electronics

As most people know, there are 3 passive electronic devices: the resistor, the capacitor and the inductor. However, already in 1971, Leon O. Chua hypothesized, purely based on symmetry arguments, that there must be a fourth basic two-terminal component, and he called this device the memristor: it acts as a resistor which has memory. From a purely linear standpoint this device would not exist and would just act as a resistor, but it is thanks to Chua's focus on non-linear circuit analysis that the existence of this special device was predicted.

For many years this idea was thought of as a hoax and was well headed into the scientific dustbin of ignorance. It is only very recently that the people of HP Labs were able to manufacture a nano-scale memristor and published the results in Nature. These memristive properties turn out to be very abundant at the nano-scale level, and in retrospect have been reported in hundreds of paper over the last decades, but they were never understood and linked to the concept of a memristor. It was thanks to the years long effort of the HP research lab that they finally could link the anomalous behavior which was perceived at the nano-scale level with the ghostly memristor device.

The device HP actually build is a quite elegant and simple device: take two conductors (nano-wires in this case) and put an TiO2 isolator between them, but make the isolator so that at one end it is depleted of O2 ions. When a significantly high voltage is put over this device, the 02 ion holes gradually migrate towards the other side of the component, and because these ion holes are conducting, in this process the conductivity of the component increases. The real key part is that these ion holes are immobile when the driving voltage is removed, and the component thus remembers its resistance (apparently for a very long time). An inverse voltage again decreases the conductivity as this forces the ion holes to move back to one edge, and turn the device back into an isolator.

But why all this fuss over a new mystery device that only lives at the nano-scale? It is at the nano-scale and it has memory!! These devices are the perfect non-volatile memory components of the future. They could also be used to drastically improve the density of FPGAs. Another very important aspect is that they allow to redefine the way Boolean logic is implemented in computers. And what I find most interesting, there is a direct link between a memristor and a synapse in a neuron! There already have been circuits build which implement this. In a recent paper is was BTW also shown that memristive properties lead to learning in a very natural way and can explain how, for example, simple single cellular creatures can learn from their environment.

On November 21, 2008 a first full-blown symposium on memristive devices was organized at the University of California, Berkeley. The (quite long) four part videos of this symposium are available if you really want to get some in-depth information on these devices which have a great future lying ahead in the nano-scale electronics era!