| GE Global Research, the centralised research organisation of the General Electric Company, announced the development of the world’s best performing diode built from a carbon nanotube, which will enable smaller and faster electronic devices with increased functionality. The nano-diode is one of the smallest functioning devices ever made. The GE Nanotechnology Advanced Technology program reported its discovery in the cover story of the July 5, 2004 edition of Applied Physics Letters. Diodes are fundamental semiconductor devices that form the basic building blocks of electronic devices, such as transistors, computer chips, sensors, and light emitting diodes (LEDs). Unlike traditional diodes, GE's carbon nanotube device has the ability for multiple functions -- as a diode and two different types of transistors -- which should enable it to both emit and detect light. “Just as silicon transistors replaced old vacuum tube technology and enabled the electronic age, carbon nanotube devices could open a new era of electronics,” said 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.” GE’s breakthrough device comes very close to the theoretical limits of performance. Measured through the ideal diode equation, developed by Nobel Laureate William Shockley, GE's new diode has an “ideality factor” very close to one, which is the best possible performance for a diode. One possible application for GE is to use the device to build the next generation of advanced sensors that will have unsurpassed levels of sensitivity. For example, next generation sensors in security applications could detect potential terrorist threats from chemical and biological hazards, even if they are present in extremely small quantities. This would enable enhanced security at airports, office buildings and other public areas. The carbon nanotube diode was developed by Dr. Ji-Ung Lee, a scientist who works in the Nanotechnology Advanced Technology Program at the GE Global Research Center in Niskayuna, N.Y. More research is underway to enhance the carbon nanotube diode and increase the yield in the manufacturing process, but GE nanotechnology researchers believe this breakthrough could enable a range of important new applications in computing, communications, power electronics, and sensors. Technical Details A diode is formed by joining a p-type and an n-type semi-conducting material. 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, GE uses an electric field to create the p and n regions. Electric field coupling is accomplished with a split gate electrode fabricated underneath the nanotube. The two coplanar gates couple to the two halves of a carbon nanotube. Essentially, 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. Since 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). Finally, the material properties of carbon nanotubes should enable the device to function as a Light Emitting Diode as well. |