Published online 30 April 2008 | Nature | doi:10.1038/news.2008.789

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Described in 1971, made in 2008: 'memristors' promise a computer revolution.

Old school: classical resistors are now joined by nanoscopic counterparts. PUNCHSTOCK

High-school physics students grappling with the delights of capacitors, inductors and resistors will be groaning into their exercise books. Electronics experts in California have finally succeeded in proving the existence of a fourth fundamental unit of electronic circuits: the 'memristor'.

The existence of the memristor, short for 'memory resistor', was first suggested in 1971, but only now have researchers succeeded in creating a real, working example. They hope that the new components could revolutionize computing, promising an end to frustrating waits for your computer to boot up.

"A memristor is essentially a resistor with memory," explains Stan Williams of HP Labs in Palo Alto, California, who reports the memristor's creation in this week's Nature1. "The actual resistance of the memristor changes depending on the amount of voltage and the time for which that voltage has been applied to the device."

That means that a computer created from memristive circuits can 'remember' what has happened to it previously, and freeze that memory when the circuit is turned off. This quality could allow computers to turn off and on again in an instant, as all the components could revert to their last state instantly, rather than having to 'boot up'.

Size problems

Williams and his colleagues created a memristor while experimenting with very tiny circuits. They sandwiched a nanoscopic film of a semiconductor (titanium dioxide) between two slivers of metal (platinum). Those are standard materials; the trick is to make the component just 5 nanometres wide — about 10,000 times thinner than a human hair.

It's only at the nanoscale that the behaviour of memristors begins to be detectable, Williams says. Any larger and they behave just like ordinary resistors, where resistance is equal to the voltage divided by the current. Electronics were originally developed at a scale far too large to see these effects and only recently have researchers been able to work at that scale.

That's probably one reason why the idea has mouldered on the shelf for 37 years, suggests Leon Chua, the electrical engineer at the University of California, Berkeley, who first postulated the existence of memristors in a 1971 paper2.

Six years after reading Chua's 1971 work, Williams and his team managed to make the tiny device. The scale of the project was not the only challenge. The mathematics underlying the principle were not simple, says Williams. "The original prediction and the papers in which the prediction appeared were very heavy mathematically, so it required a very significant investment in order to read those papers," he says.

Chua, somewhat modestly, disagrees, and thinks the idea may have struggled to find its feet simply because it is so weird. "It's not really that difficult — it is more that it is sort of heretical. Nobody would believe this was the case because it sounds unnatural in some sense."

Volatile discovery

Chua says that he is pleased that his theory has finally been proved. "I was very excited — I never thought I would live to see this happen."

Now that his calculations have been vindicated, he thinks that memristors will be a big deal. They should be crucial in developing 'non-volatile' memory — the type that doesn't decay when the power is switched off.

Most computers use 'volatile memory' to perform their running functions, because this offers faster access to data than the non-volatile memory used to store data on hard disks and flash devices such as iPods. Building computers with memristors might allow a full switch to non-volatile memory, doing away with power-sapping 'running memory' and allowing devices to consume far less power when operating. "Someday I imagine that you won't have to charge your cellphone or your laptop so often," says Chua.

But what of the poor high-school students who now have more to learn in their electronics classes? "I believe this is going to be in textbooks in the near future," Chua says. He says that the rounding out of his theory and creation of an actual memristor should make the concept easier to grasp than it was when he first proposed it. Struggling students might be more, shall we say, resistant to progress.