1 May 2008—Anyone familiar with electronics knows the trinity of fundamental components: the resistor, the capacitor, and the inductor. In 1971, a University of California, Berkeley, engineer predicted that there should be a fourth element: a memory resistor, or memristor. But no one knew how to build one. Now, 37 years later, electronics have finally gotten small enough to reveal the secrets of that fourth element. The memristor, Hewlett-Packard researchers revealed today in the journal Nature, had been hiding in plain sight all along—within the electrical characteristics of certain nanoscale devices. They think the new element could pave the way for applications both near- and far-term, from nonvolatile RAM to realistic neural networks.

By Sally Adee, IEEE’s Spectrum Issue May 2008

The memristor’s story starts nearly four decades ago with a flash of insight by IEEE Fellow and nonlinear-circuit-theory pioneer Leon Chua. Examining the relationships between charge and flux in resistors, capacitors, and inductors in a 1971 paper, Chua postulated the existence of a fourth element called the memory resistor.
Thirty years later, HP senior fellow Stanley Williams and his group were working on molecular electronics when they started to notice strange behavior in their devices. “They were doing really funky things, and we couldn’t figure out what [was going on],” Williams says. Then his HP collaborator Greg Snider rediscovered Chua’s work from 1971. “He said, ‘Hey guys, I don’t know what we’ve got, but this is what we want,’ ” Williams remembers. Williams spent several years reading and rereading Chua’s papers. “It was several years of scratching my head and thinking about it.” Then Williams realized their molecular devices were really memristors. “It just hit me between the eyes.”
The reason that the memristor is radically different from the other fundamental circuit elements is that, unlike them, it carries a memory of its past. When you turn off the voltage to the circuit, the memristor still remembers how much was applied before and for how long. That’s an effect that can’t be duplicated by any circuit combination of resistors, capacitors, and inductors, which is why the memristor qualifies as a fundamental circuit element.
Williams is in talks with several neuroscience/engineering labs that are pursuing the goal of building devices that emulate neural systems. Chua says that synapses, the connections between neurons, have some memristive behavior. Therefore, a memristor would be the ideal electronic device to emulate a synapse.
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