Big Expectations for Tiny Diode
According to Ji-Ung Lee, the GE researcher who developed the new device, his diode is not the first to be created using nanotubes, but it is the first that works well enough to actually function in real-world applications. In fact, the device comes very close to the theoretical limits of diode performance. Measured by the ideal diode equation, developed by Nobel Laureate William Shockley, GE's new diode has an "ideality factor" very close to one-the best possible performance for a diode.
Lee and other researchers are saying the device could serve in a number of ways, appearing in computing, communications and sensors applications by the end of this decade.
"Just as silicon transistors replaced old vacuum tube technology and enabled the electronic age, carbon nanotube devices could open a new era of electronics," says Margaret Blohm, GE's advanced technology leader for nanotechnology. "We are excited about this breakthrough and we're eager to start developing new applications for the GE businesses."
According to Lee, the challenge in creating nanotube diodes lies in joining the contrasting p- and n-types of semi-conducting material that make up the device. Traditionally, these are created by adding impurities or "dopants" to a bulk semiconductor. But unlike traditional semiconductors, there is not a commercially viable method to dope carbon nanotubes.
To solve this problem, Lee uses an electric field, created by a split-gate electrode fabricated under the nanotube, to create the p and n regions. The two coplanar gates couple to the two halves of a carbon nanotube; this acts as a field effect transistor where the gate is split into two independently addressable gates. By biasing one gate with a negative voltage and the other with a positive voltage, a p-n junction can be formed.
Because the doping is not fixed, the diode can dynamically change polarity from a p-n to an n-p diode and visa versa. In addition, the device also functions as a p-channel transistor (both gates are negatively biased) or an n-channel transistor (both gates are positively biased.