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Missing Link of Electronics Discovered: "Memristor"

Memory plus resistor may add up to longer-lasting batteries and faster-booting computers



Image courtesy of J. J. Yang, HP Labs.

After nearly 40 years, researchers have discovered a new type of building block for electronic circuits. And there's at least a chance it will spare you from recharging your phone every other day. Scientists at Hewlett-Packard Laboratories in Palo Alto, Calif., report in Nature that a new nanometer-scale electric switch "remembers" whether it is on or off after its power is turned off. (A nanometer is one billionth of a meter.)

Researchers believe that the memristor, or memory resistor, might become a useful tool for constructing nonvolatile computer memory, which is not lost when the power goes off, or for keeping the computer industry on pace to satisfy Moore's law, the exponential growth in processing power every 18 months.

You may dimly recall circuit diagrams from your middle school science class; those little boxes with a battery on one end and a lightbulb on the other. Ring any bells? Until now, electrical engineers had only three "passive" circuit elements (those that dissipate the energy from a power source) [see note below]: The capacitor accumulates electric charge; the resistor (represented by the light bulb) resists electric current; and the inductor converts current into a magnetic field.

In 1971 researcher Leon Chua of the University of California, Berkeley, noticed a gap in that list. Circuit elements express relationships between pairs of the four electromagnetic quantities of charge, current, voltage and magnetic flux. Missing was a link between charge and flux. Chua dubbed this missing link the memristor and created a crude example to demonstrate its key property: it becomes more or less resistive (less or more conductive) depending on the amount of charge that had flowed through it.

Physicist Stanley Williams of HP Labs says that after a colleague brought Chua's work to his attention, he saw that it would explain a variety of odd behaviors in electronic devices that his group and other nanotech researchers had built over the years. His "brain jolt" came, he says, when he realized that "to make a pure memristor you have to build it so as to isolate this memory function."

So he and his colleagues inserted a layer of titanium dioxide (TiO2) as thin as three nanometers between a pair of platinum layers [see image above]. Part of the TiO2 layer contained a sprinkling of positively charged divots (vacancies) where oxygen atoms would have normally been. They applied an alternating current to the electrode closer to these divots, causing it to swing between a positive and negative charge.

When positively charged, the electrode pushed the charged vacancies and spread them throughout the TiO2, boosting the current flowing to the second electrode. When the voltage reversed, it slashed the current a million-fold, the group reports. When the researchers turned the current off, the vacancies stopped moving, which left the memristor in either its high- or low-resistant state. "Our physics model tells us that the memristive state should last for years," Williams says.

Chua says he didn't expect anyone to make a memristor in his lifetime. "It's amazing," he says. "I had just completely forgotten it." He says the HP memristor has an advantage over other potential nonvolatile memory technologies because the basic manufacturing tools are already in place.

Williams adds that memristors could be used to speed up microprocessors by synchronizing circuits that tend to drift in frequency relative to one another or by doing the work of many transistors at once.

Whether industry will adopt it remains to be seen. In an editorial accompanying the paper, nanotech researchers James Tour and Tao He of Rice University in Houston note that "even to consider an alternative to the transistor is anathema to many device engineers, and the memristor concept will have a steep slope to climb towards acceptance."

But the memristor concept is a promising one, they wrote, adding: "It is often the simple ideas that stand the test of time."

Note: This article originally stated incorrectly that a battery is a type of capacitor.

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