One of the most hyped devices of the past few years has been the memristor, and in comparison to other circuit elements I think you’ll agree it’s pretty weird, and interesting!

The existence of the memristor was first predicted in 1971 by Leon Chua. The behavior of basic circuit elements had long been described mathematically, with each circuit element having a given relationship between two of four quantities: charge (q), current (I), voltage (V), and flux (φ). Chua noticed that the mathematical equations could be tabulated and had a symmetry of sorts, but that one equation seemed to be missing, for a device that related current to a change in magnetic flux. The behavior of such a device would change depending on how much current had passed through it, and for this reason Chua called it a ‘memristor’, a contraction of ‘memory’ and ‘resistor’. You can see the mathematical relationships represented below as the edges of a tetrahedron. Resistor behavior is quantified by the resistance R and capacitor behavior is quantified by the capacitance C, in the two equations near the top. And on either side, we have relations for the inductance L and the memristance M. It’s not crucial to understand these equations intimately, just to see that they have a certain symmetry and completeness to them as a set of relations between these four key variables. Five of these relationships had been experimentally observed in devices, and mathematically suggested the sixth, the memristance equation on the right. But having the equation doesn’t tell you how to make a device that will exhibit that behavior!

Chua’s initial proposals to physically create a memristor used external power to store the remembered information, making the component active rather than passive. However, in 2008 a real-world memristor was created using a nanoscale film with embedded free charge that could be moved by applying an electric field that exerts a force on the charge. How much of the film contains the extra charge determines how much of the device has high resistance and low resistance, which causes the total resistance to depend on how much current has passed through.

This isn’t the only implementation of a memristor available, because as many researchers realized once the first results were announced, memory of past measurements is a common nanoscale feature. Current flow can often cause small changes in materials, but while these changes may not be noticeable in the properties of a bulk material, when the material has very small thickness or feature size, the changes can affect material properties in a measurable way. Since this constitutes a form of memory that lasts for a significant amount of time, and there is a large market for non-volatile electronic memory for computers, the commercial interest in these devices has been considerable. HP expects to have their version memristor-based computer memory on the market by 2014, and it remains to be seen what other novel electronics may come from the memristor.

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