Communications - March 2011

The memristor prototype chip built
at Rice is a 1-kilobyte ReRAM with sub-
5 nm switches, according to Jim Tour,
a synthetic organic chemist at Rice
and a leading memristor researcher.
Of Rattner’s concern about manufac-
turing the devices, he says, “All so far
looks good—materials cost, fabrica-
tion needs, scalability and switching
times—except the switch voltage is a
bit higher than we’d like, but we have
some ideas to reduce that.”
Rice claims to have an edge over
HP’s silicon-and-titanium memristor
chip with its all-silicon model. “There
are lots of engineering barriers to be
overcome before this really takes off,”
says Doug Natelson, a professor of
physics and astronomy at Rice. “But
the use of all silicon makes the manu-
facturing very understandable.”
Memristance comes from reduc-
tion-oxidation chemistry, in which
atoms or molecules gain or lose their
affinity for oxygen atoms, and in which
the physical structure of materials can
change. The Rice memristor chip, a
thin layer of silicon oxide sandwiched
between two electrodes, is made to
convert back and forth between sili-
con (a conductor) and silicon oxide
(an insulator.) A sufficiently large volt-
age (up to 13 volts) applied across the
silicon oxide converts some of it into
pure silicon nanocrystals that conduct
current through the layer. The switch,
according to Natelson, shows robust
nonvolatile properties, a high ratio of
current “on” to current “off” (> 105),
fast switching (sub- 100 ns), and good
endurance ( 104 write-erase cycles).

The HP version is conceptually similar, but works by the alternating

in the short term,
memristors are most
likely to be used in
storage devices,
but eventually may
be used in artificial
neural networks.

oxidation and reduction of titanium. Titanium dioxide (TiO2) is a semiconductor and is highly resistive in its pure state. However, oxygen-deficient TiO2, which has oxygen “vacancies” where an oxygen atom would normally appear, is highly conductive. By applying a bias voltage across a thin film of semiconductor with oxygen-deficient TiO2 on one side, the oxygen vacancies move into the pure TiO2 on the other side of the semiconductor, thus lowering the resistance. Running current in the other direction will move the oxygen vacancies back to the other side, increasing the resistance of the TiO2 gain.

In the short term, memristors are
most likely to be used in storage de-
vices, but eventually may be used in
artificial neural networks, in applica-
tions such as pattern recognition or
real-time analysis of the signals from
sensor arrays, in a way that mimics
the human brain. A memristor works
like a biological synapse, with its con-
ductance varying with experience, or
with the current flowing through it
over time. Similarly, the brain learns
and configures itself by varying the
strength of synaptic connections be-
tween neurons. The ability of mem-
ristors to remember and to work as
analog devices allows them to assume
any of many values over a range, just as
synapses do.

Further Reading Chua, L. Memristor–the missing circuit element, IEEE Transactions on Circuit Theory 18, 5, Sept. 1971.

Jo, S.H., Chang, T., Ebong, I., Bhadviya, B.B., Mazumder, P., and Lu, W. nanoscale memristor device as synapse in neuromorphic systems, Nano Letters 10, 4, March 1, 2010. Strukov, D.B., Snider, G.S., Stewart, D.R., and Williams, R.S. The missing memristor found, Nature 453, May 1, 2008.

Tour, J.M. and He, T. Electronics: the fourth element, Nature 453, May 1, 2008. Yao, J., Sun, Z., Zhong, L., Natelson, D., and Tour, J.M. Resistive switches and memories from silicon oxide, Nano Letters 10, 10, Aug. 31, 2010.

Gary Anthes is a technology writer and editor based in arlington, Va.

Technology
Flexible Screens

hewlett-Packard plans to
deliver a prototype of a solar-
powered, lightweight device
with a flexible plastic screen—
which hP researchers are
affectionately terming “a Dick
tracy wristwatch”—to the u. S.
army later this year. Roughly
the size of an index card, the
low-power device will enable
soldiers to read digital maps,
directions, and other data on
a screen that won’t break or
shatter like glass.