Memristor: The Component You Never Knew

Key Takeaways

• The memristor is a component first described in the mid-20th century. While the technology is still immature, fabrication has led to the first commercial memristor products.

• The memristor operates on the relationship between a change in flux and a change in charge.

• Continued development of memristor technology could have innumerable memory and miniaturization benefits for designs.

Memristor technology is still in its infancy but offers some compelling benefits.

Introductory network analysis focuses on the flagship components of the basic circuit parameters: resistors, capacitors, and inductors. Due to parasitics, it’s possible to represent much more complex ICs with a combination of 2 or even all 3 fundamental components in various topologies. Interestingly, four distinct parameters are at play for the defining relationship of each component: voltage and current for resistance, charge and voltage for capacitors, and current and flux for inductors. The other unique arrangement of parameters are identities between flux and voltage (V = dϕ/dt) and current and charge (I = dq/dt), respectively, and a theorized but mostly unrealized association between flux and charge. The component governing this link is known as the memristor, and while it’s not an item designers could easily find in product catalogs or footprint libraries, continuing physics and manufacturing investigations hope to bring the device to further prominence.

The Four Fundamental Components at a Glance

The Memristor: Comparisons to Fundamental Components

The memristor is the hypothesized “missing link” between the fundamental circuit parameters and is well-described in theory. While a resistor exhibits a general linear (i.e., Ohmic) relationship between voltage and current around the origin, capacitors and inductors are more complex (pardon the pun). As either of these can store fields (electric for capacitors, magnetic for inductors), they can also source current or voltage (respectively) contained within the field energy. An idealized inductor or capacitor, or one purely capacitive or inductive and with no resistance (and therefore, no power loss), would be able to store and transform its energy indefinitely, meaning the voltage could become voltage (and vice versa). The I-V curve of an ideal inductor or capacitor would be a closed circle centered at the origin, with the radius representing the amount of energy stored.

A memristor would combine some attributes of both the linear resistance and conic capacitor and inductor I-V curves. A memristor would appear as a figure-8 with the cross-over occurring at the origin and both lobes of the curve occurring in the same-sign quadrants of the graph (i.e., I and III). Like the resistor, the memristor can pass through the origin (capacitors and inductors are forbidden because this would violate a stored amount of non-zero energy in the ideal model). As frequency increases, the lobes of the loop flatten out until the curve is nothing more than a linear relationship around the origin–just like a resistor. A simple (apologies in advance for the poor mathematical rigor) integration for memristance:

where M(q) is the measure of memristance, given in units of ohms. Because there are two paths over which the I-V curve traces through a shared point, memristance shows hysteresis on its I-V curve. However, practical models of memristors show three distinct regions in the memristor functionality that suggest a slightly different operation than the model would propose:

• A very rapid initial setting of the voltage level of the memristor.
• A second set state that depends on the current level.
• A voltage-controlled reset state that returns the voltage level to the initial set and closes the loop.

What Is the Purpose of Another Fundamental Component?

For network analysis, the memristor offers an exciting mix of capabilities. It is similar to a resistor in operation, except the current through the device modifies the resistance (instead of the voltage). Also, like a resistor, a memristor cannot store or generate power – it is a passive device. The memristor value indicates the current passage through the device over time: one direction will increase the memristance, while the other decreases the memristance. Unlike a resistor whose voltage reading immediately updates with an instantaneous change in the current, the change in memristance is incremental, and even power cycling with extended off-periods will not theoretically change the memristance value recalled by the device. For this reason, the memristor offers a new potential avenue for data storage with some significant advantages over modern memory storage devices.

Memristor Developments and Uses

Potential applications for memristor systems are varied and offer much mutability. For device manufacturers, memristors' two most valuable aspects are their latching capabilities and nanometer scale, enabling greater package miniaturization for improved function density. While logistic efforts remain to bring these technologies to a competitive footing with other methods, significant optimism exists for new technologies:

• Non-volatile memory - Memristor functionality perfectly suits a latching element for data storage. Unfortunately, current approaches still make switching speeds a magnitude slower than DRAM solutions. One solution may be resistive RAM (ReRAM): the memory uses high and low resistance states alongside a transistor-resistor cell model to support greater storage density than the theoretical max of DRAM architecture. 

• Crossbar - A massive junction of memristor switches implementable at the nanometer scale. Due to the sheer number of memristors available within a small package area, it is highly defect-tolerant and remains highly manufacturable despite its small size. The small size also dramatically reduces the power consumption of the active switching elements. 3D arrangement of crossbar structures may offer additional package functionality and scale for memristor chips. 

• Storage density - Memristor stacks offer superior storage density, and current projected capabilities would dwarf hard drive capabilities, lending to further electronic miniaturization without sacrificing performance. Combined with improvements in non-volatile memory, computers (and other complex electronics) could instantly regain user progress from power disruption at the moment of power-up. 

Of course, many of these implementations hinge on further material breakthroughs and developments to better approximate ideal memristor performance. 

Cadence Offers Solutions for Today’s and Tomorrow’s Components

Memristors exist between proof-of-concept and the accepted reliability of more regular assembly components. As an adapting technology still facing many pratfalls during its inception, designers are less likely to encounter memristor usage. However, cutting-edge designs require supporting layout technology that can simulate common and unique features to fully characterize signal response before finalizing the layout. Fortunately, Cadence’s PCB Design and Analysis Software suite gives design teams an all-in-one solution for ECAD modeling that reduces the number of DFM revisions necessary. Combined with the fast and functional  OrCAD PCB Designer, the layout has never been quicker or easier.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.