Researchers have constructed a light-emitting transistor that has set a
new record with a signal-processing modulation speed of 4.3 gigahertz,
breaking the previous record of 1.7 gigahertz held by a light-emitting
diode. But, the researchers didn’t stop there. By internally
connecting the base and collector of a light-emitting transistor, they
created a new form of light-emitting diode, which modulates at up to 7
gigahertz, breaking the speed record once again.
In a pair of papers published in the June 15 issue of Applied Physics
Letters, researchers at the University of Illinois and at U. of I.
licensee Quantum Electro Opto Systems in Melaka, Malaysia, report the
fabrication and testing of the new high-speed light-emitting transistor
and the new “tilted-charge” light-emitting diode.
 |
| 4.3
GHz optical bandwidth light-emitting transistor, top view, cross
section and collector current-voltage characteristics | Image courtesy
Feng and Holonyak. |
“Simple in design and construction, the tilted-charge
light-emitting diode offers an attractive alternative for use in
high-speed signal processing, optical communication systems and
integrated optoelectronics,” said Nick Holonyak Jr., a John
Bardeen Chair Professor of Electrical and Computer Engineering and
Physics at Illinois, and a co-author of both papers.
The modulation speed of either a light-emitting diode or a
light-emitting transistor is limited by the rate at which electrons and
holes (the minus and plus charges – the carriers of current)
recombine. The recombination lifetime is important in determining
device speed.
With a usual “slow” recombination process, the speed of a
light-emitting diode is limited to approximately 1.7 gigahertz, which
corresponds to a carrier lifetime of 100 picoseconds. For more than 40
years, scientists thought breaking the 100-picosecond barrier was
impossible.
Recombination speeds of less than 100 picoseconds are not readily
achieved in light-emitting diodes because equal number densities of
electrons and holes are injected into the active region to preserve
charge neutrality, said Holonyak, who invented the first practical
visible light-emitting diode more than 40 years ago.
These charges become stuck, stacked-up waiting to recombine, Holonyak
said. To achieve high recombination speeds, an extremely high injection
level and a very high charge population are required in light-emitting
diodes. These conditions are not necessary in transistors, however.
“Unlike a diode, a transistor does not store charge,” said
Milton Feng, the Holonyak Chair Professor of Electrical and Computer
Engineering, and a co-author of the two papers. “Charges are
delivered to the transistor’s quantum well active region, where
they either recombine almost instantly, or they are kept moving on out
of the device. The charges do not become stacked-up, waiting to
recombine with their oppositely charged twins.”
To increase the modulation speed of their light-emitting transistor,
the researchers reduced the emitter size, increased the so-called
collector thickness (the third terminal region), and utilized a special
internal common collector design. These changes resulted in a faster
signal at a very low current level, and at low heat dissipation.
Having a “fast” recombination process, the modulation speed
of the light-emitting transistor was measured at 4.3 gigahertz, which
corresponds to a recombination lifetime of 37 picoseconds, well under
the “100-picosecond barrier.”
“In the light-emitting transistor, the third terminal – the
collector – effectively ‘tilts’ the charge and
removes carriers with slower recombination lifetimes,” said
Holonyak, who also is a professor in the university’s Center for
Advanced Study, one of the highest forms of campus recognition.
“As opposed to the charge ‘pile-up’ condition found
in a normal diode, the dynamic ‘tilted’ charge flow
condition in the transistor base is maintained with the collector in
competition with the base recombination process,” Holonyak said.
“If the charge doesn’t recombine and generate a photon fast
enough, it is swept away by the current in the collector.”
By preventing the build-up of “slow” charges in the base,
the “fast” picosecond recombination dynamics also provided
the basis for the researchers’ light-emitting transistor rewired
internally as a new type of light-emitting diode.
The tilted-charge light-emitting diode achieved a record-breaking
modulation speed of 7 gigahertz, corresponding to a recombination
lifetime of 23 picoseconds. “The tilted-charge light-emitting
diode is simple to make, low cost, and easy to package and use,”
Holonyak said.
Because of the tilted base population in the device, current flow,
which is a function of the slope of the charge distribution, makes
possible high current densities without requiring extreme carrier
densities.
“That’s the trick of the transistor,” Holonyak said.
“And now we’ve incorporated it into a diode. The physics
has been there all along. It just wasn’t recognized.”
With Feng and Holonyak, co-authors of the paper are lead author Gabriel
Walter (chief executive officer at Quantum Electro Opto Systems), and
graduate students Chao-Hsin Wu and Han Wui Then.
Funding was provided by the U.S. Army Research Office and the Brain
Gain Malaysia Diaspora Program. Device fabrication and testing was
performed at the university’s Micro and Nanotechnology
Laboratory.
Quantum Electro Opto Systems is a company formed by Walter, Feng and
Holonyak to commercialize the light-emitting transistor and
tilted-charge light-emitting diode technology.
back
top
