Sunday, February 08, 2009


Anyone who has played around with electric circuits knows resistors, capacitors, and inductors.  Few people knows about memristors though.  The memristor idea was thought up by Professor Leon Chua in 1971 and physical devices were just intentionally created in the lab in 2008.

An adventure story of how physical memristors were created is given by R. Stanley Williams, one of the inventors, here.  Below are the excerpts about what a memristor is and how a 31-year-old, mostly-forgotten theory provided a breakthrough in understanding the experiments:

Memristor is a contraction of “memory resistor,” because that is exactly its function: to remember its history. A memristor is a two-terminal device whose resistance depends on the magnitude and polarity of the voltage applied to it and the length of time that voltage has been applied. When you turn off the voltage, the memristor remembers its most recent resistance until the next time you turn it on, whether that happens a day later or a year later.


We were frustrated and burned out. Here we were, in late 2002, six years into our research. We had something that worked, but we couldn’t figure out why, we couldn’t model it, and we sure couldn’t engineer it. That’s when Greg Snider, who had worked with Kuekes on the Teramac, brought me the Chua memristor paper from the September 1971 IEEE Transactions on Circuits Theory. “I don’t know what you guys are building,” he told me, “but this is what I want.”

To this day, I have no idea how Greg happened to come across that paper. Few people had read it, fewer had understood it, and fewer still had cited it. At that point, the paper was 31 years old and apparently headed for the proverbial dustbin of history. I wish I could say I took one look and yelled, “Eureka!” But in fact, the paper sat on my desk for months before I even tried to read it. When I did study it, I found the concepts and the equations unfamiliar and hard to follow. But I kept at it because something had caught my eye, as it had Greg’s: Chua had included a graph that looked suspiciously similar to the experimental data we were collecting.

But what memristors can be used for?  The author suggests super-dense, super fast digital memory that does not requires electrical current to keep its state (for computers that turns instantly on and off), and an analog simulation of the brain (by encoding synaptic strengths using memristors.)   

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