Resistance to memory

1 May 2008

It's taken more than 35 years to find, but it looks as though HP Labs has found a cousin to the resistor and the capacitor hiding in the delicate thin films of metal oxides.

Naturally, HP Labs talks up the prospects of the memristor. One application the researchers have put forward is as a potential successor to devices such as the venerable DRAM. On the face of it, this is arguably the worst place to go. The industry is littered with 'nearly there' memories that have better properties than those apparently on offer with the memresistive approach. The HP labs team namechecks a bunch of materials in the Nature paper that show memresistive-like effects. However, at least three major categories are already in production or have multiple teams working on them, with varying degrees of success.

Chalcogenides are the materials that go into phase-change memories of the kind being pushed by Numonyx - the JV formed by Intel and STMicroelectronics. To give you an idea of how long it can take to get a memory technology off the ground, phase-change memories have been around about as long as Leon Chua's theory of the memristor. And you still can't buy one in the shops. On top of that, the phase-change memory is meant to be non-volatile: it doesn't forget stuff when you take out the battery.

The memristor will not be a non-volatile memory but only semi-non-volatile, according to the researchers. It seems that, like a capacitor, these things 'leak' a little. Leave it too long, and it will have forgotten what you told it.

This is a problem that afflicts the latest new memory technology: metal oxide, which is also being touted as a possible candidate for the memristor treatment. Metal-oxide memories are programmed by heating. Unfortunately, right now, just storing them at room temperature provides enough energy after a few days, or even hours, for them to reset themselves.

Then you have the perovskites, such as barium titanate. These are already in use in ferroelectric memories. You can go out and buy these but it's another memory technology that never quite made it to the mainstream.

However, it seems that something like memristor behaviour has been seen in organic materials. This may be the way forward as it points to the possibility of being able to print memory devices using organic chemicals. These kinds of material make pretty rubbishy transistors, but they might perform better as memristors.

The part that might lead to radical changes in computer design is the observation that memristors work in a similar way to the Hodgkin-Huxley model of the neuron.

One big problem with nanoscale electronics is variability: these things are so small that there's way too much of it. This makes it tough to build reliable binary switches: the primary use of a conventional transistor. But, what if you don't want to make a switch? This is the kind of work being performed by researchers such as Professor Steve Furber's group at the University of Manchester with the EPSRC-funded Spinnaker project. The idea behind the project is that you dump binary logic in favour of a system that lies on statistics. In that kind of environment, manufacturing variability is not necessarily your friend, but it's way less of an enemy.

The inspiration for the work is the brain and the way that neurons communicate with each other. In Prof Furber's model, you use a bunch of them together to effectively vote on a calculation. The overall elements wind up bigger but you use the elements to process more information than just binary bits. Right now, the team is using arrays of ARM processors to model neurons. However, if the work pays off, it might point to a simplified system that could be implemented using either nanonscale transistors or elements such as memristors, which have the advantage of working more like a neuron out of the box, as it were.