Researchers at the University of Illinois at Urbana-Champaign have demonstrated the laser operation of a heterojunction bipolar light-emitting transistor. The scientists describe the fabrication and operation of their transistor laser in the Nov. 15 issue of the journal Applied Physics Letters.
"By incorporating quantum wells into the active region of a light-emitting transistor, we have enhanced the electrical and optical properties, making possible stimulated emission and transistor laser operation," said Nick Holonyak Jr., a John Bardeen Professor of Electrical and Computer Engineering and Physics at Illinois.
The same principle making possible the transistor - negative and positive charge annihilation in the active region (the source of one of the transistors three currents) - has been extended and employed to make a transistor laser, he said. Holonyak invented the first practical light-emitting diode and the first semiconductor laser to operate in the visible spectrum.
Unlike a light-emitting diode, which sends out broadband, incoherent light, the transistor laser emits a narrow, coherent beam. Modulated at transistor speeds, the laser beam could be sent through an optical fiber as a high-speed signal. "This is a true, three-terminal laser, with an electrical input, electrical output and an optical output, not to mention a coherent optical output," said Milton Feng, the Holonyak Professor of Electrical and Computer Engineering at Illinois. "It is a device that operates simultaneously as a laser and as a transistor." Feng is credited with creating the worlds fastest bipolar transistor, a device that operates at a frequency of 509 gigahertz.
At laser threshold - where the light changes from spontaneous emission to stimulated emission - the transistor gain decreases sharply, but still supports three-port operation, Feng said. "The electrical signal goes down, but the optical signal goes up."
Earlier this year, Feng and Holonyak reported their discovery of a three-port, light-emitting transistor. Building upon that work, the researchers fabricated the transistor laser in the universitys Micro and Nanotechnology Laboratory. Unlike traditional transistors, which are built from silicon and germanium, the transistor laser is made from indium gallium phosphide, gallium arsenide and indium gallium arsenide, but can employ other materials in this family (the so-called III-V compounds). "This work is still very much in its infancy," Holonyak said. "There is much more to be learned, including how to separate and optimize the transistor laser output between electrical signals and light signals."
Down the road, ultra-fast transistor lasers could extend the modulation bandwidth of a semiconductor light source from 20 gigahertz to more than 100 gigahertz. Used as optoelectronic interconnects, transistor lasers could facilitate faster signal processing, higher speed devices and large-capacity seamless communications, as well as a new generation of higher performance electrical and optical integrated circuits.
Co-authors of the paper with Feng and Holonyak are postdoctoral research associate Gabriel Walter and graduate research assistant Richard Chan. The Defense Advanced Research Projects Agency funded the work.
James E. Kloeppel | UIUC - News Bureau
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