Team designs world's smallest transistor
Nano switch A team of Australian physicists has created the world's first functioning single-atom transistor, which could prove a critical building block toward the development of super-fast computers.
The tiny electronic device, described today in a paper published in the journal Nature Nanotechnology, uses as its active component an individual phosphorus atom patterned between atomic-scale electrodes and electrostatic control gates.
While single-atom devices have been developed before, these had an error of about 10 nanometres in positioning of the atoms, which is large enough to affect functionality.
Professor Michelle Simmons, group leader and director of the ARC Centre for Quantum Computation and Communication at the University of New South Wales (UNSW), says it is the first time "anyone has shown control of a single atom in a substrate with this level of precise accuracy".
"Several groups have tried this, but if you want to make a practical computer in the long-term you need to be able to put lots of individual atoms in," she says.
"Then you find the separation between the atoms is quite critical so you need to have atomic precision to do that, so then you can also bring electrodes in to address each of those individual atoms."
The UNSW team used a scanning tunnelling microscope (STM) to see and manipulate atoms at the surface of the crystal inside an ultra-high vacuum chamber.
Using a lithographic process, they patterned phosphorus atoms into functional devices on the crystal, then covered them with a non-reactive layer of hydrogen.
Hydrogen atoms were removed selectively in precisely defined regions with the super-fine metal tip of the STM.
A controlled chemical reaction then incorporated phosphorus atoms into the silicon surface.
Finally, the structure was encapsulated with a silicon layer and the device contacted electrically using an intricate system of alignment markers on the silicon chip to align metallic connects.
The electronic properties of the device were in excellent agreement with theoretical predictions for a single phosphorus atom transistor.
Lead author Dr Martin Fueschle says this individual position is very important if you want to use the transistor as a future quantum bit (or qbit).
"If you want to have precise control at this level you need to position the individual atoms with atomic precision with respect to control gates and electrodes," he says.
The device is also remarkable, says Dr Fuechsle, because its electronic characteristics exactly match theoretical predictions undertaken with Professor Gerhard Klimeck's group at Purdue University in the United Stastes and Professor Hollenberg's group at the University of Melbourne, the joint authors on the paper.
Limits of Moore's Law
The team also believes the use of silicon to encase the transistor increases its potential for future manufacturing.
It is predicted that transistors will reach the single-atom level by about 2020 to keep pace with Moore's Law, which describes an ongoing trend in computer hardware that sees the number of chip components double every 18 months.
"We really decided 10 years ago to start his program to try and beat that law," says Simmons.
"So here we are in 2012 and we've made a single-atom transistor about 8-10 years ahead of where industry is going to be."