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Optical, electrical bistability study sheds light on next-gen high speed data transfer
Today, electrical bistable devices are the foundation of
digital electronics, serving as building blocks of switches, logic gates and
memories in computer systems. However, the bandwidth of these electronic computers
is limited by the signal delay of time constants important to electronic logic
operations. In an attempt to mitigate these problems, scientists have
considered the development of an optical digital computer, and one team has
gone so far as to demonstrate the optical and electrical bistability for
switching in a single transistor.
Today, electrical bistable devices
are the foundation of digital electronics, serving as building blocks of
switches, logic gates and memories in computer systems. However, the bandwidth
of these electronic computers is limited by the signal delay of time constants
important to electronic logic operations. In an attempt to mitigate these
problems, scientists have considered the development of an optical digital
computer, and one team has gone so far as to demonstrate the optical and
electrical bistability for switching in a single transistor.
This week, in the Journal of
Applied Physics, from AIP Publishing, a research team from the University
of Illinois at Urbana-Champaign present their findings regarding the optical
and electrical bistability of a single transistor operated at room temperature.
Prior to this work, quantum-wells
were incorporated near the collector in the base of a III-V heterojunction
bipolar transistor, resulting in a heavily reduced radiative spontaneous
recombination lifetime of the device. The laser current modulation bandwidth is
related to the electron-hole radiation recombination lifetimes, photon
lifetimes and cavity photon density.
In a method patented by two of the
article's authors, often referred to as Feng and Holonyak's idea, the optical
absorption can be further enhanced by the cavity coherent photon intensity of
the transistor laser. Using the unique property of intra-cavity photon-assisted
tunneling modulation, the researchers were able to establish a basis of direct
laser voltage modulation and switching at high gigahertz speeds.
The researchers found the transistor
laser electrical and optical bistabilities to be controllable by base current
and collector voltage. The current switching was found to be due to the
transistor base operation shift between stimulated and spontaneous
electron-hole recombination process at the base-quantum-well.
According to Milton Feng, of the
research group, this was the first time this has been done.
"We put a transistor inside of
an optical cavity, and the optical cavity controls the photon density in the
system. So, if I use tunneling to absorb the photon, and then the quantum-well
to generate the photon, then I basically can voltage-tune and current-control
the electrical and optical switching between coherent and incoherent state for
the light, and between stimulated and spontaneous recombination for the
current," Feng said.
Compared to prior investigations,
which contained optical hysteresis in cavities containing nonlinear absorptive
and dispersive gain media, the operation principles as physical processes and
operating mechanisms in transistor laser electro-optical bistabilities are
considerably different. In this case, different switching paths between optical
and electrical energy states results in different thresholds of input collector
voltage, resulting in this considerable difference in method and results.
"Because of the switching path differences
between coherent and incoherent cavity photon densities reacting with collector
voltage modulation via Feng-Holonyak intra-cavity photon-assisted tunneling
resulting in the collector voltage difference in switch-UP and switch-DOWN
operations, the transistor laser bistability is realizable, controllable and
usable," Feng said.
It is the researchers' belief the
operations of the electro-optical hysteresis and bistability in the compact
form of the transistor laser can be utilized for high speed optical logic gate
and flip-flop applications.
"I hope the new domain for
research will be extended from electronics -- from bodies in electronics which
transport in solid-state -- into electronic-optical domain into an integrated
circuit, which is going to be the big breakthrough for the future generation of
high-speed data transfer," Feng said.
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