IBM proves nanotubes can beat silicon

IBM has improved its carbon nanotube (CNT) transistors to the point where they can outperform the most advanced silicon transistors in prototype today, it said Monday.

CNTs are tube-shaped molecules made of carbon atoms, one nanometer (around three atoms) thick and 50,000 times thinner than a human hair, IBM Research Scientist Shalom Wind said in an interview.

Scientists from IBM's T.J. Watson Research Center in Yorktown Heights, New York, have achieved the highest transconductance (a measure of current-carrying capability) of any CNT transistor to date, and more than twice the transconductance, per unit width, of the top-performing silicon transistors available, IBM said.

CNTs are expected to replace silicon when chip development hits the physical barrier where silicon just can't be made any smaller, in about 10 to 15 years, IBM said.

Wind was lead author, along with colleagues Joerg Appenzeller, Richard Martel, Vincent Derycke and Phaedon Avouris, of an article published in Monday's Applied Physics Letters journal describing the effects of "Vertical scaling in carbon nanotube field-effect transistors using top gate electrodes."

The research involved creating carbon nanotube field-effect transistors (CNFETs) with a similar structure to conventional metal-oxide semiconductor field-effect transistors (MOSFETs). That allowed the CNTs to be compared with their silicon equivalent, IBM said.

The devices showed excellent electrical characteristics, turning on and off easily and conducting well at low voltages. Previous attempts at CNT transistors needed high voltages of up to 20 volts to turn on and off, Wind said, compared to the 1 volt used for silicon.

These previous transistor attempts were based on a silicon wafer, using the silicon itself as the gate or switch to control the flow of electricity. That worked, but meant all of the transistors had to be turned on and off at the same time, he said.

The new structure puts the gate on top of the nanotube with a thin insulating layer of silicon dioxide in between, "so that it looks just like a silicon transistor, but with a narrow nanotube instead of a flat sheet of silicon," Wind said.

The new carbon nanotube transistor can turn on with power of 1 volt or less, as a result of the insulating layer, and operate independently.

"Imagine we had previously developed a new type of light bulb that worked, but needed high voltages, wasn't very bright, and you have to turn on all the lights in your house at once. Now we can make them brighter, using less power, and turn them on and off individually."

This research takes carbon nanotube technology a step closer to becoming a viable option for replacing silicon, Wind said.

"It's one thing to develop lots of little devices using new technology, but if they don't operate as well as older technology it's not worth it. But it looks like these operate even better than the silicon transistors being developed, and that's pretty exciting."

The CNFETs have yet to be optimized, with a thinner silicon dioxide layer and shorter channel lengths (so that the electrons don't have to travel so far), so Wind expects even better results to be produced in future.

Not that carbon nanotube transistors will be in the shops anytime soon, Wind stressed. Researchers still have to find ways to control how the carbon nanotubes are placed in specific places so that they work properly, and there are "subtle issues in terms of how the current gets in and out. It's slightly different to how it works in silicon and we're trying to understand that."

Getting nanotube technology up to speed before silicon meets its physical limits will be "tricky," but the latest research is one step further towards that goal, Wind said.

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