 optical signals." A transistor usually has two ports: one for input and one for output. "Our new device has three ports: an input, an electrical output, and an optical output," says Milton Feng, professor of electrical and computer engineering at Illinois. "This means that we can interconnect optical and electrical signals for display or communication purposes." Feng is credited with creating the world's fastest bipolar transistor, a device that operates at a frequency of 509 GHz. Graduate student Walid Hafez fabricated the light-emitting transistor at the University of Illinois' micro and nanotechnology lab. Unlike traditional tran- sistors, which are built from silicon and germanium, the light-emitting transis- tors are made from indium gallium phosphide and gallium arsenide. "In a THE FUTURE IS FIBER-OPTICAL TECHNOLOGIES bipolar device, there are two kinds of injected carriers: negatively charged electrons and positively charged holes," Holonyak says. "Some of these car- riers will recombine rapidly, supported by a base current that is essential for the normal transistor function." The recombination process in indium gallium phosphide and gallium arsenide materials also creates infrared photons, the "light" in the researchers' light-emitting transistors. "In the past, this base current has been regarded as a waste current that generates unwanted heat," says Holonyak. "We've shown that for a certain type of transistor, the base current creates light that can be modulated at transistor speed." Although the recombination process is the same as that which occurs in light-emitting diodes, the photons in light-emitting transistors are generated under much higher speed conditions. So far, the researchers have demon- strated the modulation of light emission in phase with a base current in tran- sistors operating at a frequency of 1 MHz. Much higher speeds are considered certain. "At such speeds, optical interconnects could replace electrical wiring between electronic components on a circuit board," says Feng. This wo