The Mysterious Memristor

by Sally Adee

First Published May 2008

Researchers at HP have solved the mystery of the resistor 37 years of memory, considered the 4th element of electrical circuits absent.

First of May, 2008 - Someone who is familiar with electricity and electronics know that there is a triad of key elements in electricity: the resistor, capacitor, and inductor. In 1971, the University of California at Berkeley, an engineer predicted that there should be a fourth element electric: a type of resistor with memory, or Memristor. But until then nobody knew how to build that element. Now, 37 years later, the machine was finally small enough to reveal the secrets of that fourth electric element. The Memristor built by researchers at Hewlett-Packard (HP) revealed today in Nature, was being hidden from the sight of all for a long time hidden in the electrical characteristics of certain nanoscale devices. They believe that the new element can open a new path for applications in random access memory nonvolatile, and also dreamed of realistic neural networks, instead of artificial neural networks (one of the current techniques applied in Artificial Intelligence).

The history of Memristor starts almost four decades with an insight into the pioneer of the theory of nonlinear circuit Leon Chua. Examining the relationship between electric charge and magnetic flux in electrical resistors, capacitors and inductors in an article written in 1971, Chua postulated the existence of a fourth element which he called electric resistor memory. Such a device should provide a link between the magnetic flux and electric charge, similar to an electrical resistor that would do when subjected to voltage and current. In practice this would mean that he served as an electrical resistor whose electrical resistance value can vary depending on the electric current passing through it, and that it would remember the value of electric current even after the electric current has disappeared.

But the hypothetical device was assessed as the lack of compatibility of constructive time with mathematics that explains the effect Memristivo, but that was not visible to the device 1971. Thirty years later, a colleague at HP, Stanley Williams, and his research group working on molecular electronics when they began to notice strange behavior in their devices. "They were really things that we could not understand," says Williams. So your employee of HP, Greg Snider, rediscovered Chua's article of 1971. He said, "Hey guys, I do not know what we have, but this is what we want." Remembers Williams. Williams spent several years reading and rereading the pages of the article Chua. "He (the article) spent several years knocking on my head." Then Williams was able to build their molecular devices and getting a really Memristors. "He hit me between the eyes."

The reason that the memristor is radically different from other electric elements is fundamental that, unlike them, it carries a memory of its past. When you switch the electrical circuit, the memristor still remembers how much was the value applied to it before the shutdown. This is an effect that can not be duplicated by any circuit combination of resistors, capacitors or inductors, which is what differentiates the Memristor and qualifies as a fundamental circuit element.

The classic analogy for an electrical resistor is a pipe through which water (electricity) passes. The width of the tube is analogous to electrical resistance that allows the flow of electric current passing through the tube - the narrower the tube, the greater the electrical resistance. The resistors have a normal pipe size invariant under the effects of current and voltage. A Memristor, on the other hand, changes with the amount of water flowing through the pipe, or cable. If you push the water through the pipe in a given direction, the tube becomes larger (less resistive). If you push the water in the other direction, the tube becomes smaller (more resistive), is characteristic of a memristor. When the flow of water stops moving, the pipe size does not change more. This mechanism may be technically replicated using transistors and capacitors, but, says Williams, "he uses many transistors and capacitors to make the use of a single Memristor."

The memory of the Memristor has consequences: the normal digital computers must be restarted each time they are connected, since their logic circuits are unable to keep their bits after power is interrupted. But because a memristor can remember the old tensions applied on it, a computer built with new components like Memristor never need to be restarted. "You can leave all your files, documents and spreadsheets open, turn off your computer and go drink a cup of coffee or go on vacation for two weeks," says Williams. "When you turn, you turn on your computer and everything will be immediately on the screen exactly the way you left it."

Chua deduced the existence of the Memristor of mathematical relationships between the four basic quantities of electrical circuits. The four basic circuit quantities (charge, current, voltage and magnetic flux) can be related to each other in six different ways. Two quantities are covered by the basic laws of physics, and three are covered by the basic elements of electrical circuits known (resistor, capacitor and inductor), says electrical engineering professor David Vallancourt to Columbia University. Based on this, Chua proposed the memristor purely as a class of circuit elements based on a relationship between the electric charges and magnetic flux.

Chua interprets the work of the staff of HP as a paradigm shifted in time. He compares the addition of Memristor, the army of circuit elements, just as you would the sum of a new element in the periodic table, "now all the manuals of Electrical Engineering (EE) have to be modified," he says.

So why someone had not noticed before the Memristência? Chua has indeed produced a Memristor in 1970 with an unworkable combination of resistors, capacitors, inductors, and amplifiers only as proof of concept. But Memristor as a property of an electrical equipment was, until recently, too subtle to make use of it. The Memristência is obscured by other electrical effects of greater relevance, until you see this effect in materials and other devices that are built in nanometric sizes.

The absence of an application for Memristência was particularly complicated, there was no need to apply Memristência. No engineer said, "If only we had a Memristor, we can do such a thing!," Says Vallancourt. In fact, Vallancourt, who has been teaching techniques of electrical circuits for years, had never heard to speak of Memristência before this week.

The smaller the scale at which the engineers and scientists build their devices more noticeable the effect of Memristor, says Chua, who is now senior lecturer at Berkeley.

There were several clues to the existence of the Memristor the beginning. "We knew funny voltage characteristics in the literature during the past 50 years," says Williams. "I went to these old articles and seen the pictures and said, 'Yes, they have Memristência, and we did not know how to interpret it'."

"Without the equations of Chua's circuit, you can not make use of this device," says Williams. We used all the circuit equations incorrect. Comparing it with a motor of a washing machine, would be the same as replacing the electric motor of the machine by a combustion engine, wondering why he will not run. "Memristor Williams found an ideal in the titanium dioxide - material and white paint sunscreen. As silicon, titanium dioxide (TiO2) is a semiconductor in its pure form is highly resistive. However, it can be doped with other elements to make it very conductive. The TiO2 doped is not stationary in a high electric field, it tends to drift in the direction of electric current. Such mobility is a poison to a transistor, but results in exactly the desired effect in a Memristor. The placement of a voltage level through a thin film of TiO2 doped semiconductor on one side only, because the decrease in electrical resistance. The displacement of electric current in another direction then push back the dopant, thus increasing the electrical resistance of TiO2.

The HP Labs has been researching how to manufacture the Memristors of TiO2 and other materials in addition to trying to understand the physics behind them. They also have the perspective to integrate Memristors on a single silicon chip. The group of HP is a type of hybrid silicon CMOS memristor in tests.

The implications for aspects of electrical circuits may be the feelings of the moment. The applications have to be identified because the unique characteristics of Memristor offer possibilities not yet covered by components of today.

Williams is in negotiations with several engineering laboratories and neuroscience that have long been pursuing devices that emulate biological neural systems. Chua says that synapses, connections between neurons, have a bit of behavior Memristivo. Therefore, a Memristor be the ideal electronic device to emulate a biological synapse.

Redesigning certain types of circuits to include Memristors, Williams expects the same function with fewer components, making the circuit itself is cheaper and significantly reduced, including as its consumption of energy. In fact, he hopes Memristors combine with elements of traditional circuit to produce a device that makes computing non-Boolean. "We do not know if we are building an electronic brain, but we want something that will work as a human brain," says Williams. They think they can disregard "the basic idea of the synapse" to make a computer primarily analog and efficient. "Some things that a digital computer would take an almost infinite time, an analog computer based Memristor would take only a slight breeze," he says.

The HP group also is studying the development of a non-volatile memory based on Memristor. "A memory-based Memristors can be 1,000 times faster than magnetic disks and use much less energy," says Williams, looking like a kid in a candy store.

Chua believes that the non-volatile memory is the immediate application of Memristor. "I'm very happy for this break through the enemy lines," he says. "In reality the nanoscale allows this effect becomes dominant, and you'll find anywhere like it or not, I am pleased to show people the right direction."

Adapted from the text of IEEE Spectrum Online on the Mysterious Memristors by Prof. Dr. Luis Filipe Barbosa Wiltgen - LRA / FEAU / UNIVAP - 06 May 2008.