Researchers at HP Labs have solved a decades-old mystery by proving the existence of a fourth basic element in integrated circuits that could make it possible to develop computers that turn on and off like an electric light.
The memristor — short for memory resistor - could make it possible to develop far more energy-efficient computing systems with memories that retain information even after the power is off, so there's no wait for the system to boot up after turning the computer on. It may even be possible to create systems with some of the pattern-matching abilities of the human brain.
A mathematical model and a physical example that prove the memristor's existence appear in a paper published in the April 30 issue of the journal Nature.
"To find something new and yet so fundamental in the very mature field of electrical engineering is a big surprise," said R. Stanley Williams, an HP Senior Fellow and director of the Information and Quantum Systems Lab (IQSL).
A memristor effectively stores information because the level of its electrical resistance changes when current is applied. A typical resistor provides a stable level of resistance. By contrast, a memristor can have a high level of resistance, which can be interpreted as a computer as a "1" in data terms, and a low level can be interpreted as a "0." Thus, data can be recorded and rewritten by controlling current. In a sense, a memristor is a variable resistor that, through its resistance, reflects its own history, Williams said.
Fundamental circuit element
The memristor first appeared in a 1971 paper published by Professor Leon Chua, a distinguished faculty member in the Electrical Engineering and Computer Sciences Department of the University of California Berkeley.
Chua described and named the memristor, arguing that it should be included along with the resistor, capacitor and inductor as the fourth fundamental circuit element. The memristor has properties that cannot be duplicated by any combination of the other three elements.
Although researchers had observed instances of memristance for more than 50 years, the proof of its existence remained elusive - in part because memristance is much more noticeable in nanoscale devices. The crucial issue for memristance is that the device' atoms need to change location when voltage is applied, and that happens much more easily at the nanoscale.
Proving memristor in the lab
Williams and co-authors Dmitri B. Strukov, Gregory S. Snider and Duncan R. Stewart were able to formulate a physics-based model of a memristor and build nanoscale devices in their lab that demonstrate all of the necessary operating characteristics to prove that the memristor was real.
"This is an amazing development," Chua says. "It took someone like Stan Williams with a multi-disciplinary background and deep insights to conceive of such a tiny memristor only a few atoms in thickness."
Williams has a background in physical chemistry. Strukov is a theoretical physicist, Snider is a computer architect and Stewart is an experimental physicist.
Possible replacement for D-RAM
By providing a mathematical model for the physics of a memristor, the team makes possible for engineers to develop integrated circuit designs that take advantage of its ability to retain information.
"This opens up a whole new door in thinking about how chips could be designed and operated," Williams says.
Engineers could, for example, develop a new kind of computer memory that would supplement and eventually replace today's commonly used dynamic random access memory (D-RAM). Computers using conventional D-RAM lack the ability to retain information once they are turned off. When power is restored to a D-RAM-based computer, a slow, energy-consuming "boot-up" process is necessary to retrieve data stored on a magnetic disk required to run the system.
Memristor-based computers wouldn't require that process, using less power and possibly increasing system resiliency and reliability. Chua believes the memristor could have applications for computing, cell phones, video games - anything that requires a lot of memory without a lot of battery-power drain.
As for the human brain-like characteristics, memristor technology could one day lead to computer systems that can remember and associate patterns in a way similar to how people do.
This could be used to substantially improve facial recognition technology or to provide more complex biometric recognition systems that could more effectively restrict access to personal information.
These same pattern-matching capabilities could enable appliances that learn from experience and computers that can make decisions.
Nanoscale electronics experience
In the memristor work, the researchers built on their extensive experience - Williams founded the precursor lab to IQSL in 1995 - in building and studying nanoscale electronics and architectures.
One goal of this work has been to move computing beyond the physical and fiscal limits of conventional silicon chips. For decades, increases in chip performance have come about largely by putting more and more transistors on a circuit. Higher densities, however, increase the problems of heat generation and defects and affect the basic physics of the devices.
"Instead of increasing the number of transistors on a circuit, we could create a hybrid circuit with fewer transistors but the addition of memristors - and more functionality," Williams says. Alternately, memristor technologies could enable more energy-efficient high-density circuits.
In 2007, the team developed an architecture for such a hybrid chip using conventional CMOS technology and nanoscale switching devices.
"What we now know," Williams says, "is that these switches have a name - memristor."