2008年5月2日 星期五

Missing Link of Electronics Discovered: "Memristor"


The memristor, an electrical resistor with memory properties, may also make it possible to fashion advanced logic circuits, a class of reprogrammable chips known as field programmable gate arrays, that are widely used for rapid prototyping of new circuits and for custom-made chips that need to be manufactured quickly.






憶阻器的概念最早在1971年由柏克萊加州大學的華裔科學家蔡少堂 Leon Chua提出。他近日接受訪問時表示,已經數十年未研究這項技術,惠普研究人員數個月前連絡他,讓他大感驚喜。



H.P. Reports Big Advance in Memory Chip Design

Published: May 1, 2008

Hewlett-Packard scientists reported Wednesday in the science journal Nature that they have designed a simple circuit element that they believe will make it possible to build tiny powerful computers that could imitate biological functions.

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Paul Sakuma/Associated Press

R. Stanley Williams, Hewlett-Packard’s director of the quantum science research group, and his team designed a circuit element that may make it possible to build tiny powerful computers.

The device, called a memristor, would be used to build extremely dense computer memory chips that use far less power than today’s DRAM memory chips. Manufacturers of today’s chips are rapidly reaching the limit on how much smaller chips can be.

The memristor, an electrical resistor with memory properties, may also make it possible to fashion advanced logic circuits, a class of reprogrammable chips known as field programmable gate arrays, that are widely used for rapid prototyping of new circuits and for custom-made chips that need to be manufactured quickly.

Potentially even more tantalizing is the ability of the memristors to store and retrieve a vast array of intermediate values, not just the binary 1s and 0s conventional chips use. This allows them to function like biological synapses and makes them ideal for many artificial intelligence applications ranging from machine vision to understanding speech.

Independent researchers said that it seemed likely that the memristor might relatively quickly be applied in computer memories, but that other applications could be more challenging. Typically, technology advances are not adopted unless they offer large advantages in cost or performance over the technologies they are replacing.

“Whether it will be useful for other large-scale applications is unclear at this point,” said Wolfgang Porod, director of the Center for Nano Science and Technology at the University of Notre Dame.

The technology should be fairly quickly commercialized, said R. Stanley Williams, director of the quantum science research group at Hewlett-Packard. “This is on a fast track.”

The memristor was predicted in 1971 by Leon Chua, an electrical engineer at the University of California, Berkeley. There have been hints of an unexplained behavior in the literature for some time, Mr. Chua said in a phone interview on Tuesday.

He noted, however, that he had not worked on his idea for several decades and that he was taken by surprise when he was contacted by the Hewlett-Packard researchers several months ago. The advance clearly points the way to a prediction made in 1959 by the physicist Richard Feynman that “there’s plenty of room at the bottom,” referring to the possibility of building atomic-scale systems.

“I can see all kinds of new technologies, and I’m thrilled,” he said.

The original theoretical work done by Mr. Chua was laid out in a paper, “Memristor — The Missing Circuit Element.” The paper argued that basic electronic theory required that in addition to the three basic circuit elements — resistors, capacitors and inductors — a fourth element should exist.

The Hewlett-Packard research team titled their paper, “The Missing Memristor Found.”

The Hewlett-Packard researchers said that the discovery of the memory properties in tiny, extremely thin spots of titanium dioxide came from a frustrating decade-long hunt for a new class of organic molecules to serve as nano-sized switches. Researchers in both industry and academia have hoped they would be able to fashion switches as small as the size of a single molecule to someday replace transistors once the semiconductor industry’s shrinking of electronic circuits made with photolithographic techniques reached a technological limit.

The memristor is a radically different approach from another type of solid state storage called phase-change memory that is being pursued by I.B.M., Intel and other companies. In phase-change memory, heat is used to shift a glassy material from an amorphous to a crystalline state and back again. The switching speed of these systems is both slower and requires more power, according to the Hewlett-Packard scientists.

The Hewlett-Packard team has successfully created working circuits based on memristors that are as small as 15 nanometers (the diameter of an atom is roughly about a tenth of a nanometer.) Ultimately, it will be possible to make memristors as small as about four nanometers, Mr. Williams said. In contrast the smallest components in today’s semiconductors are 45 nanometers, and the industry currently does not see a way to shrink those devices below about 20 nanometers.

Because the concept of a memristor was developed almost 40 years ago by Mr. Chua, it is in the public domain. The Hewlett-Packard scientists, however, have applied for patents covering their working version of the device.

The most significant limitation that the Hewlett-Packard researchers said the new technology faces is that the memristors function at about one-tenth the speed of today’s DRAM memory cells. They can be made in the same kinds of semiconductor factories that the chip industry now uses, however.

Missing Link of Electronics Discovered: "Memristor"

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

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By JR Minkel

SET IT AND FORGET IT: A new device called a memristor "remembers" whether it's on or offwhether or not it has power. Shown here 17 memristors in a row, formed by crossing 17 platinum nanowires with another wire, with a thin dab of titanium dioxide sandwiched between each junction. Each wire is 50 nanometers wide, equivalent to about 150 atoms.
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? To an electrical engineer, the battery is a capacitor—a device for storing electric charge—and the lightbulb is a resistor—an obstacle to electric current. Until now, engineers have had only one other basic element to work with—the inductor, which turns 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."