HP breakthrough to hasten flash memory's demise?
Memristor secrets revealed
HP scientists have made a breakthrough in the development of memristors, a fundamental circuit type that looks increasingly likely to replace NAND flash and possibly DRAM.
Essentially, they've figured out the physical and chemical mechanisms that make memristors work.
"We were on a path where we would have had something that works reasonably well, but this improves our confidence and should allow us to improve the devices such that they are significantly better," the leader of the HP research team, R. Stanley Williams, told IDG News.
Memristors are the fourth fundamental type of passive circuitry, along with the resistor, capacitor and inductor. Like flash, memristors are nonvolatile – they "remember" their state when power isn't applied to them.
The core advantage of memristors is that they can theoretically achieve speeds 10 times that of flash at one-tenth the power budget per cell. They can also be stacked, enabling exceptionally dense memory structures.
Memristor cells can also be built using exceedingly small fabrication processes. Using a grid of nanowires whose crossing points form the memristors, Williams told The New York Times that HP Labs has working devices with three-nanometer memristors that switch on and off in about a nanosecond and could store 20GB in a square centimeter.
HP has known for some time how memristors can store bits. Simply put, a charge applied to a memristor moves what Williams calls a "oxygen vacancy" from one layer of a titanium dioxide semiconductor to the other, changing its conductivity by a factor of a million. Another charge moves it back. That conductivity change stores the bit of information in the memristor.
Though the HP scientists knew that their memristors worked, they were still in the dark as to the actual physical and chemical details behind their function – but now they've figured it out. That breakthrough in understanding was revealed on Monday in the journal Nanotechnology (free registration required).
As the paper's summary explains in refined boffin-speak:
Here we correlate device electrical characteristics with local atomic structure, chemistry and temperature. We resolved a single conducting channel that is made up of a reduced phase of the as-deposited titanium oxide. Moreover, we observed sufficient Joule heating to induce a crystallization of the oxide surrounding the channel, with a peculiar pattern that finite element simulations correlated with the existence of a hot spot close to the bottom electrode, thus identifying the switching location.
According to the paper's lead author, HP's John Paul Strachan, this is a Very Big Deal in the effort to discover "the microscopic picture for how [memristors] undergo such tremendous and reversible change in resistance."
As Strachan told the Institute of Physics, publisher of Nanotechnology: "We now have a direct picture for the thermal profile that is highly localized around this [conductive] channel during electrical operation, and is likely to play a large role in accelerating the physics driving the memristive behavior."
It's important to note that HP's work on memristors is not merely pie-in-the-sky boffinry. Late last year, the company joined forces with the Korean memory manufacturer Hynix to fabricate ReRAM (resistive random access memory) modules, with a planned ship date of 2013. ®
Although the first practical application of memristors is likely to be as a replacement for flash memory, they can also mimic the synapses that coordinate communications among neurons in the brain. One researcher at the University of Michigan, in fact, is working to exploit that synapse-like behavior by using memristors to model a cat's brain.
"The idea is to use a completely different paradigm compared to conventional computers," the researcher, Wei Lu, told Science Daily last year. "The cat brain sets a realistic goal because it is much simpler than a human brain but still extremely difficult to replicate in complexity and efficiency."
Now that HP Labs has uncovered the physical underpinnings of the memristor's function, Lu may be one step closer towards creating our future cyber-kitty overlords.