Science > Physics > Physics News Update -- Update #667 (December 30, 2003)
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"Sam Wormley" |
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30 Dec 2003 01:03:02 PM |
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Physics News Update -- Update #667 (December 30, 2003) |
Ref: http://www.aip.org/enews/physnews/2003/667.html
Physics News Update -- Update #667 (December 30, 2003)
by Phillip F. Schewe, Ben Stein, and James Riordon
World's First Light Emitting Transistor
Researchers at the University of Illinois at Urbana-Champaign have
developed the world's first light emitting transistor (LET). Unlike
conventional transistors, which include an electrical input port and an
electrical output port, the new LET also has an infrared optical output
port (see image).
The LET is built of indium gallium phosphide and gallium arsenide,
rather than the silicon and germanium used in many conventional
transistors. Although the LET produces light in essentially the same
way that light emitting diodes (LEDs) operate, the transistor can
modulate light at much higher speeds. To date, the researchers (N.
Holonyak, Jr., blpayne@.uiuc.edu, 217-333-4149) have managed to
modulate the optical LET output at a frequency of one megahertz, but
much higher speeds are theoretically possible.
Although it's too early to predict the various applications for LETs,
the hybrid device should help integrate electrical and optical
circuitry designs with one convenient, high-speed package. It is only
fitting that the research team that developed the LET include the
inventor of the first visible LED (Holonyak) and the developer of the
world's fastest bipolar transistor (Feng). (M. Feng et al., Applied
Physics Letters, 5 January 2004).
Glial Cells and Epilepsy
Is there a connection? Neurons are not the only cells in the brain. In
fact, 90% of brain matter consists of glial cells. Astrocytes, the most
common glial cell type, don't have enough sodium channels to carry on
the active electrochemical signaling characteristic of neurons, but
they can communicate with other cells through the diffusion of
messenger molecules.
Furthermore, astrocytes can partially or wholly enwrap neuronal
synapses, the message sending or receiving ends of the neuron. This
facilitates neutron-astrocyte interactions, and even neuron-neuron
communications via astrocytes. Formerly glia were thought to play a
passive role in the nervous system---cleaning up the potassium needed
in the neural firing mechanism. But increasingly scientists believe
that glia play a more active role in enhancing or inhibiting action in
the synapse.
Suhita Nadkarni and Peter Jung at Ohio University believe that glia
participate in the making of epilepsy. There is no accepted theory of
epilepsy; does it arise from neurons talking in synchrony or is it a
sort of "thunderstorm" of spontaneous activity among neurons? Jung
argues that under some conditions the neuron might "listen" so much to
its astrocyte environment (by an overexpression of certain receptor
molecules) that it enters into a bistable state; even in the absence of
outside (normal) stimulation the neuron could fire indiscriminately in
the manner characteristic of epilepsy. It is therefore necessary to
undertake a sort of electrical engineering study of neural-glial
circuitry.
Jung, a physicist (presently at the Kavli Institute for Theoretical
Physics at UC Santa Barbara, 805-893-7333, jungp@kitp.ucsb.edu), has
demonstrated some of this glial-neural behavior in computer simulations
and is working with neurobiologists who might shortly put the model to
an experimental test. (Nadkarni and Jung, Physical Review Letters, 31
December 2003.)
Improved Tandem Organic LEDs
Stacking organic light emitting diodes (OLEDs) leads to brighter,
stabler, longer lived light sources than individual OLEDs.
Unfortunately, the metal layers typically used to connect the
individual elements are not very transparent, reducing the resulting
brightness of underlying OLEDs in a tandem configuration.
Researchers in the Display Technology Laboratory at Eastman Kodak
Company have now managed to stack OLEDs that are connected through
optically transparent, organic semiconductor materials. The improvement
in brightness in the new, tandem OLED is essentially linearly related
to the number of individual light emitting segments included in the
device, that is, a three-segment tandem OLED is roughly three times as
bright as a conventional OLED.
High brightness, high efficiency tandem OLEDs could lead to brighter
TV's and computer screens. They could also make it easier to read cell
phone displays in bright sunlight, which often renders existing cell
phone displays unintelligible.
The researchers (contact: L. S. Liao, liang-sheng.liao@kodak.com)
propose that tandem OLEDs may also be useful as lighting sources for
liquid crystal display backlighting or as solid-state room lights. In
addition, varying the number of units in a tandem OLED stack changes
the operating voltage, allowing the possibility of tailoring the
devices to match different electrical sources, such as household 110
volt systems. Conventional LED lighting, on the other hand, typically
requires transformers to adjust power sources to meet the lighting
element's electrical specifications. (L. S. Liao et al., Applied
Physics Letters, upcoming article)
Physics News Update is a digest of physics news items arising from
physics meetings, physics journals, newspapers and magazines, and other
news sources. Subscriptions are free as a way of broadly disseminating
information about physics and physicists. Feel free to post it where
others can read it; please credit the American Institute of Physics.
Physics News Update appears approximately once a week. Questions?
Contact the editors at physnews@aip.org.
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