## Will memristors prove irresistible?

Tuesday, September 2nd, 2008

Will memristors prove irresistible? I’ve mentioned memristors before, but perhaps a refresher’s in order:

Technically, a memristor is a passive circuit element that relates flux to charge in the same way resistors relate voltage to current, capacitors relate voltage to charge and inductors relate flux to current. The fact that this fourth combination has been ignored in electronic-circuit theory was discovered by EE professor Leon Chua at the University of California, Berkeley, who wrote a seminal paper about the memristor in 1971.

“Memristors represent a fundamental change in electronic-circuit theory,” said Sung-Mo Kang, chancellor of the Engineering School and an EE professor at the University of California at Merced. The most important items in electronics are the voltage, the current, the electrical charge and the flux linkage, he said. “If you consider those four variables as constitutive relations, then you get the equations that describe the resistor, inductor and capacitor.”

But there is a fourth combination that everybody had overlooked, said Kang. “Chua’s genius was realizing that combination defined a new passive-device type — the memristor,” he said. “Chua’s argument was mathematical, but what he was saying is that the memristor had just as much a fundamental right as resistors, inductors and capacitors.”

Chua called his discovery a memristor because of its behavior: The device acts as a variable resistance that “remembers” how much current has flowed through it by changing the voltage across its terminals. Thus, it can serve as a memory element that can be flipped “on,” with a current in one direction, and “off,” with a current in the reverse direction.

“A resistor relates voltage to current and the memristor relates flux to charge,” said Notre Dame’s Porod. “However, if you sum up flux over time, it becomes a voltage, and if you sum up charge over time it becomes a current. So a device that relates flux to charge, like the memristor, will over time relate voltage to current like a variable resistor that changes its value depending on how much, and in which direction, current has flowed through it.”

The memristor may follow the pattern set by the transistor decades ago:

“The memristor’s history is similar to that of the transistor, which was invented 35 years before its first major application,” said Wolfgang Porod, an EE professor at Notre Dame University. Created in the 1920s by physicist Julius Edgar Lilienfeld, the device was not developed to its full potential until it came to the attention of Bell Labs researchers William Bradford Shockley, John Bardeen and Walter Houser Brattain, who were awarded the 1956 Nobel Prize for their pioneering work.

The first application, Porod said, was in-ear hearing aids, where “its small size justified its higher cost in those days compared with vacuum tubes.” Transistor radios soon followed.

In just the same way, HP sees RRAMs [resistive random-access memories] as only the beginning for the memristor. HP Labs foresees its use in neural networks that could learn to adapt by allowing current to flow in either direction, as needed.