Switchable nanotube diodes

Single-walled carbon nanotubes (SWNTs) can be either metallic conductors or semiconductors and have been used as transistors, sensors, and memory devices. Now, researchers at General Electric’s Global Research Center in Niskayuna, New York, have created a room-temperature five-terminal device with a semi-conducting SWNT. Most transistors are three-terminal devices: Current comes in at (i) the source and exits at (ii) the drain so long as (iii) the gate carries a certain voltage. That voltage can electrostatically clear a road along which charge carriers flow. In the GE device, the silicon substrate was another terminal, and the gate, located beneath the SWNT, was split into two. That arrangement allowed the physicists to electrostatically dope the two ends of the SWNT separately. They could thus make their device either unipolar—conducting electrons or holes in a single direction only—or ambipolar, in which case they could switch from hole- to electron-conduction by changing voltage. Even more interesting, by biasing the gates with opposite polarities, the researchers turned the device into a switchable p-n junction diode. The researchers expect their device to find uses as both a field-effect transistor and a light-emitting diode. It might also find applications in power electronics, where huge currents and voltages are to be found. (J. U. Lee, P. P. Gipp, C. M. Heller, Appl. Phys. Lett. 85 , 145, 2004 http://dx.doi.org/10.1063/1.1769595 .)