The Memristor is Almost Ready For Prime Time.

With the amount of NVRAM that’s used these days the amount of innovation in the sector has been comparatively little. For the most part the advances have come from the traditional avenues, die shrinks and new gate technologies, with the biggest advance in 3D construction only happening last week. There’s been musings about other kinds of technology for a long time like memristors which had their first patent granted back in 2007 and were supposed to making their way into our hands late last year, but that never eventuated. However news comes today of a new memory startup that’s promising a lot of things and whilst they don’t say it directly it looks like they might be one of the first to market with memristor based products.

Crossbar-Simple-CMOS-Integration-080213

Crossbar is a new company that’s been working in stealth for some time on a new type of memory product which, surprisingly, isn’t anything particularly revolutionary. It’s called Resistive RAM (RRAM) and a little research shows that there’s been companies working on this idea as far back as 2009. It’s based around a fairly interesting phenomena whereby a dielectric, an electric insulator, can be made to conduct through the application of high voltage. This forms a filament of low resistance which can then be reset, breaking the connection, and then set again using another high voltage jolt. This idea lends itself well to applications in memory as the two states translate perfectly to binary and if the specifications are anything to go by the performance that will come out of them should be quite spectacular.

If this is sounding familiar then you’re probably already familiar with the idea of memristors. These are the 4th fundamental component of electronic circuits that were postulated back in 1971 by Leon Chua and were made real by HP in 2007. In a basic sense their resistance is a function of the current following through them and when the current is removed that resistance is remembered, hence their name. As you can see this describes the function of RRAM pretty well and there is a solid argument to be made that all RRAM technologies are in fact memristors. Thus whilst it’s pretty spectacular that a start up has managed to perfect this technology to the point of producing it on a production fab it’s actually technology that’s been brewing for quite some time and one that everyone in the tech world is excited about.

Crossbar’s secret sauce could likely come from their fabrication process as they claim that the way they create their substrate means that they should be able to stack them, much in the same way that Samsung can now do with their VNAND. Now this is exciting because previously HP alluded to the fact that memristor based storage could be made much more dense than NAND, several orders of magnitude more dense to be precise, and considering the density gains Samsung got with their 3D chips a layered memristor device’s storage capacity could be astronomical. Indeed Crossbar claims this much with up to 1TB for a standard chip that could be stacked multiple times, enabling terabytes on a single chip. That puts good old fashioned spinning rust disks on notice as they just couldn’t compete, even when it comes to archival storage. Of course the end price will be a big factor in this but that kind of storage potential could drive the cost per GB through the floor.

So the next couple months are going to be quite interesting as we have Samsung, the undisputed king of NAND, already in the throws of producing some of the most dense storage available with Crossbar (and multiple other companies) readying memristor technology for the masses. In the short term I give the advantage to Samsung as they’ve got the capital and global reach to get their products out to anyone that wants them. However if memristor based products can do even half of what they’re claimed to be capable of they could quickly start eating Samsung’s lunch and I can’t imagine it’d be too long before they either bought the biggest players in the field or developed the technology themselves. Regardless of how this all plays out the storage market is heading for a shake up, one that can’t come quick enough in my opinion.

 

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  1. Resistive RAM (RRAM or ReRAM) is not memristor!

    What is called “resistance switching” is a sort of phenomena. Under certain conditions, “resistance switching” behavior can be brought about in various metal/insulator/metal structures after a soft-breakdown of the insulating material has occurred. Such effects could offer the potential for nonvolatile memory applications (ReRAM or RRAM). “Resistance switching” phenomena are well known since decades and are in no way related to the concept of memristor/memristive systems. Nevertheless, there is ongoing research because there are still a lot of questions with respect to the underlying physical mechanisms. Understanding into the probabilistic nature of the “resistance switching” is, for example, crucial to get grip on reliability issues of ReRAM devices.

    What is called “memristor” is a hypothetical concept. “Memristors” are conceptually defined by a unique set of characteristic mathematical state equations – based on the mathematical framework proposed by L. O. Chua. Thus, solid state memory devices should only be labeled “memristors” if one is able to propose a reasonable physical model that satisfies these state equations. However, any scientific evidence that “memristors” might exist in physical reality is missing so far. Moreover, there are now severe arguments that the “nonvolatile memristor” concept in itself cannot be brought in line with fundamental laws of information processing (see: “Fundamental Issues and Problems in the Realization of Memristors” by P. Meuffels and R. Soni ( http://arxiv.org/abs/1207.7319 ) and “On the physical properties of memristive, memcapacitive, and meminductive systems” by M. Di Ventra and Y. V. Pershin, Nanotechnology, vol. 24, 2013 ( http://iopscience.iop.org/0957-4484/24/25/255201 ).

  2. The belief that nonvolatile “memristor/memristive” systems might exist in physical reality is merely based on HP’s “memristor” model which was presented in 2008 in the NATURE paper “The missing memristor found” (Nature 453, (2008) 80-83). In the meantime, however, it has become obvious that HP’s “memristor” claims are based on severe electrochemical misconceptions (see “Fundamental Issues and Problems in the Realization of Memristors” by P. Meuffels and R. Soni): One cannot derive the characteristic dynamical state equations of a “memristor” on base of HP’s dopant drift model, i.e., no memory devices can operate in accordance with the model because the model is by itself in conflict with fundamentals of electrochemistry. Thus, up to now nobody has invented or found a memory device which operates like a genuine nonvolatile ”memristor”.

    I suppose that most of the former “memristor” enthusiasts are now aware that they have been taken in by an erroneous publication. The stochastic nature of the complex resistance switching effects which are observed on various metal/insulator/metal structures sometimes involves current-voltage characteristics which might remind of the pinched hysteresis loops that are thought to be the fingerprints of “memristors”. Such observations, however, are not sufficient to claim that a “memristor” has been found, i.e., curve fitting on base of polynomials with arbitrary degrees is no substitute for a thorough physical understanding. One should be careful with “memristor” claims in case one is not able to present a valid physical model that satisfies the “memristor” state equations according to Chua’s mathematical framework.

    I have no idea why Chua came to the conclusion that all resistance switching devices should be considered “memristors”. Maybe, the following quotation from the science2.0 blog entry “The Memristor: Another Science Scandal” contains some clues:

    “One wonders why, and particular suspicions are strengthened by the flood of papers that was generated under the catchy “memristor” label (as well as the number of patent applications that rest with certain, extremely competitive companies that can draw on large financial resources – the memristor is not called “HP Memristor” for no reason!)”. (http://www.science20.com/alpha_meme/memristor_another_science_scandal-92795 )

  3. Chua obviously has a self-serving incentive to claim all resistance switching devices are memristors. It seems to me incredible hypocrisy for Chua to claim this since he was arguing for 37 years that the memristor was a missing circuit element. Chua was certainly aware of the research being done in MRAM, ReRAM, and phase change memory but never claimed these memory types to be memristors until after HP’s initial claim.

    Perhaps Chua may have more concern for his reputation than accurate science or consistency with his own definition?

  4. I’ve had a read through the blog posts and journals linked here and whilst I’ll abstain from speculating about Chua himself it does appear that there’s a case to be made as to why these device should not be classified as memristors. Satisfying one part of the equation (resistance changing in the presence of current) does leave other parts unsatisfied and some argue that the mechanism these devices use to achieve that also negates them being classified as memristors.

    Chua argues otherwise in that article I linked but I lack the expertise to make a firm case for either side at the moment.

    Thanks for bringing those articles to my attention, it’s always good to see both sides of scientific debate.

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