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HP pulls memory Missing Link from bottle of beer

Better than flash

More than 35 years ago, when the world assumed that circuits were crafted from three basic building blocks, a man named Leon Chua predicted the existence of a fourth.

The capacitor, the resistor, and the inductor, he said, would be joined by something called the memristor. Today, scientists at HP Labs announced that this prediction was right on the money.

After a good five years of work, HP Labs Fellow R. Stanley Williams and his team have actually built a memristor - a resistor that stores information even after losing power. With this new electrical building block, chip manufacturers could reinvent modern memory technology, delivering machines that are far more efficient and that boot instantly.

Naturally, HP is trumpeting this as a cure-all for all those data centers up in the cloud.

Leon Chua's memristor wasn't much more than a math project. Looking for a unified mathematical theory for electrical circuits, the University of California prof noticed a gap in the equations that traditionally describe such circuits.

"He saw that there were patterns in those circuit equations, and in looking at those patterns, he noticed a hole," Williams tells us. "But he could drop a new equation into this hole that completed the symmetry - the aesthetic view - of his theory. And this equation told him that there must be a fourth fundamental device we don't yet have."

Chua could predict the behavior of this fourth building block - he knew it could remember charges without power - but he couldn't actually build one. That would require the advent of nanotechnology - and some extra work from Williams and company.

HP's nano-scale memristor is fashioned from two layers of the semiconductor titanium dioxide - one that includes tiny "oxygen vacancies" and one that doesn't. The top layer - with the vacancies - is conductive. The bottom layer - without - is not.

If you send a voltage across the device, you can push the vacancies from one layer to the other. That's your switch.

"You can switch the bottom layer of titanium oxide from being highly insulating to being highly conductive," Williams says. "You can make the resistance of the device decrease by a factor of a million." Then, with different voltage, you can push the vacancies back into the first layer, flipping your switch the other way.

Williams compares this to a bottle of beer turned upside-down. "A vacancy moving through titanium dioxide is sort of like a bubble moving through beer," he explains. "If you turn a bottle of beer upside-down, gravity pulls the beer down and the bubbles float up. It's the same with our vacancies - except we're using a voltage rather than gravity."

But the key here is that HP's memristor can remember its state even without power. "It knows how much voltage you put on the device, in which direction you held it, and how long you held it there."

In other words, it's nonvolatile memory - with a few advantages over flash. "It holds its memory longer," Williams says. "It's simpler. It's easier to make - which means it's cheaper - and it can be switched a lot faster, with less energy."

But even Williams admits that flash maintains one large advantage over his brand new building block. "Flash is a real product. You can go out and buy it now." The memristor may drive the cloud. But not for awhile. ®

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