Physicist Stanley Williams of HP Labs says that after a colleague broughtChuas work to his attention; he saw that it would explain a variety of oddbehaviors in electronic devices that his group and other nanotech researchershad built over the years. He says when he realized that "to make a purememristor you have to build it so as to isolate this memory function."So he and his colleagues inserted a layer of titanium dioxide (TiO2) as thin asthree nanometers between a pair of platinum layers [see image above]. Part ofthe TiO2 layer contained a sprinkling of positively charged divots (vacancies)where oxygen atoms would have normally been. They applied an alternatingcurrent to the electrode closer to these divots, causing it to swing between apositive and negative charge.When positively charged, the electrode pushed the charged vacancies andspread them throughout the TiO2, boosting the current flowing to the secondelectrode. When the voltage reversed, it slashed the current a million fold, thegroup reports. When the researchers turned the current off, the vacanciesstopped moving, which left the memristor in either its high or low resistantstate? "Our physics model tells us t at the memristive state should last for years”Chua says that, HP memristor has an advantage over other potential nonvolatilememory technologies because the basic manufacturing tools are already inplace.Williams adds that memristors could be used to speed up microprocessors bySynchronizing circuits that tend to drift in frequency relative to one another orby doing the work of many transistors at once. MEMRISTOR
Fig 3: Symbol of memristorMemristors are basically a fourth class of electrical circuit, joining the resistor,the capacitor, and the inductor, that exhibit their unique properties primarily atthe nanoscale. Theoretically, Memristors, a concatenation of “memoryresistors”, are a type of passive two terminal circuit elements that maintain arelationship between the time integrals of current and voltage across a twoterminal element. Thus, a memristors resistance varies according to a devicesmemristance function, allowing, via tiny read charges, access to a “history” ofapplied voltage. The material implementation of memristive effects can bedetermined in part by the presence of hysteresis (an accelerating rate of changeas an object moves from one state to another) which, like many other non-linear“anomalies” in contemporary circuit theory, turns out to be less an anomalythan a fundamental property of passive circuitryWhy it is different from other fundamental circuit element?The definition of the memristor is based solely on fundamental circuitvariables, similarly to the resistor, capacitor, and inductor. Unlike those threeelements, which are allowed in linear time invariant or LTI system theory,memristors are nonlinear and may be described by any of a variety of timevarying functions of net charge.There is no such thing as a generic memristor. Instead, each device implementsa particular function, wherein either the integral of voltage determines theintegral of current, or vice versa. A linear time invariant memristor is simply aconventional resistor.
The reason that the memristor is radically different from the other fundamentalcircuit elements is that, unlike them, it carries a memory of its past. When youturn off the voltage to the circuit, the memristor still remembers how much wasapplied before and for how long. Thats an effect that cant be duplicated by anycircuit combination of resistors, capacitors, and inductors, which is why thememristor qualifies as a fundamental circuit element.Analogy of Memristor with a PipeThe classic analogy for a resistor is a pipe through which water (electricity)runs. The width of the pipe is analogous to the resistance of the flow of current--the narrower the pipe, the greater the resistance. Nor mal resistors have anunchanging pipe size. A memristor, on the other hand, changes with the amountof water that gets pushed through. If you push water through the pipe in onedirection, the pipe gets larger (less resistive). If you push the water in the otherdirection, the pipe gets smaller (more resistive). And the memristor remembers.When the water flow is turned off, the pipe size does not change. Such amechanism could technically be replicated using transistors and capacitors, but,Williams says, ´it takes a lot of transistors and capacitors to do the job of asingle memristor.´ Φ Φm
Consequences of Memristors MemoryThe memristors memory has consequences: the reason computers have to berebooted every time they are turned on is that their logic circuits are incapableof holding their bits after the power is shut off. But because a memristor canremember voltages, a memristor-driven computer would arguably never need areboot. ´You could leave all your Word files and spreadsheets open, turn offyour computer, and go get a cup of coffee or go on vacation for two weeks, saysWilliams. ´When you come back, you turn on your computer and everything isinstantly on the screen exactly the way you left it.´For some memristors, applied current or voltage will cause a great change inresistance. Such devices may be characterized as switches by investigating thetime and energy that must be spent in order to achieve a desired change inresistance. Here we will assume that the applied voltage remains constant andsolve for the energy dissipation during a single switching event. For amemristor to switch from Ron to Roff in time Ton to Toff, the charge mustchange by ΔQ = Qon-Qoff.
To arrive at the final expression, substitute V=I(q)M(q), and then dq/V = ¨ Q/Vfor constant V. T is power characteristic differ s fundamentally from th at of ametal oxide semiconductor transistor, which is a capacitor-based de ice. Unlikethe transistor, the final state of the memristor in terms of charge does notdepend on bias voltage.The type of memristor described by Williams ceases to be ideal after switchingover its entire resistance range and enters hysteresis, also called the "hard-switching regime." Another kind of switch would ha e a cyclic M(q) so thateach off-on event would be followed by an on-off event under constant bias.Such a device would act as a memristor under all conditions, but would be lesspractical.