at Purdue using nanoHUB technology, an online community resource site for researchers in computational nanotechnology.

Gerhard Klimeck, who directed the Purdue group that ran the simulations, says this is an important development because it shows how small electronic components can be engineered.

"To me, this is the physical limit of Moore's Law," Klimeck says. "We can't make it smaller than this."

Although definitions can vary, simply stated Moore's Law holds that the number of transistors that can be placed on a processor will double approximately every 18 months. The latest Intel chip, the "Sandy Bridge," uses a manufacturing process to place 2.3 billion transistors 32 nanometers apart. A single phosphorus atom, by comparison, is just 0.1 nanometers across, which would significantly reduce the size of processors made using this technique, although it may be many years before single-atom processors actually are manufactured.

The single-atom transistor does have one serious limitation: It must be kept very cold, at least as cold as liquid nitrogen, or minus 391 degrees Fahrenheit (minus 196 Celsius). "The atom sits in a well or channel, and for it to operate as a transistor the electrons must stay in that channel," Klimeck says. "At higher temperatures, the electrons move more and go outside of the channel. For this atom to act like a metal you have to contain the electrons to the channel.

"If someone develops a technique to contain the electrons, this technique could be used to build a computer that would work at room temperature. But this is a fundamental question for this technology."

Although single atoms serving as transistors have been observed before, this is the first time a single-atom transistor has been controllably engineered with atomic precision. The structure even has markers that allow researchers to attach contacts and apply a voltage, says Martin Fuechsle, a researcher at the University of New South Wales and lead author on the journal paper.

"The thing that is unique about what we have done is that we have, with atomic precision, positioned this individual atom within our device," Fuechsle says.

Simmons says this control is the key step in making a single-atom device. "By achieving the placement of a single atom, we have, at the same time, developed a technique that will allow us to be able to place several of these single-atom devices towards the goal of a developing a scalable system." 

The single-atom transistor could lead the way to building a quantum computer that works by controlling the electrons and thereby the quantum information, or qubits. Some scientists, however, have doubts that such a device can ever be built.

"Whilst this result is a major milestone in scalable silicon quantum computing, it does not answer the question of whether quantum computing is possible or not," Simmons says. "The answer to this lies in whether quantum coherence can be controlled over large numbers of qubits. The technique we have developed is potentially scalable, using the same materials as the silicon industry, but more time is needed to realize this goal."

Klimeck says despite the hurdles, the single-atom transistor is an important development. "This opens eyes because it is a device that behaves like metal in silicon. This will lead to many more discoveries," he said.

"When I established this program 10 years ago, many people thought it was impossible with too many technical hurdles. However, on reading into the literature I could not see any practical reason why it would not be possible," Simmons says. "Brute determination and systemic studies were necessary - as well as having many outstanding students and postdoctoral researchers who have worked on the project."

Klimeck notes that modern collaboration and community-building tools such as nanoHUB played an important role. "This was a trans-Pacific collaboration that came about through the community created in nanoHUB. Now Purdue graduate students spend time studying at the University of New South Wales, and their students travel to Purdue to learn more about nanotechnology. It has been a rewarding collaboration, both for the scientific discoveries and for the personal relationships that were formed."