Email to a friend
New light-emitting transistor
could revolutionize electronics industry
Kloeppel, Physical Sciences Editor
— Put the inventor of the light-emitting diode and the maker of
the world’s fastest transistor together in a research laboratory
and what kinds of bright ideas might surface? One answer is a light-emitting
transistor that could revolutionize the electronics industry.
Professors Nick Holonyak Jr. and Milton Feng at the University of Illinois
at Urbana-Champaign have uncovered a light-emitting transistor that
could make the transistor the fundamental element in optoelectronics
as well as in electronics. The scientists report their discovery in
the Jan. 5 issue of the journal Applied Physics Letters.
“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,” said Holonyak,
a John Bardeen Professor of Electrical
and Computer Engineering and Physics at 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,” Holonyak said. “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.”
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,” said 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 credited with creating the world’s fastest
bipolar transistor, a device that operates at a frequency of 509 gigahertz.
Graduate student Walid Hafez fabricated the light-emitting transistor
in the university’s Micro
and Nanotechnology Laboratory. Unlike traditional transistors, which
are built from silicon and germanium, the light-emitting transistors
are made 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
said. “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 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,” Holonyak said. “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
light-emitting diodes, the photons in light-emitting transistors are
generated under much higher speed conditions. So far, the researchers
have demonstrated the modulation of light emission in phase with a base
current in transistors operating at a frequency of 1 megahertz. Much
higher speeds are considered certain.
“At such speeds, optical interconnects could replace electrical
wiring between electronic components on a circuit board,” Feng
said. This work could be the beginning of an era in which photons are
directed around a chip in much the same fashion as electrons have been
maneuvered on conventional chips.
“In retrospect, we could say the groundwork for this was laid
more than 56 years ago with John Bardeen and Walter Brattain and their
first germanium transistor,” said Holonyak, who was Bardeen’s
first graduate student. “But the direct recombination involving
a photon is weak in germanium materials, and John and Walter just wouldn’t
have seen the light – even if they had looked. If John were alive
and we showed him this device, he would have to have a big grin.”