urce electrode and positive charges (holes) from a drain elec- trode into a single carbon nanotube. When the electrons and holes meet in the nanotube, they neutralize each other and generate light. Because it is a transistor, IBM's light emitter can be switched on and off depending on the voltage applied to the gate of the device. Electrical control of the light emission of individual nanotubes allows detailed investigations of the optical physics of these unique one-dimensional materials. IBM researchers compared the characteristics of the emitted light with theoretical predictions to prove that the light was created by the electron-hole recombi- nation mechanism. Although optical emission from individual molecules has been measured before, that light emission was induced by laser irradiation of samples of molecules. In the case of carbon nanotubes, light emission is from a collection of a large number of nanotubes suspended in a liquid irradiated with a laser. 4.6.2 Light-Emitting Transistor Researchers at the University of Illinois-Urbana/Champaign have created a light-emitting transistor that could make the transistor the fundamental element in optoelectronics as well as in electronics (Fig. 4-2). LIGHT EMITTERS Light Emitting Transistor Collector Emitter (contact) Figure 4-2 Light-emitting transistor. "We have demonstrated light emission from the base layer of a hetero- junction bipolar transistor and showed that the light intensity can be con- trolled by varying the base current," says Nick Holonyak, a John Bardeen professor of electrical and computer engineering and physics at the Univer- sity of Illinois. Holonyak invented the first practical light-emitting diode and the first semiconductor laser to operate in the visible spectrum. "This work is still in the early stage, SO it is not yet possible to say what all the applications will be," says Holonyak. "But a light-emitting transistor opens up a rich domain of integrated circuitry and high-speed signal processing that involves both electrical signals and