Scientists have assembled the first ever transistor that could simultaneously emit light and produce electronic signals. Discover the implications:

The inventor of the light-emitting diode and the maker of the world’s fastest transistor have teamed up and developed the first light-emitting transistor—a breakthrough that could transform the electronics industry. Professors Nick Holonyak Jr. and Milton Feng at the University of Illinois at Urbana-Champaign have made a light-emitting transistor (or LET) that could establish the transistor as the fundamental element in optoelectronics as well as in electronics.

The new device could match the impact of Holonyak’s 1960s invention—the light-emitting diode (LED), which has become the key component of a global optoelectronics industry valued at billions of dollars. According to Russell Dupuis, an electrical engineer and optoelectronics expert at the Georgia Institute of Technology in Atlanta, the new discovery could significantly simplify devices that currently use both LEDs and conventional transistors. “You now have light emission and light modulation in the same physics—that could turn out to be very useful,” he tells IEEE Spectrum Online.

“This work is still in the early stage, so it is not yet possible to say what all the applications will be,” Holonyak says. “But a light-emitting transistor opens up a rich domain of integrated circuitry and high-speed signal processing that involves both electrical signals and optical signals.” He thinks the new device could alter the way microelectronic circuits are designed and configured. While other research groups have previously observed light emissions from transistors, “nobody has been able to make a transistor that produces both electronic and optical signals at the same time,” he notes.

“We have demonstrated light emission from the base layer of a heterojunction bipolar transistor, and showed that the light intensity can be controlled by varying the base current,” explains Holonyak, a John Bardeen Professor of Electrical and Computer Engineering and Physics at Illinois. The heterojunction bipolar transistor is commonly utilized as an amplifier in communications devices such as cell phones. “Heterojunction” is defined as the use of at least two semiconductors, allowing designers to manipulate the band gaps at the junctions within them.

A typical transistor 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 Feng, the Holonyak 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 known for creating the world’s fastest bipolar transistor, a device that operates at a frequency of 509 gigahertz.

Graduate student Walid Hafez built the light-emitting transistor in the university’s Micro and Nanotechnology Laboratory. Unlike traditional transistors, which are constructed from silicon and germanium, the light-emitting transistors are assembled from indium gallium phosphide and gallium arsenide.

“In a bipolar device, there are two kinds of injected carriers: negatively charged electrons and positively charged holes,” Holonyak says. “Some of these carriers 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 generates 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,” Holonyak explains. “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 created under much higher speeds. So far, the researchers have made the light switch on and off in sync with a base current in transistors operating at a frequency of 1 megahertz. Much higher speeds are expected in the future.

“At such speeds, optical interconnects could replace electrical wiring between electronic components on a circuit board,” Feng explains. This work could usher in an era in which photons are manipulated around a chip in much the same way as electrons have been maneuvered on conventional chips.

The scientists described their work in the Jan. 5 issue of the journal Applied Physics Letters.

Sources:

New Light-Emitting Transistor Could Revolutionize Electronics Industry
James E. Kloeppel, Physical Sciences Editor
News Bureau, University of Illinois at Urbana-Champaign, January 5, 2004
www.news.uiuc.edu/news/04/0105LET.html

First Light-Emitting Transistor
Justin Mullins
IEEE Spectrum Online, January 21, 2004
www.spectrum.ieee.org/WEBONLY/wonews/jan04/0104let.html