The memristor seems to generate fairly polarized debate, especially here on this website in the comments on stories covering the technology. The controversy seems to fall along the lines that the device that HP Labs’ Stan Williams and Greg Snider developed back in 2008 doesn’t exactly line up with the original theory of the memristor proposed by Leon Chua back in 1971.
While this debate will not likely abate, research is continuing in developing two-terminal non-volatile memory devices based on resistance switching.
Along these lines, researchers at Northwestern University have pushed the envelope of the two-terminal device—which can only control one voltage channel—by creating a third terminal. The researchers believe that this will expand the capabilities of memristors into more complex electronics, paving the way for computers to more closely mimic the neurons of the human brain.
“Computers are very impressive in many ways, but they're not equal to the mind," said Mark Hersam, of Northwestern University's McCormick School of Engineering, in a press release. "Neurons can achieve very complicated computation with very low power consumption compared to a digital computer.”
To achieve this biomimetic capability, sometimes called neuromorphic computing, Hersam believes that the use of memristors as a memory element in an integrated circuit or computer could be the key because of their stability and ability to remember their state even if there’s no power.
The Northwestern researchers turned to the two-dimensional material molybdenum disulfide (MoS2) to create the third terminal for the memristor.
In research published in the journal Nature Nanotechnology, the team found that the grain boundaries of MoS2 provided a better way to modulate resistance than can be achieved with memristors consisting of metal–insulator–metal structures with insulating oxides.
Grains are essentially the direction that atoms are arranged in a material; the grain boundaries are the interface where these grains come together and meet. In MoS2 these grain boundaries are well-defined.
"Because the atoms are not in the same orientation, there are unsatisfied chemical bonds at that interface," Hersam explained in the release. "These grain boundaries influence the flow of current, so they can serve as a means of tuning resistance."
In actual practice, when a voltage is applied to the MoS2-based memristor the grain boundaries physically move, which is the mechanism that changes the device’s resistance. The result is a three-terminal memristive device that can be tuned by a gate electrode.
Hersam added: "With a memristor that can be tuned with a third electrode, we have the possibility to realize a function you could not previously achieve,” Hersam said.
He added that with a three-terminal memristor in hand, his research team will be actively pursuing the potential for brain-like computing.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.