Study: Carbon nanotubes make the best semiconductors

 Carbon nanotubes, the experimental materials seen as possible replacements for conventional chip-making materials, conduct electricity better than any other material at room temperature, according to researchers at the University of Maryland in College Park.

The mobility of carbon nanotubes is about 70 times higher than that of silicon used to currently manufacture processors and 25 percent higher than any other known semiconductor material, the researchers said in a report published online this week in Nano Letters, a scientific journal.

Mobility is calculated by dividing the conductivity of a particular material by the number of charges it carries, or the amount of current flowing through the material. The result is a measure of how fast electrons move through a transistor.

“It’s probably the most useful number to know about a semiconductor,” said Michael Fuhrer, assistant professor of physics at the University of Maryland, and leader of the team of researchers.

Chip manufacturers use transistors built atop silicon wafers to carry electrical current, which trips a gate as it flows through a transistor in order to produce the millions of bits of information that run a computer. As chip materials continue to grow smaller, more and more current is able to escape from the transistors, producing heat and causing transistors to fail.

Designers are examining the use of carbon nanotubes as an alternate material to carry electrical current across silicon wafers. Nanotubes are cylinders with walls that are only as wide as a single carbon atom.

Because the carbon atoms are more tightly bound together than the metals currently used in transistor production, electrons flowing through the tubes have less room to veer off track, Fuhrer said. This allows a greater amount of current to move even faster through carbon nanotubes than through the copper interconnects of today’s chips, he said.

The carbon nanotubes not only carry current at higher speeds than silicon transistors, but also detect electrical changes with a greater degree of precision than silicon, Fuhrer said. This allows the nanotube to function as a highly responsive sensor, he said.

For instance, a specific nanotube could be constructed to detect the weak electrical signal given off by a single molecule of a particular chemical or strand of DNA, and send a signal broadcasting the location of that molecule, Fuhrer said.

Further research is needed to determine whether carbon nanotubes will eventually serve as a replacement for CMOS (complementary metal-oxide semiconductor) manufacturing techniques used today, but the discovery of the material’s high mobility shows that it could work as a semiconductor, Fuhrer said.