15 August 2005

Nanotube Transistor Created

by Kate Melville

Researchers from the University of California, San Diego (UCSD) and Clemson University, writing in Nature Materials, said specially synthesized Y-shaped carbon nanotubes were shown to behave as electronic switches similar to conventional MOS (metal oxide semiconductor) transistors, the key element of modern microprocessors. They added that the carbon nanotube transistors exhibited superior electronic properties compared to the conventional transistors used in modern computers.

"This is the first time that a transistor-like structure has been fabricated using a branched carbon nanotube," said researcher Prabhakar Bandaru. "This discovery represents a new way of thinking about nano-electronic devices, and I think people interested in creating functionality at the nanoscale will be inspired to explore the ramifications of these Y-junction elements in greater detail."

The discovery may speed the development of new types of nano-electronic devices, overcoming the fundamental technological and financial limits that existing MOS technologies must soon face. Currently, the size of conventional MOS transistors is around 100 nanometers, and while that can be expected to reduce further in the future, MOS technology is approaching critical limits. The new Y-shaped nanotubes, however, can be made as small as a few nanometers.

The nanotube transistors were initially grown as straight nanotube elements. Titanium-modified iron catalyst particles added to the synthesis mixture were then attached to the straight nanotubes, nucleating additional growth, which continued in a fashion similar to branches growing from a tree trunk. The nascent nanotubes assumed a Y-shape with the catalyst particle gradually becoming absorbed at the junction of the stem and two branches. When electrical contacts are attached to the nanotube structures, electrons travel into one arm of the Y, hop onto the catalyst particle, and then hop to the other arm and flow outward. Experiments conducted in Bandaru's lab at UCSD's Jacobs School of Engineering showed that the movement of electrons through the Y-junction can be finely controlled, or gated, by applying a voltage to the stem, a replication of the function of existing transistors.

Bandaru said the phenomenon effectively makes Y-shaped nanotubes the smallest ready-made transistor yet, with rapid switching speeds and possible three-way gating capability. In earlier attempts to make carbon nanotube-based transistors, separate gates were added rather than built in. "We can synthesize functionality at the nanoscale, in this case to include the three elements of a circuit - the gate, source, and drain - and we don't have to go to the trouble of making them separately and assembling them," he explained.

Bandaru and his co-researchers now plan to experiment with various other catalyst particles in order to tailor the three-way gating properties of the Y-junctions. "If we can easily fabricate, manipulate, and assemble these nano-devices on a large scale they could become the basis of a new kind of transistor and nanotechnology," Bandaru concluded.

Ref: Novel electrical switching behaviour and logic in carbon nanotube Y-junctions
P.R. Bandaru, C. Daraio, S. Jin and A.M. Rao
Nature Materials, September 2005