What Are Memristors?

What is a memristor? Memristors are basically a fourth class of electrical circuit, joining the resistor, the capacitor, and the inductor, that exhibit their unique properties primarily at the nanoscale. Theoretically, Memristors, a concatenation of “memory resistors”, are a type of passive circuit elements that maintain a relationship between the time integrals of current and voltage across a two terminal element. Thus, a memristors resistance varies according to a devices memristance function, allowing, via tiny read charges, access to a “history” of applied voltage. The material implementation of memristive effects can be determined in part by the presence of hysteresis (an accelerating rate of change as an object moves from one state to another) which, like many other non-linear “anomalies” in contemporary circuit theory, turns out to be less an anomaly than a fundamental property of passive circuitry.

Until recently, when HP Labs under Stanley Williams developed the first stable prototype, memristance as a property of a known material was nearly nonexistant. The memristance effect at non-nanoscale distances is dwarfed by other electronic and field effects, until scales and materials that are nanometers in size are utilized. At the nanoscale, such properties have even been observed in action prior to the HP Lab prototypes.

But beyond the physics of electrical engineering, they are a reconceptualizing of passive electronic circuit theory first proposed in 1971 by the nonlinear circuit theorist Leon Chua. What Leon Chua, a UC Berkeley Professor, contended in his 1971 paper Transactions on Circuit Theory, is that the fundamental relationship in passive circuitry was not between voltage and charge as assumed, but between changes-in-voltage, or flux, and charge. Chua has stated: “The situation is analogous to what is called “Aristotle’s Law of Motion, which was wrong, because he said that force must be proportional to velocity. That misled people for 2000 years until Newton came along and pointed out that Aristotle was using the wrong variables. Newton said that force is proportional to acceleration–the change in velocity. This is exactly the situation with electronic circuit theory today. All electronic textbooks have been teaching using the wrong variables–voltage and charge–explaining away inaccuracies as anomalies. What they should have been teaching is the relationship between changes in voltage, or flux, and charge.”

As memristors develop, its going to come down to, in part, who can come up with the best material implementation. Currently IBM, Hewlett Packard, HRL, Samsung and many other research labs seem to be hovering around the titanium dioxide memristor, but there are quite a few other types of memristors with vectors of inquiry. Additionally, work is ongoing on a variety of related non-volatile memory applications including neuromorphic computing, memtransistors (what are memtransistors?), and neural network applications.