SCIENTISTS at IBM in New York have successfully put 8000 transistors
and four photodetectors onto a single gallium-arsenide chip. They believe
that this is the densest optoelectronic chip – one that integrates electronic
circuits with optical devices – yet produced. It can detect and process
data at speeds of a billion bits per second.
In their search for more powerful computers and faster telecommunications
systems, designers are using fibre optics to transmit data faster than electronic
signals. IBM will use the optoelectronic chips as transmitters and receivers
to convert the light to electronic signals and vice versa.
The company decided on gallium arsenide for the chips because light-emitting
components, such as lasers and photodiodes, cannot be made from silicon.
The drawback is that gallium arsenide cannot take as many components as
silicon.
Computer circuits are faster and more reliable when the electronics
and optical devices are integrated on single chips. Integration also makes
it cheaper to mass-produce the chips. Another advantage noted by John Crow
of the IBM Thomas J. Watson Research Center at Yorktown Heights in New York
is that integrated transmitters and receivers can fit ‘right into the processor,
(where) the real estate gets very expensive’. Until this development, integrated
chips of only tens of electronic components and a few photodiodes for high-speed
fibre-optic receivers were available. This has limited developments in computers
more than telecommunications.
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Designers of high-speed telecommunications systems need only to integrate
the few electronic components that interface directly with a detector or
laser; they can use a separate chip for electronic functions. Research at
AT&T Bell Labs, for example, is aimed at integrating more optical components
to process optical signals at even higher speeds. To transfer data between
and within computers, it pays IBM to have a single chip perform many electronic
functions as well as optical ones.
Millions of electronic components can be integrated on silicon chips
because the components themselves are flat, and are made easily by altering
the surface. Gallium-arsenide components are faster than silicon ones. But
because the compound is harder to process than silicon, electronic integration
has reached only tens of thousands of components per chip. Optical components
such as semiconductor lasers and light detectors are particularly hard to
integrate with conventional electronics because most are made by depositing
a series of layers to form thick structures.
IBM integrated optical devices and electronics on the same chip using
Schottky diodes as photodetectors. Schottky detectors are not as sensitive
as conventional types, but they are made with the same flat-surface technology
as highly integrated electronic circuits. Planar lasers are not yet perfected,
so few lasers can be integrated on a chip.
