COLUMBUS, Ohio – Ohio State University researchers
have invented a new organic polymer tunnel diode – an
electronic component that could one day lead to plastic
computer memory and plastic logic circuits on computer
chips. Today, computer chips use mainly inorganic
silicon.
The diode transmits electrical current at room temperature,
and its design lends itself to easy, inexpensive
manufacturing for smart cards and other memory devices,
said Paul Berger, a professor of electrical
and computer engineering and professor of physics at
Ohio State.
In tests, the team was able to fashion two diodes
into a simple computer chip device called a logic
switch, which was powered by the voltage equivalent
to an ordinary watch battery.
Berger and his students describe their patent-pending
invention in the current issue of the scientific
journal Applied
Physics Letters .
Most plastics don't conduct electricity, Berger
explained. That fact hasn't stopped researchers from
trying to build plastic computer chips, which could
be used in lightweight, flexible electronic devices.
In their most successful efforts, some groups have
coaxed modest numbers of electrons through conducting
plastics using quantum mechanical effects – but then
only by painstakingly manipulating individual molecules
of plastic at cryogenic temperatures. Those experiments
have also been difficult to replicate.
Berger and his students got around that problem
by taking the opposite approach. Instead of working
with one plastic molecule at a time, they painted
a thick layer of plastic on top of traditional chip
materials, with a specially-designed layer of titanium
oxide sandwiched in-between.
They got the idea in 2003 when Sita Asar – then
an undergraduate physics student at Ohio State – was
designing a plastic solar cell in Berger's lab. The
device was designed to convert solar energy to electrical
energy.
When he looked at the results of one of Asar's experiments,
Berger noticed something unusual – a tiny blip in
an otherwise smooth graph line charting the amount
of electrical current passing through the material.
At low voltages, the current spiked, and then returned
to normal.
On closer inspection, he saw that the plastic was
showing an effect called “negative differential resistance,” in
which the current actually decreases over a particular
range of increasing voltage. The effect resembled
that shown by a semiconductor device called a tunnel
diode.
Diodes are one-way conductors that typically power
amplifiers for devices such as stereo speakers. Tunnel
diodes are so named because they transmit electricity
via a quantum mechanical effect known as tunneling,
which lets electrons pass through barriers unhindered.
Berger later worked with graduate students Woo-Jun
Yoon and Sung-Yong Chung to refine a polymer tunnel
diode design that demonstrates the same kind of blip
that the solar cell did, but more robustly and more
reliably. They also embarked on a study to uncover
the physical mechanism which created this effect.
The team is still trying to fully understand why
the design works as well as it does, but they have
been able to achieve strong quantum mechanical effects
in the plastic without manipulating individual molecules.
“The titanium oxide is the key,” Berger said. “In
our experiments, when we replace it with anything
else, the phenomenon goes away.”
Other research groups have tried to marry polymer
tunnel diodes to titanium oxide without success.
The breakthrough came when Berger decided to deposit
a layer of pure titanium on a chip, and then carefully
oxidize it later, instead of depositing titanium
oxide all at once, as the others had done. He suspects
that it's the extra control he exerted over the oxidation
process that ultimately led to the diode's enhanced
performance.
He and the students were able to combine two plastic
tunnel diodes to form a simple logic gate – the structure
that writes data on a computer chip.
The gate operated on a mere 1.5 volts of electricity – an
amount equal to a watch battery. It also worked at
room temperature. These results suggest that it could
one day be easily incorporated into traditional computer
chips for devices that required low power, Berger
said.
Moreover, he and his students created their plastic
layer through a process called spin casting, which
costs significantly less than the processes currently
used to machine silicon chips. In spin casting, drops
of a liquid plastic solution are deposited on a surface,
and the surface is spun at high speed to spread a
thin coating evenly across the surface.
For all of the diode's good points, Berger stopped
short of saying that it could lead to electronics
made entirely of plastic.
“Plastic isn't going to replace silicon – at least,
I don't advocate that. I think that plastic is going
to augment silicon,” he said.
He envisions lightweight, portable electronics that
knit silicon and plastic together to form a kind
of hybrid computer logic circuit or memory. Devices
like smart cards and other memory devices would be
somewhat bendable and run on less power.
“This new diode is just another tool for the circuit
designer's toolbox,” he said.
This research was partly supported by the National
Science Foundation , including an award Berger
received from its Research Experience for Undergraduates
program.
Contact: Paul Berger, (614) 247-6235; pberger@ieee.org
Written by Pam Frost Gorder, (614) 292-9475; Gorder.1@osu.edu
|
