University of Maryland physicists have found that semiconducting carbon nanotubes have the highest mobility of any known material at room temperature. Mobility is a measure of how well a semiconductor conducts electricity.
A team of researchers led by Michael Fuhrer, assistant professor of physics in the university's Center for Superconductivity Research, have fabricated a semiconducting nanotube transistor that shows a mobility almost 25 percent higher than any previous semiconducting material and more than 70 times higher than the mobility of the silicon used in today's computer chips. The results, published online in the journal Nano Letters, provide new evidence that semiconducting carbon nanotubes hold great promise for replacing conventional semiconductor materials in applications ranging from computer chips to biochemical sensors.
"This is the first measurement of the intrinsic conduction properties of semiconducting nanotubes," said Fuhrer, who heads the university's Nanoelectronics Research Group. "It is an important step forward in efforts to develop nanotubes into the building blocks of a new generation of smaller, more powerful electronics."
Mobility is the conductivity of a material divided by the number of charges, which carry the current, and is the number typically used to compare the conduction properties of one semiconductor to another. Fuhrer's group, in research supported by the National Science Foundation, found that the mobility of their carbon nanotube exceeds 100,000 square-centimeters per volt-second at room temperature. The previous record for room temperature mobility was 77,000 square-centimeters per volt-second in indium antimonide and was first measured in 1955. The mobility in the silicon used to make today's computer chips is only about 1,500 square-centimeters per volt-second.
To perform their measurements, the team had to prepare extremely long nanotubes, and be able to place metal wires precisely on one single nanotube. They synthesized nanotubes with lengths up to 0.3 millimeters, or about 100,000 times the nanotubes' diameter. This is some 100 times longer than nanotubes previously studied in electronic measurements. The nanotubes were grown directly on flat silicon chips. A special technique using a scanning electron microscope had to be developed in order to locate the nanotubes on the chip so that wires could be connected to them.
Carbon nanotubes can be thought of as a single atom-thick sheet of graphite, rolled into a seamless cylinder. Nanotubes were discovered in 1991 by Sumio Iiijima (NEC, Japan), and since then have been the subject of research around the world. Today nearly every major research university has at least one group studying carbon nanotubes.
Nanotubes are being considered for many applications in electronic devices including field-effect transistors, memory cells, and chemical and biochemical sensors. In each of these applications mobility is the key to how well the device can perform. Mobility dictates how fast the charges move through a device, so it determines the ultimate speed of a transistor. Mobility also determines the change in conductivity that is caused by a nearby electrical charge. Thus, mobility also is a measure of the sensitivity of a transistor for detecting charge (as in a memory cell) or detecting a nearby molecule (as in a chemical or biochemical sensor).
Fuhrer's group demonstrated last year that high-mobility semiconducting nanotube transistors could detect single electrons in a memory cell. This suggests that chemical sensors made from nanotubes could detect a single molecule of a target compound.