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Hidden structure revealed in characteristics of transistor laser
James E.
Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@uiuc.edu
4/5/2006
|
CHAMPAIGN, Ill.
— The transistor laser, invented by scientists at the University
of Illinois at Urbana-Champaign, has been full of surprises. Researchers
recently coaxed the device to reveal fundamental properties of the transistor,
and of the transistor laser, moving it a step closer to commercialization.
As reported in the April 3 issue of the journal Applied Physics Letters,
Nick Holonyak Jr., Milton Feng, and colleagues at the U. of I. explored
the current-voltage relationship in a transistor laser. During stimulated
emission, the laser light allowed the scientists to see into the device
and study its elusive electronic structure.
“We were able to look at the transistor’s operating characteristics,
look inside of the transistor, and see features and behaviors that we
couldn’t see before,” said Holonyak, a John Bardeen Chair
Professor of Electrical and Computer
Engineering and Physics.
“The current-voltage characteristics were clearly distorted under
stimulated recombination, compared to ordinary 58-year-old-transistor
spontaneous recombination.”
The transistor laser employs a quantum well and a resonator in the base
to control electron-hole recombination and electrical gain. By blocking
the laser resonator with white paste, the researchers converted the
device into an ordinary transistor. Because the process is reversible,
the researchers could compare collector characteristics when the device
was functioning as a normal transistor and when it was functioning as
a transistor laser, something that was never before possible.
“We found significant structure in the current-voltage characteristics
of the transistor laser, that can be mapped in detail and related to
the quantum-well carrier recombination,” said Holonyak, who also
is a professor in the university’s Center
for Advanced Study, one of the highest forms of campus recognition.
“We were also able to correlate optical measurements with electrical
measurements of quantum-well properties,” Holonyak said.
The transistor laser combines the functionality of both a transistor
and a laser by converting electrical input signals into two output signals,
one electrical and one optical. Photons for the optical signal are generated
when electrons and holes recombine in the base, an intrinsic feature
of transistors.
“When we weaken the strength of the photon generation process,
we change the nature of the process connecting the electron and the
hole, and we change their behavior in an electrical sense,” Holonyak
said. “When we let the device operate as a transistor laser, however,
the photons streaming out let us look inside and see more of the mechanics
that goes on. We see features of the transistor never revealed before.”
The change in gain and laser wavelength corresponding to stimulated
recombination on quantum-well transitions can be compared to operation
in spontaneous recombination and used with conventional transistor charge
analysis to determine some of the dynamic properties of the transistor
laser.
“This transistor laser is letting us see the properties and mechanics
of how fast the electrons and holes generate photons, and we can turn
laser photon generation on and off,” said Feng, the Holonyak Chair
Professor of Electrical and Computer Engineering at Illinois. “This
allows us to alter the processes and see how the speed and time factors
are changing. This is the first time we could directly determine the
lifetime, the speed of stimulated recombination. The transistor has
now made certain laser measurements easier or more convenient.”
This capability opens the door to developing transistor lasers that
operate at different speeds for a variety of commercial applications,
Feng said.
“Until now, we had missed something important and fundamental
about the boundaries of what the photon can do, of what the electron
and hole can do, and of what the semiconductor can do,” Holonyak
said. “We found those boundaries to be much further out than we
had ever imagined, which now makes our prognosis for the transistor
laser much more optimistic.”
Co-authors of the paper with Feng and Holonyak are postdoctoral research
associates Richard Chan and Gabriel Walter, and graduate student Adam
James. The Defense Advanced Research Projects Agency funded the work.
To reach Nick Holonyak,
call 217-333-4149.
To reach Milton Feng, call 217-333-8080; e-mail: mfeng@uiuc.edu.
