HP: Memristors now a reality

April 9, 2010 – New insights into memristors could offer an offramp to the increasingly challenging navigations of Moore’s Law scaling, and some very interesting applications in biomed research.

A memristor ("memory resistor") is the fourth fundamental circuit element (along with resistors, capacitors, and inductors), which together could form a complete set of functions in a wide variety of electronics circuits. Incorporating memristors could eliminate the need for much of the transistors & capacitors, in some cases by an order-of-magnitude, according to Stan Williams, director of HP’s Information & Quantum Systems Lab and lead researcher on the project. And using fewer transistors to achieve the same functionality sidesteps the problem of maintaining Moore’s Law by scaling devices to physics-altering sizes. Memristors also are seen as a potential replacement for memory chips — they are nonvolatile, consume less energy, store twice as much data in the same space, and are highly radiation-resistant.

Two years ago, HP claimed proof of existence of memristors, though some argue that it’s been a popular field of study for years. And speaking last Nov. at a Silicon Valley Engineering Council event, Williams had teased about upcoming new achievements in memristors.

And now, in a paper published in Nature, HP researchers say that they have improved their initial memristor devices so that they are closer in performance to today’s silicon transistors yet at a fraction of the size — and they indeed can perform logic functions, and thus could be used in future microprocessors, made with conventional materials and processes. More immediate use would be in memory chips; HP Labs claims to have a "development-ready architectures" that stack multiple layers of memristor memory on top of each other in a single chip.

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Figure 1. A possible and highly simplified implication logic circuit architecture based on a linear array of memristive switches. (a) A schematic illustration of the architecture with two demultiplexers and two voltage sources, one to deliver the voltage VCOND to the source memristive switch and the other to deliver the voltage VSET to the target switch. Note that with this architecture, the demultiplexers can arbitrarily address any switch to set or clear it and any two switches in the array to implement an IMP operation. (b) A diagram of the linear array of memristive switches shown as green, and the connection pattern of permanently fused (brown) and open (blue) junctions in a crossbar that act as the VCOND and VSET demultiplexers for the switches. Note that the width of the demultiplexers scales as the base two logarithm (log2[N]) of the number of memristive switches in the linear array N, and that in principle there may be many rows of memristive switches M all connected by the same demultiplexers. By loading different initial values into each of the M rows of switches, the same computational sequence can be carried out in parallel to yield M different results. (Source: Nature)

From the Nature paper abstract:

Recently, ultra-dense resistive memory arrays built from various two-terminal semiconductor or insulator thin film devices have been demonstrated. Among these, bipolar voltage-actuated switches have been identified as physical realizations of ‘memristors’ or memristive devices, combining the electrical properties of a memory element and a resistor. Such devices were first hypothesized by Chua in 1971, and are characterized by one or more state variables that define the resistance of the switch depending upon its voltage history.

Here we show that this family of nonlinear dynamical memory devices can also be used for logic operations: we demonstrate that they can execute material implication (IMP), which is a fundamental Boolean logic operation on two variables p and q such that pIMPq is equivalent to (NOTp)ORq. Incorporated within an appropriate circuit, memristive switches can thus perform ‘stateful’ logic operations for which the same devices serve simultaneously as gates (logic) and latches (memory) that use resistance instead of voltage or charge as the physical state variable.

"Memristive devices could change the standard paradigm of computing by enabling calculations to be performed in the chips where data is stored rather than in a specialized central processing unit," stated Williams. "We anticipate the ability to make more compact and power-efficient computing systems well into the future, even after it is no longer possible to make transistors smaller via the traditional Moore’s Law approach.

Moreover, there’s a clear path to some interesting applications for memristors — whose functionality is basically the same as how a brain works. "The flood gate is now open for commercialization of computers that would compute like human brains, which is totally different from the von Neumann architecture underpinning all digital computers," added UC/Berkeley’s Leon Chua, who was involved in the pioneering work in memristors in the 1970s.

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Figure 2. (a) Atomic force microscope micrograph of a nanocircuit. (b) Idealized memristive electrical characteristics, with abrupt voltage thresholds for opening and closing the switch between the low-resistance switch-closed state (logical value ‘1’) and the high-resistance switch-open state (logical value ‘0’). (c) Experimental direct-current current-voltage switching characteristics (four-probe method). Traces b-f are offset. Trace a shows a closed-to-open transition, trace b shows stability and trace c shows an open-to-closed transition. Traces d-f repeat this cycle. (d) Switch toggling by pulsed voltages (2μs long; VSET = -5V and VCLEAR = +9V). Non-destructive reads at -0.2V test the switch state. (Source: Nature)



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