1. Physicist Stanley Williams of HP Labs says that after a colleague brought
Chua's work to his attention; he saw that it would explain a variety of odd
behaviors in electronic devices that his group and other nanotech researchers
had built over the years. He says when he realized that "to make a pure
memristor 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 as
three nanometers between a pair of platinum layers [see image above]. Part of
the TiO2 layer contained a sprinkling of positively charged divots (vacancies)
where oxygen atoms would have normally been. They applied an alternating
current to the electrode closer to these divots, causing it to swing between a
positive and negative charge.
When positively charged, the electrode pushed the charged vacancies and
spread them throughout the TiO2, boosting the current flowing to the second
electrode. When the voltage reversed, it slashed the current a million fold, the
group reports. When the researchers turned the current off, the vacancies
stopped moving, which left the memristor in either its high or low resistant
state? "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 nonvolatile
memory technologies because the basic manufacturing tools are already in
place.
Williams adds that memristors could be used to speed up microprocessors by
Synchronizing circuits that tend to drift in frequency relative to one another or
by doing the work of many transistors at once.
MEMRISTOR

2. Fig 3: Symbol of memristor
Memristors are basically a fourth class of electrical circuit, joining the resistor,
the capacitor, and the inductor, that exhibit their unique properties primarily at
the nanoscale. Theoretically, Memristors, a concatenation of “memory
resistors”, are a type of passive two terminal circuit elements that maintain a
relationship between the time integrals of current and voltage across a two
terminal element. Thus, a memristors resistance varies according to a devices
memristance function, allowing, via tiny read charges, access to a “history” of
applied voltage. The material implementation of memristive effects can be
determined in part by the presence of hysteresis (an accelerating rate of change
as 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 anomaly
than a fundamental property of passive circuitry
Why it is different from other fundamental circuit element?
The definition of the memristor is based solely on fundamental circuit
variables, similarly to the resistor, capacitor, and inductor. Unlike those three
elements, which are allowed in linear time invariant or LTI system theory,
memristors are nonlinear and may be described by any of a variety of time
varying functions of net charge.
There is no such thing as a generic memristor. Instead, each device implements
a particular function, wherein either the integral of voltage determines the
integral of current, or vice versa. A linear time invariant memristor is simply a
conventional resistor.

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.
Analogy of Memristor with a Pipe
The 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 an
unchanging pipe size. A memristor, on the other hand, changes with the amount
of water that gets pushed through. If you push water through the pipe in one
direction, the pipe gets larger (less resistive). If you push the water in the other
direction, the pipe gets smaller (more resistive). And the memristor remembers.
When the water flow is turned off, the pipe size does not change. Such a
mechanism could technically be replicated using transistors and capacitors, but,
Williams says, ´it takes a lot of transistors and capacitors to do the job of a
single memristor.´
Φ
Φm

4. Consequences of Memristors Memory
The memristors 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.´
For some memristors, applied current or voltage will cause a great change in
resistance. Such devices may be characterized as switches by investigating the
time and energy that must be spent in order to achieve a desired change in
resistance. Here we will assume that the applied voltage remains constant and
solve for the energy dissipation during a single switching event. For a
memristor to switch from Ron to Roff in time Ton to Toff, the charge must
change by ΔQ = Qon-Qoff.

5. To arrive at the final expression, substitute V=I(q)M(q), and then dq/V = ¨ Q/V
for constant V. T is power characteristic differ s fundamentally from th at of a
metal oxide semiconductor transistor, which is a capacitor-based de ice. Unlike
the transistor, the final state of the memristor in terms of charge does not
depend on bias voltage.
The type of memristor described by Williams ceases to be ideal after switching
over 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 that
each 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 less
practical.