## Tuesday, May 6, 2008

### "MEMRISTOR" ...... New Circuit Element

As all science students know, there are only three basic elements that make up an electrical circuit:the resistor, the capacitor and the inductor.

Sorry guys!It may be time to tear up your textbooks and write the new ones:scientists have realised physical samples of a fourth fundamental element which they call a memristor - short for memory resistor.

In a paper published in the latest issue of NATURE MAGAZINE(”The missing memristor found”,may 1st ,2008;vol no.453 ; pp 80-83), researchs at hewlett packard labs, report that the missing fourth element of the circuitry that professor Leon Chua of the University of California in Berkeley predicted in 1971 is indeed realisable.

PRACTICAL UNITS :

The team, led by R.Stanley Williams, believes that using nano technology one can soon build practical units of the resistor-with-memory that cannot be created by a mere combination of the three basic circuit elements.

Such element could fuel a new class of computer memory that would ‘remember’, even if the machine were switched off……in other words, tomorrow’s PCs could boot up and spring to life instantly.

The engineers are busy building memristors using Titanium Dioxide and have already realised a few hybrid versions in silicon.

Memory banks built using memristors could be a thousand times faster than todays magnetic disk systems , and consume a fraction of the power, the scientists suggest.

In electrical circuit theory, the memristor is a passice circuit element. It has been described as the fourth basic type of passive circuit element, alongside the well-known capacitor, resistor, and inductor. The name is a portmanteau of memory resistor.

Although the memristor was predicted and described in 1971 by Leon Chua of UC Berkeley, in a paper in IEEE Transactions on Circuit Theory, it was a hypothetical device for 37 years, with no known physical examples.

According to Chua, the “era of nanoscale electronics will be enabled by the memristor. This is not just an invention, it is a basic scientific discovery.

In April 2008, a working device with similar characteristics to a memristor was announced by a team of researchers at HP Labs. The new circuit element may enable the development of a new class of high-density non-volatile digital memory. Performance of memristors improves as they are scaled down, and they generate less heat than transistors. The memristor also has unique analog properties that may lead to the invention of other devices.

The memristor is an element in which the magnetic flux Φm is a function of the accumulated electric charge q in the device. The rate of change of flux with charge

$M(q)=\frac{\mathrm d\Phi_m}{\mathrm dq}$

is known as memristance. This is comparable to the other three fundamental circuit elements:

• Resistance: $R(I)=\frac{\mathrm dV}{\mathrm dI}$
• Inductance: $L(I) = \frac{\mathrm d\Phi_m}{\mathrm dI}$
• Capacitance: $\frac{1}{C(q)}=\frac{\mathrm dV}{\mathrm dq}$

Here q is electrical charge, I is electrical current, V is electrical potential and Φm is magnetic flux. The differential forms of these equations are used because we are comparing non-linear circuit elements; a linear memristor would be uninteresting, as explained below.

Applying Faraday’s law of electromagnetic induction and the chain rule to the equation defining the memristance, one obtains that the voltage V across a memristor is related to the current I by the instantaneous value of the memristance:

$V(t) = M(q(t)) I(t) \,$

Thus at any given instant, a memristor behaves like an ordinary resistor. However, its “resistance” M(q) is a value which depends on the charge accumulated in the device. This differs from ordinary resistors where the resistance is determined by fixed physical properties and transistors where the resistance is controlled by either the voltage at or current through a gate electrode. A linear memristor (one for which M is constant) would thus be indistinguishable from an ordinary linear resistor (one for which R is constant), with M = R. Memristance can be said to depend on the history of the charge that has flowed through the device in the same way that the voltage of capacitors does.

The memristor is capable of “remembering” how much electrical charge most recently passed through it in one direction versus another. Thus, it can “remember” the state it was last in.

Fabrication :

Interest in the memristor revived in 2007 when an experimental solid-state version was reported by Stanley Williams of hewlett packard. A solid-state device could not be constructed until the unusual behavior of nanoscale materials made it possible. The device does not use magnetic flux as the theoretical memristor suggested, nor stores charge as a capacitor does, but instead achieves a resistance dependent on the history of current using a chemical mechanism.

The HP device is composed of a thin (5 nm) titanium dioxide film between two electrodes. Initially, there are two layers to the film, one of which has a slight depletion of oxygen atoms. The oxygen vacancies act as charge carriers, meaning that the depleted layer has a much lower resistance than the non-depleted layer. When an electric field is applied, the oxygen vacancies drift, changing the boundary between the high-resistance and low-resistance layers. Thus the resistance of the film as a whole is dependent on how much charge has been passed through it in a particular direction, which is reversible by changing the direction of current.

Samsung has a pending U.S. patent application for a memristor similar to that described by Williams.

Potential applications :

Williams’s solid-state memristors can be combined into devices called crossbar latches, which would replace transistors in future computers, taking up a much smaller area. They can also be fashioned into non-volatile solid-state memory, which would allow greater data density than hard drives with access times potentially similar to DRAM, replacing both components. HP prototyped a crossbar latch memory using the devices that can fit 100 gigabits in a square centimeter. For comparison, the highest-density flash memories at this time (2008) hold 16 gigabits. HP has reported that its version of the memristor is about one tenth the speed of DRAM.

The devices’ resistance would be read with alternating current so that they do not affect the stored value.