Memristor: The thinking resistor

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They switch, save and use very little electricity. But because they also behave like synapses, all the hype surrounding neural networks has put the mysterious memristor back into the public eye.

The principle of the exotic “resistor with a memory” is old. In the 1960s, Berkley Professor Leon O. Chua worked on a theory for passive components. However, resistors, capacitors and coils weren’t enough for him. He still needed a component whose resistance was the result of changes in electric charge and magnetic flux.

In 1971, he ultimately published a scientific paper in which he predicted the fourth passive component, i.e. the memristor. It was a nice theory, but one that quickly disappeared in his desk drawer because no one could manufacture the “thing.” And if an employee of Hewlett Packard (HP) didn’t happen across the “drawer,” perhaps no one would have ever heard of the mysterious component again.

Mysterious memristor

To start, a memristor is merely a resistor, albeit one that depends on current flow and current direction. The resistance value can be adjusted continuously by changing the amount of current applied. In a de-energized state, the last electrical state (resistance value) is maintained, or remembered. Thus the term “mem(ory) r(es)istor.” Admittedly, even then Professor Chua recognized that the component he was looking for was very similar to the way synapses in our brains work.

Memristor (Image: HP)
Array of 17 oxygen depleted titanium dioxide memristors.(Image: HP).

At HP, first they wanted to use the exotic resistors to replace flash and DRAM memories. High packing densities and low current consumption promised a gold mine. HP made every effort to quickly develop initial prototypes for memristors, and finally succeeded in 2007.

Because memristors can also be used to replace transistors in areas where no amplification is necessary, researchers were also working on an entirely new computer architecture at the same time. Combining the memory and CPU on a single chip was supposed to increase processing speed enormously. However, the world’s first memristor, which was embedded in a microcomputer, was developed by Panasonic in 2013.

At HP, collaborating with flash memory manufacturer SanDisk Corp. was supposed to give the memristor’s stubborn history another boost. But as is so often the case: SanDisk was taken over by Western Digital, and Hewlett Packard is still busy restructuring. So it may be a while.

First commercial chip from a start-up

A breath of fresh memristor air came last year from a start-up, something that also happens frequently. Knowm’s online shop was selling a chip with eight memristors for 220 US dollars, allowing researchers to implement algorithms for machine learning.

“Nature Transistors,” as Knowm calls it, uses the “AhaH” (anti-Hebbian and Hebbian) concept that follows well-known Hebbian learning rules for linking neurons that have common synapses. Among other things, they say that neurons that are frequently active with one another at the same time will also tend to interact with another (“what fires together, wires together”).

Knowm’s memristor technology differs from the HP approach, but according to the EE Times, is has supposedly met with the approval of Leon Chua, the “father of the memristor.”

Now he has shown up here in Germany, where he sits on the scientific advisory board of the Chua Memristor Center (CMC) in Dresden that bears his name. The new network wants to bring researchers from various disciplines in Germany’s scientific and industrial sectors together to further explore the scientific fundamentals of the memristor and to finally make industrial applications possible.

So after 45 years, the story has sort of gone full circle. Or as Victor Hugo said: “Nothing is more powerful than an idea whose time has come.”


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