A review paper on the electronics of carbon nanotubes, presented in the emerging-technologies session of the International Solid-State Circuits Conference here this week, pointed out that despite their typical minimum dimensions of about 1.4 nanometers, carbon nanotubes can display many conventional semiconductor characteristics.

The extremely small size of the carbon molecules holds out the prospect of terahertz switching, said the paper's author, Alan (Charlie) Johnson of the University of Pennsylvania. It also promises extremely low power consumption, he said, although the behavior of the material is not yet completely understood and the technology must still overcome some hurdles.

"These are real molecular circuits," said Johnson. "It's becoming an experimental field of research rather than a theoretical one."

Built like a honeycomb sheet of graphite rolled into a tube, these recently discovered tubes of hexagonally bonded carbon can be as small as 1.4 nm and as long as 10 microns.

"They could be very fast because they are very small," said Johnson.

This form of carbon has only been reliably synthesized since about 1996, but already Johnson's research group and others are coming to grips with its strange electrical characteristics.

Johnson reported that among the devices built is a so-called TubeFET, in which a carbon nanotube is laid across source and drain metallic contacts, and serves as the semiconducting channel between them. The TubeFET operates at room temperature and displays a 106 change in conductance with a 10-V change in gate voltage. The gate is either a third lead held near the tube or the substrate of the wafer. Johnson also reported diodelike action, probably because of an impurity lying across a nanotube under study.

Unfortunately, the devices display a very high surface barrier resistance between the leads and the tube-on the order of 1 megohm-that limits the FET gain and speed.

"We don't know for sure why that is," said Johnson. "It might be to do with contact between the highly regular tube and the atomically rough metallic surface." He said different arrangements of the nanotubes and metallic contacts yielded different resistances, holding out the prospect that resistance can be reduced by an order of magnitude or more. If resistance can be trimmed to the 6-kohm quantum limit, 10-THz switching speeds should be possible, Johnson said.

If that should prove difficult, the tubes offer an alternative mode of operation, Johnson observed.

For example, carbon nanotubes can be created in the form of either metallic or semiconducting material, depending on the pitch of the spiral winding of the carbon lattice in the tube.

For metallic forms of nanotube at temperatures close to absolute zero, Johnson's group has seen single-electron transistor action.

Johnson's talk also included the intriguing prospect of "devices on a tube." Since the carbon nanotubes can be characterized as metals and semiconductors, there is a possibility of metal-semiconductor combination tubes as well. The dislocations between a metallic and semiconducting section of tube can be characterized as the equivalent of dopants in conventional semiconductor devices, Johnson said.