A memristor, or memory resistor, is a resistor that "remembers" how much current has flowed through it. Unlike other modern-day electronics memories like those made from CMOS devices, memristors are able to “remember” their state (that is the information stored in them), even if you lose power. They also use much less energy.

So-called resistance-switching memristors made from transition metal oxides are promising for electronic components in a variety of future device applications thanks to the fact that their conductance reversibly changes in response to an applied stimulus. Such devices include high-density non-volatile memory as well as emerging applications such as “stateful” logic and bio-inspired neuromorphic devices.

However, one of the drawbacks of such memristors is the electrical power that is wasted during the data writing process. This occurs because of the additional current that flows through cells other than the addressed cell.

Solving the 'sneak path' problem

“This sneak path problem, as it is known, limits the size of the memory array that can be used and thus the total memory density since large sneak path currents make it difficult to successfully write to the addressed cell,” explains team member Kyung Min Kim of Hewlett Packard Labs in Palo Alto. “To overcome this challenge, researchers have introduced a ‘selector’ component in series with the memristor in each cell but since selector technology is not as mature as memristor technology we are struggling to develop appropriate selectors.

“Our team has taken a different approach to solving the sneak path problem without introducing a separate selector. We have developed a memristor with a large intrinsic current-rectifying characteristic that acts as an internal or integral selector. What is more, we have described how to use this self-selecting memristor to reduce power consumption to just 8% of that in conventional crossbar circuits.”

In principle, when the memristor is combined with a series selector in each cell of the crossbar, the net power consumption could be reduced to 0.31% of the power consumed in standard crossbars, he adds.

Technology is fully CMOS compatible

The new device is based on titanium (Ti) ion electron traps in a niobium oxide (NbOx) matrix. The researchers deposited the active material in the structure using a technique called atomic layer deposition (ALD), which allows them to control the sub-atomic monolayers in the structure and so precisely control how the Ti traps are distributed in the NbOx matrix. The resistance in the memristor can be switched from high to low (and vice versa) using much lower currents (and thus power) than previously demonstrated devices, Kim tells nanotechweb.org.

According to the team, led by Stanley Williams, also of Hewlett Packard Labs, the new memristor could be used to make embedded memories for low-power chips, such as ASICS. “Since the technology is fully CMOS compatible, it might also be used to store data in or near sensors at the edge of IoT devices,” says Kim. “Eventually, it might find use as a stand-alone non-volatile memory for low-power systems.”

The researchers, reporting their work in Nano Letters DOI: 10.1021/acs.nanolett.6b01781, say that they will now be fabricating full crossbar arrays while continuing to improve the individual cell performance of the memristors, in terms of both the resistance state retention lifetime and reducing the writing voltage.