Switchable Nanotube Diodes

Switchable nanotube diodes, made by scientists at the research arm of General Electric, combine the practical electrical properties of carbon nanotubes (ability to carry high currents; ability to emit light) with the flexibility of being changed over from a p-n type of diode (allowing current to flow in one direction only) to an n-p diode type (allowing current only in the opposite direction).

Most solid state transistors are three-terminal devices: current comes in at one terminal (the source) and exits at a second terminal (the drain) if a third terminal (the gate) carries a certain voltage, which has the effect of electrostatically clearing out a realm for charge carriers to flow through.

In the GE device, the "realm" is a single-walled carbon nanotube (NT), while the "gate" is actually two separate gates located beneath the NT. These split gates can electrostatically dope the two ends of the NT in such a way that current will flow in only one direction or only in the other depending on the gate voltages.

If you count the source, drain, two gate electrodes, and another electrode attached to an underlying silicon substrate, the device overall has five terminals. Diodes are intrinsically simpler than transistors, but up till now more work has gone into developing NT transistors than for NT diodes.

The GE researchers (contact Ji-Ung Lee, leeji@research.ge.com) expect their device to function as both a field effect transistor (FET) or as a light emitting diode (LED). Because of its ability to carry high currents, and because the company in question is GE, it might also find applications in power electronics, where huge currents and voltages are to be found. (Lee et al., Applied Physics Letters, 5 July 2004.)