A couple of days ago, I noted that Bryon Moyer at Electronic Engineering Journal had interviewed me and quoted me about the recent brush up over the term “memristor” in his article “A Memristor By Any Other Name?” Moyer wrote:
“Of note, however, is that in many of these, the drifting of oxygen vacancies plays an important role. Sound familiar? It did to me, but, maddeningly, in almost none of the RRAM papers was there any mention of the word “memristor.” Was TiO2 the only “true” (a loaded word, as we’ll see) memristor? Was there some detail that made the other materials not memristive? Or were they also memristive but for some reason no one was mentioning that?
So I looked around for expert opinion to try to referee the situation – which proved harder than I thought (since much of industry focuses on technologies that are manufacturable today – or at least soon, which this is not). I had one conversation with Cadence’s Steve Leibson, who’s been watching the technology.
His view is somewhat practical, by his own admission: he doesn’t really care what you call anything; the question is, is it manufacturable, cost- and power-effective, etc.? If you take RRAM to mean anything that uses a shift in resistance to determine state, then even things like MRAM and phase-change memory (PCRAM) are included.
More interestingly, he alluded to some controversy as to whether what HP found was even a memristor at all – making sure to clarify that this wasn’t his position, just that he was aware that there was some debate; we left it there.”
Now R. Stanley Williams of HP Labs has written a position paper on this tempest in a memristor pot. He writes:
“As a result of his work on nonlinear circuit elements, Chua made an interesting observation. For traditional linear circuits, there are only three independent two-terminal passive circuit elements: the resistor R, the capacitor C and the inductor L. However, when he generalized the mathematical relations to be nonlinear, there was another independent differential relationship that in principle coupled the charge q that flowed through a circuit and the flux φ in the circuit, dφ = M dq, that was mathematically different from the nonlinear resistance that coupled the voltage v to the current i, dv = R di. As a strictly mathematical exercise, he explored the properties of this potentially new nonlinear circuit element, and found that it was essentially a resistor with memory – it was a device that changed its resistance depending on the amount of charge that flowed through the device, and thus he called this hypothetical circuit element M a memristor. This conclusion was independent of any physical mechanism that might couple the flux and charge, and in fact he did not postulate any mechanism at all.”
“This issue was made much clearer in a second paper published with his then student Sung Mo Kang [L. O. Chua & S. M. Kang, Memristive devices and systems, Proc. IEEE 64, 209-223 (1976)]… This 1976 paper showed many other properties of the generalized memristor and also discussed possible examples, but again this was a mathematical exercise that was independent of any physical mechanism at the time. The key result was that any electronic circuit element that displayed a pinched hysteresis loop in its current-voltage characteristic could be described mathematically by the two memristive system equations.”
“Examples of memristors include bipolar and unipolar resistive switches, often called RRAM or ReRAM; ‘atomic switches’; spin-torque transfer RAM devices, phase-change memory devices, and several other systems based on a wide variety of materials and mechanisms [L. Chua, Resistance switching memories are memristors, Appl. Phys. A 102, 765-783 (2011)]. For the most part, we have chosen to use the term memristor to describe the devices in our papers, not because we are trying to impose an ‘HP brand’ (especially since the term was invented by Leon Chua), but because we feel the general term connotes a broader range of applications. ‘RAM’ means random access memory, and that is certainly one application for memristors, but we find that much too restrictive, since they can also be used in a wide variety of other electronic circuits, including logic, FPGAs, and various types of ‘synaptic’ or ‘neural’ applications – memristors are much more than memory.”
“In summary, the memristor was a discovery – it is a rigorous mathematical model that can be used to predict the behavior of a wide variety of physical devices. There have been many developments of different types of memristors, now called by many different names, based on different materials and physical mechanisms, but they are all described by the same general mathematical formalism developed by Chua.”