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MEMRISTOR

INTRODUCTION

1.	Generally when most people think about electronics, they may initially think of products such as cell phones, radios, laptop computers, etc. others, having some engineering background, may think of resistors, capacitors, etc. which are the basic components necessary for electronics to function. Such basic components are fairly limited in number and each having their own characteristic function.

2.	Memristor theory was formulated and named by Leon Chua in a 1971 paper. Chua strongly believed that a fourth device existed to provide conceptual symmetry with the resistor, inductor, and capacitor. This symmetry follows from the description of basic passive circuit elements as defined by a relation between two of the four fundamental circuit variables. A device linking charge and flux (defined as time integrals of current and voltage), which would be the memristor, was still hypothetical at the time. However, it would not be until thirty-seven years later, on April 30, 2008, that a team at HP Labs led by the scientist R. Stanley Williams would announce the discovery of a switching memristor. Based on a thin film of titanium dioxide, it has been presented as an approximately ideal device.

3.	The reason that the memristor is radically different from the other fundamental circuit elements is that, unlike them, it carries a memory of its past. When you turn off the voltage to the circuit, the memristor still remembers how much was applied before and for how long. That's an effect that can't be duplicated by any circuit combination of resistors, capacitors, and inductors, which is why the memristor qualifies as a fundamental circuit element.

4.	The arrangement of these few fundamental circuit components form the basis of almost all of the electronic devices we use in our everyday life. Thus the discovery of a brand new fundamental circuit element is something not to be taken lightly and has the potential to open the door to a brand new type of electronics. HP already has plans to implement memristors in a new type of non-volatile memory which could eventually replace flash and other memory systems.

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MEMRISTOR THEORY AND ITS PROPERTIES

Definition of Memristor

5.	The memristor is formally defined as a two-terminal element in which the magnetic flux Φm between the terminals is a function of the amount of electric charge q that has passed through the device. Chua defined the element as a resistor whose resistance level was based on the amount of charge that had passed through the memristor

Figure 2. Symbol of Memristor. Memristance

6.	Memristance is a property of an electronic component to retain its resistance level even after power had been shut down or lets it remember (or recall) the last resistance it had before being shut off.

WORKING OF MEMRISTOR Figure 3. Al/TiO2 or TiOX /Al “Sandwich”

7.	The memristor is composed of a thin (5 nm) titanium dioxide film between two electrodes as shown in figure 5(a) above. 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.

FEATURES

8.	The reason that the memristor is radically different from the other fundamental circuit elements is that, unlike them, it carries a memory of its past. When you turn off the voltage to the circuit, the memristor still remembers how much was applied before and for how long. That's an effect that can't be duplicated by any circuit combination of resistors, capacitors, and inductors, which is why the memristor qualifies as a fundamental circuit element.

New Memristor  Could Make Computers Work like Human Brains

9.	After the resistor, capacitor, and inductor comes the memristor. Researchers at HP Labs have discovered a fourth fundamental circuit element that can't be replicated by all combination of the other three. The memristor (short for "memory resistor") is unique because of its ability to, in HP's words, "[retain] a history of the information it has acquired." HP says the discovery of the memristor paves the way for anything from instant on computers to systems that can "remember and associate series of events in a manner similar to the way a human brain recognizes patterns." Such brain-like systems would allow for vastly improved facial or biometric recognition, and they could be used to make appliances that "learn from experience."

10.	 In PCs, HP foresees memristors being used to make new types of system memory that can store information even after they lose power, unlike today's DRAM. With memristor-based system RAM, PCs would no longer need to go through a boot process to load data from the hard drive into the memory, which would save time and power especially since users could simply switch off systems instead of leaving them in a "sleep" mode

Memristors Make Chips Cheaper

10.	The first hybrid memristor-transistor chip could be cheaper and more energy efficient. Entire industries and research fields are devoted to ensuring that, every year,computers continue getting faster. But this trend could begin to slow down as the components used in electronic circuits are shrunk to the size of just a few atoms.Researchers at HP Labs in Palo Alto, CA, are betting that a new fundamental electronic component--the memristor--will keep computer power increasing at this rate for years to come. They are nanoscale devices with unique properties: a variable resistance and the ability to remember the resistance even when the power is off.Increasing performance has usually meant shrinking components so that more can be packed onto a circuit. But instead, Williams's team removes some transistors and replaces them with a smaller number of memristors. "We're not trying to crowd more transistors onto a chip or into a particular circuit," Williams says. "Hybrid memristor-transistor chips really have the promise for delivering a lot more performance."12 A memristor acts a lot like a resistor but with one big difference: it can change resistance depending on the amount and direction of the voltage applied and can remember its resistance even when the voltage is turned off. These unusual properties make them interesting from both a scientific and an engineering point of view. A single memristor can perform the same logic functions as multiple transistors, making them a promising way to increase computer power. Memristors could also prove to be a faster, smaller, more energy-efficient alternative to flash storage.

Memristor as Digital and Analog

11.	 A memristive device can function in both digital and analog forms, both having very diverse applications. In digital mode, it could substitute conventional solid-state memories (Flash) with high-speed and less steeply priced nonvolatile random accessmemory (NVRAM). Eventually, it would create digital cameras with no delay between photos or computers that save power by turning off when not needed and then turning back on instantly when needed.

No Need of Rebooting

12.	The memristor's memory has consequences:The reason computers have to be rebooted every time they are turned on is that their logic circuits are incapable of holding their bits after the power is shut off. But because a memristor can remember voltages, a memristor-driven computer would arguably never need a reboot. “You could leave all your Word files and spreadsheets open, turn off your computer, and go get a cup of coffee or go on vacation for two weeks,” says Williams. “When you come back, you turn on your computer and everything is instantly on the screen exactly the way you left it.”that keeps memory powered. HP says memristor-based RAM could one day replace DRAM altogether.

FUTURE OF MEMRISTOR

13.	Although memristor research is still in its infancy, HP Labs is working on a handful of practical memristor projects. And now Williams's team has demonstrated a working memristor-transistor hybrid chip. "Because memristors are made of the same materials used in normal integrated circuits," says Williams, "it turns out to be very easy to integrate them with transistors." His team, which includes HP researcher Qiangfei Xia, built a field-programmable gate array (FPGA) using a new design that includes memristors made of the semiconductor titanium dioxide and far fewer transistors than normal.Engineers commonly use FPGAs to test prototype chip designs because they can be reconfigured to perform a wide variety of different tasks. In order to be so flexible,however, FPGAs are large and expensive. And once the design is done, engineers generally abandon FPGAs for leaner "application-specific integrated circuits." "When you decide what logic operation you want to do, you actually flip a bunch of switches and configuration bits in the circuit," says Williams. In the new chip, these tasks are performed by memristors. "What we're looking at is essentially pulling out all of the configuration bits and all of the transistor switches," he says. According to Williams, using memristors in FPGAs could help significantly lower costs. "If our ideas work out, this type of FPGA will completely change the balance," he says. Ultimately, the next few years could be very important for memristor research.

When is it coming?

14.	Researchers say that no real barrier prevents implementing the memristor in circuitry immediately. But it's up to the business side to push products through to commercial reality. Memristors made to replace flash memory (at a lower cost and lower 14 power consumption) will likely appear first; HP's goal is to offer them by 2012. Beyond that, memristors will likely replace both DRAM and hard disks in the 2014-to-2016 time frame. As for memristor-based analog computers, that step may take 20-plus years