Tag Archives: Memristors

Mimicking Nerves with Memristors

Researchers are planning to build a computer mimicking the monumental computational power of the human brain. For this, they prefer to use memristors, because these devices vary their electrical resistance on the basis of the memory of their past activity. Memristors are semiconductor devices, and at NIST, the National Institute of Standards and Technology, researchers demonstrate the long and mysterious manner of the inner workings of memristors, explaining their ability to behave as the short-term memory of human nerve cells.

Nerve cells signal one another, but how well they do so depends on the frequency of their recent past communication. In the same way, the resistance of a memristor also depends on the current flow that went through it very recently. The best part is memristors remember even with their electrical power switched off.

Researchers read the memristor with the help of an electron beam. As the beam impinges on various parts of the memristor, it induces currents depending on the resistance value of that part. Traversing the entire device, this yields a complete image of variations of current throughout the device. By noticing the nature of the current variations, it is possible to indicate the places that may fail, as these show overlapping circles within the titanium dioxide filament.

So far, during their study of memristors, scientists have not been able to understand their working, and neither could they develop standard tool-sets for studying them. Now, for the first time, scientists at NIST have been able to create a tool-set that can probe the working of memristors deeply. They envisage their findings will pave the way for operating memristors more efficiently, and minimize current leaks from them.

For exploring the electrical functioning of memristors, the scientists focused a beam of electronics at various locations on the device. The beam was able to knock some of the electronics from the titanium dioxide surface of the device. The free electrons formed an ultra-sharp image of each of the locations. The beam also caused four clear-cut levels of currents to flow through the device. According to the researchers, several interfaces of materials within the memristor were the cause. Typically, a memristor has an insulating layer separating two conducting metal layers. As the researchers could control the position of the electron beam inducing the currents, they were able to know the location of each of the currents.

By imaging the device, researchers located several dark spots on the memristor. They surmised these spots to be regions of enhanced conductivity. These were the places from where there was a greater probability of currents leaking out of the memristor during its normal operations. However, they found the leaking pathways to be beyond the core of the memristor, and at points where it could switch between high and low resistance levels.

Their finding opened up a possibility of reducing the size of the device to eliminate some of the unwanted current leaking pathways. Until now, the researchers were only able to speculate on the current leakages, but had no means of quantifying the size reduction necessary.

What are Memristors?

What are Memristors and where are they used?

Professor Leon Chua developed the memristor theory in 1971. A scientist discovered the behavior of memristors when he was working in a lab at HP, trying to figure out crossbar switches. Memristors are also known as matrix switches, as they behave as a switch when connecting multiple inputs to several outputs. When Professor Chua looked at the assortment of resistors, capacitors, and inductors for the switching, he noticed a vital missing component, calling it the memory resistor or memristor. In 2006, Stanley Williams developed the practical model of a memristor.

One can consider the memristor as the fourth class of electrical component, after the familiar resistor, capacitor, and inductor. However, unlike the others, memristors exhibit their unique properties only at the nanoscale. Theoretically, memristors maintain a relationship between the time integrals of voltage across and current through two terminals of an element, as they are passive circuit elements.

The resistance of a memristor, or memristance, varies according to its function. It allows access to the history of the applied voltage via tiny read charges. The presence of hysteresis defines the material implementation of its memristive effects. This is its fundamental property, which looks more like a non-linear anomaly. Therefore, memristance is a simple charge dependent resistance, with a unit of Ohm. However, it has its own advantages.

The memristor technology offers lower heat generation as it utilizes less energy. Used in data centers, it offers greater resilience and reliability under interruptions of power. While not consuming any power when idle, memristors are compatible with CMOS interfaces. It is possible to store additional information as memristors allow higher densities to be achieved.

Physically, the memristor has two platinum electrodes across a resistive material, and its resistance depends on its polarity, magnitude, and length. The device retains its resistance even when the voltage is turned off, which makes it a non-volatile memory device. The resistive material can be titanium dioxide or silicon dioxide. As voltage is applied across the terminals, the oxygen atoms within the material disperse towards one of the electrodes. This activity stretches or contracts the material depending on the polarity of the applied voltage, thereby changing the resistance of the memristor.

Depending on their build, memristors are of two types—Iconic thin film and molecular memristors, and magnetic and spin based thermistors.

The material property of iconic thin film and molecular memristors rely more on different material properties of the thin film atomic lattices and application of charge makes these display hysteresis. Using these materials scientists make memristors of Titanium dioxide, ionic or polymers, resonant tunneling diodes, and manganite.

In contrast, magnetic and spin based memristors rely more on the property of the degree of electron spin. Therefore, these systems are aware of the polarization of electronic spin. There are two major types of such memristors—the spintronic memristors, and the spin torque transistor memristors.

With the practical demonstration of memristor manufacturing, their potential application has led to a rapid increase in research for using them in analog and digital circuits, such as programmable logic controllers, computers, and sensors. This has also led to development of theoretical models of memristors—Verilog-A, MATLAB, and Spice.