M memristor doc.

CHAPTER 1

INTRODUCTION

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 (themselves 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.

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.

1.1. Need For Memristor
A memristor is one of four basic electrical circuit components, joining the resistor,
capacitor, and inductor. The memristor, short for "memory resistor" was first theorized by
student Leon Chua in the early 1970s. He developed mathematical equations to represent the
memristor, which Chua believed would balance the functions of the other three types of circuit
elements.

The known three fundamental circuit elements as resistor, capacitor and inductor relates
four fundamental circuit variables as electric current, voltage, charge and magnetic flux. In that
we were missing one to relate charge to magnetic flux. That is where the need for the fourth
fundamental element comes in. This element has been named as memristor.

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Memristance (Memory + Resistance) is a property of an Electrical Component that
describes the variation in Resistance of a component with the flow of charge. Any two terminal
electrical component that exhibits Memristance is known as a Memristor. Memristance is
becoming more relevant and necessary as we approach smaller circuits, and at some point when
we scale into nano electronics, we would have to take memristance into account in our circuit
models to simulate and design electronic circuits properly.
An ideal memristor is a passive two-terminal electronic device that is built to express
only the property of memristance (just as a resistor expresses resistance and an inductor
expresses inductance). However, in practice it may be difficult to build a 'pure memristor,' since
a real device may also have a small amount of some other property, such as capacitance (just as
any real inductor also has resistance). A common analogy for a resistor is a pipe that carries
water. The water itself is analogous to electrical charge, the pressure at the input of the pipe is
similar to voltage, and the rate of flow of the water through the pipe is like electrical current. Just
as with an electrical resistor, the flow of water through the pipe is faster if the pipe is shorter
and/or it has a larger diameter.

1.2. HISTORY

The story of the memristor is truly one for the history books. When Leon Chua, now an
IEEE Fellow, wrote his seminal paper predicting the memristor, he was a newly minted and
rapidly rising professor at UC Berkeley. Chua had been fighting for years against what he
considered the arbitrary restriction of electronic circuit theory to linear systems. He was
convinced that nonlinear electronics had much more potential than the linear circuits that domi-
nate electronics technology to this day.

Memristance was first predicted by Professor Leon Chua in his paper “Memristor—The
missing circuit element” published in the IEEE Transactions on Circuits Theory (1971).

In that paper, Prof. Chua proved a number of theorems to show that there was a 'missing'
two-terminal circuit element from the family of "fundamental" passive devices: resistors (which

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provide static resistance to the flow of electrical charge), capacitors (which store charges), and
inductors (which resist changes to the flow of charge)—, or elements that do not add energy to a
circuit. He showed that no combination of resistors, capacitors, and inductors could duplicate
the properties of a memristor. This inability to duplicate the properties of a memristor with the
other passive circuit elements is what makes the memristor fundamental. However, this original
paper requires a considerable effort for a non-expert to follow. In a later paper, Prof. Chua
introduced his 'periodic table' of circuit elements.

Fig.1.1 : Describing the relation between charge,
current, voltage and magnetic flux to one another

The pair wise mathematical equations that relate the four circuit quantities—charge,
current, voltage, and magnetic flux—to one another. These can be related in six ways. Two are
connected through the basic physical laws of electricity and magnetism, and three are related by
the known circuit elements: resistors connect voltage and current, inductors connect flux and
current, and capacitors connect voltage and charge. But one equation is missing from this group:
the relationship between charge moving through a circuit and the magnetic flux surrounded by
that circuit. That is what memristor, connecting charge and flux.

Even before Chua had his eureka moment, however, many researchers were reporting
what they called “anomalous” current-voltage behavior in the micrometer-scale devices they had
built out of unconventional materials, like polymers and metal oxides. But the idiosyncrasies
were usually ascribed to some mystery electrochemical reaction, electrical breakdown, or other
spurious phenomenon attributed to the high voltages that researchers were applying to their
devices.

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Leon’s discovery is similar to that of the Russian chemist Dmitri Mendeleev who created
and used a periodic table in 1869 to find many unknown properties and missing elements.

1.2.1. HP’s first step:

Even though Memristance was first predicted by Professor Leon Chua, Unfortunately,
neither he nor the rest of the engineering community could come up with a physical
manifestation that matched his mathematical expression.

Thirty-seven years later, a group of scientists from HP Labs has finally built real working
memristors, thus adding a fourth basic circuit element to electrical circuit theory, one that will
join the three better-known ones: the capacitor, resistor and the inductor.

Interest in the memristor revived in 2008 when an experimental solid state version was
reported by R. Stanley Williams of Hewlett Packard. HP researchers built their memristor when
they were trying to develop molecule-sized switches in Teramac (tera-operation-per-second
multiarchitecture computer). Teramac architecture was the crossbar, which has since become the
de facto standard for nanoscale circuits because of its simplicity, adaptability, and redundancy.

A solid-state device could not be constructed until the unusual behavior of nanoscale
materials was better understood. The device neither uses magnetic flux as the theoretical
memristor suggested, nor do stores charge as a capacitor does, but instead achieves a resistance
dependent on the history of current using a chemical mechanism.

The HP team’s memristor design consisted of two sets of 21 parallel 40-nm-wide wires
crossing over each other to form a crossbar array, fabricated using nanoimprint lithography. A
20-nm-thick layer of the semiconductor titanium dioxide (TiO2) was sandwiched between the
horizontal and vertical nanowires, forming a memristor at the intersection of each wire pair. An
array of field effect transistors surrounded the memristor crossbar array, and the memristors and
transistors were connected to each other through metal traces.

The crossbar is an array of perpendicular wires. Anywhere two wires cross, they are
connected by a switch. To connect a horizontal wire to a vertical wire at any point on the grid,

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you must close the switch between them. Note that a crossbar array is basically a storage system,
with an open switch representing a zero and a closed switch representing a one. You read the
data by probing the switch with a small voltage. Because of their simplicity, crossbar arrays have
a much higher density of switches than a comparable integrated circuit based on transistors
electrodes.

Stanley Williams found an ideal memristor in titanium dioxide—the stuff of white paint
and sunscreen. In TiO2, the dopants don't stay stationary in a high electric field; they tend to drift
in the direction of the current. Titanium dioxide oxygen atoms are negatively charged ions and
its electrical field is huge. This lets oxygen ions move and change the material’s conductivity, a
necessity for memristors.

The researchers then sandwiched two thin titanium dioxide layers between two 5 nm
thick Applying a small electrical current causes the atoms to move around and quickly switch the
material from conductive to resistive, which enables memristor functionality.

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. Since the HP device displays fast ion conduction
at nanoscale, it is considered a nanoionic device.

In the process, the device uses little energy and generates only small amounts of heat.
Also, when the device shuts down, the oxygen atoms stay put, retaining their state and the data
they represent.

On April 30, 2008, the Hewlett-Packard research team proudly announced their
realization of a memristor prototype.

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CHAPTER 2

MEMRISTOR

2.1. MEMRISTOR FEATURES:

Memristor is passive two-terminal element that maintains functional relation between
charge flowing through the device (i.e. time integral of current) and flux or A memristor is a
two-terminal semiconductor device whose resistance depends on the magnitude and polarity of
the voltage applied to it and the length of time that voltage has been applied. When you turn off
the voltage, the memristor remembers its most recent resistance until the next time you turn it on,
whether that happens a day later or a year later.

Fig.2.1: An atomic force microscope image shows 17 memristors

As its name implies, the memristor can "remember" how much current has passed
through it. And by alternating the amount of current that passes through it, a memristor can also
become a one-element circuit component with unique properties. Most notably, it can save its
electronic state even when the current is turned off, making it a great candidate to replace today's
flash memory.

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A common analogy to describe a memristor is similar to that of a resistor. Think of a
resistor as a pipe through which water flows. The water is electric charge. The resistor’s
obstruction of the flow of charge is comparable to the diameter of the pipe: the narrower the
pipe, the greater the resistance. For the history of circuit design, resistors have had a fixed pipe
diameter. But a memristor is a pipe that changes diameter with the amount and direction of water
that flows through it. If water flows through this pipe in one direction, it expands (becoming less
resistive). But send the water in the opposite direction and the pipe shrinks (becoming more
resistive). Further, the memristor remembers its diameter when water last went through. Turn off
the flow and the diameter of the pipe “freezes” until the water is turned back on. , the pipe will
retain it most recent diameter until the water is turned back on. Thus, the pipe does not store
water like a bucket (or a capacitor) – it remembers how much water flowed through it.

Fig.2.2: Schematic diagram of pipe and current example

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.

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Technically such a mechanism can be replicated using transistors and capacitors, but, it
takes a lot of transistors and capacitors to do the job of a single memristor.

Memristance is measured by the electrical component memristor. The way a resistor
measures resistance, a conductor measures conduction, and an inductor measures inductance, a
memristor measures memristance. An ideal memristor is a passive two-terminal electronic
device that expresses only memristance. However it is difficult to build a pure memristor, since
every real device contains a small amount of another property.

Two properties of the memristor attracted much attention. Firstly, its memory
characteristic, and, secondly, its nanometer dimensions. The memory property and latching
capability enable us to think about new methods for nano-computing. With the nanometer scale
device provides a very high density and is less power hungry. In addition, the fabrication process
of nano-devices is simpler and cheaper than the conventional CMOS fabrication, at the cost of
extra device defects.

At the architectural level, a crossbar-based architecture appears to be the most promising
nanotechnology architecture. Inherent defect-tolerance capability, simplicity, flexibility,
scalability, and providing maximum density are the major advantages of this architecture by
using a memristor at each cross point.

Memristors are passive elements, meaning they cannot introduce energy into a circuit. In
order to function, memristors need to be integrated into circuits that contain active elements,
such as transistors, which can amplify or switch electronic signals. A circuit containing both
memristors and transistors could have the advantage of providing enhanced functionality with
fewer components, in turn minimizing chip area and power consumption.

This new circuit element shares many of the properties of resistors and shares the same
unit of measurement (ohms). However, in contrast to ordinary resistors, in which the resistance is
permanently fixed, memristance may be programmed or switched to different resistance states
based on the history of the voltage applied to the memristance material. This phenomena can be
understood graphically in terms of the relationship between the current flowing through a

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memristor and the voltage applied across the memristor. In ordinary resistors there is a linear
relationship between current and voltage so that a graph comparing current and voltage results in
a straight line.However, for memristors a similar graph is a little more complicated.

Fig.2.3: Current voltage characteristic of resistor and memristor

2.2. Physics of the device:

Fig.2.4: Memristor Symbols

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2.3. Definition of Memristor

“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.

2.4. MEMRISTANCE

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.

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. Each memristor is characterized by its memristance function describing the charge-
dependent rate of change of flux with charge.

Noting from Faraday's law of induction that magnetic flux is simply the time integral of voltage,
and charge is the time integral of current, we may write the more convenient form

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It can be inferred from this that memristance is simply charge-dependent resistance. If
M(q(t)) is a constant, then we obtain Ohm's Law R(t) = V(t)/ I(t). If M(q(t)) is significant,
however, the equation is not equivalent because q(t) and M(q(t)) will vary with time. Solving for
voltage as a function of time we obtain

This equation reveals that memristance defines a linear relationship between current and
voltage, as long as charge does not vary. The power consumption characteristic recalls that of a
resistor, I2R.

As long as M(q(t)) varies little, such as under alternating current, the memristor will
appear as a resistor. If M(q(t)) increases rapidly, however, current and power consumption will
quickly stop. Furthermore, the memristor is static if no current is applied. If I(t) = 0, we find V(t)
= 0 and M(t) is constant. This is the essence of the memory effect.

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CHAPTER 3

MANUFACTURING AND WORKING

3.1. MANUFACTURING

Manufacturers could make memristors in the same chip fabrication plants used now, so
companies would not have to undertake expensive retooling or new construction. And
memristors are by no means hard to fabricate. The titanium dioxide structure can be made in any
semiconductor fab currently in existence. The primary limitation to manufacturing hybrid chips
with memristors is that today only a small number of people on Earth have any idea of how to
design circuits containing memristors.

Fig.3.1: Schematic of our fabrication approach.

One of the key fabrication advantages of the crossbar architecture is that the structure is a well
ordered, periodic and simple structure. However, to achieve Nanoscale resolutions the standard
lithography approaches are insufficient. The manufacturing techniques for the Nanoscale
crossbar devices developed by Hewlett-Packard include nanoimprint lithography, which uses a
stamp-like structure with nanometer resolution to transfer a pattern of Nanoscale resolution to a
substrate.

Additional nanoscale fabrication approaches can include self-assembly techniques in
which a mixture of polymers or other materials can form periodic structures on a surface based

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on processes of energy minimalization. These self-assembly techniques can be used to form a
periodic mask structure over a metal film which can act as a resist to control removal of metal
layers in regions not covered by the mask resulting in the desired metal nanowires required for
the crossbar structure.

Fig.3.2: Images of a 1 × 21 array of memristors.

(a) Optical microscope image.
(b) SEM image of the junction area.
(c) AFM image of part of the array.

3.2. HOW MEMRISTOR WORKS???

3.2.1. Appearance – HP Labs' memristor has Crossbar type memristive circuits contain a lattice
of 40-50nm wide by 2-3nm thick platinum wires that are laid on top of one another perpendicular
top to bottom and parallel of one another side to side. The top and bottom layer are separated by
a switching element approximately 3-30nm in thickness. The switching element consists of two
equal parts of titanium dioxide (TiO2). The layer connected to the bottom platinum wire is
initially perfect TiO2 and the other half is an oxygen deficient layer of TiO2 represented by TiO2-
x where x represents the amount of oxygen deficiencies or vacancies. The entire circuit and
mechanism cannot be seen by the naked eye and must be viewed under a scanning tunneling
microscope, as seen in Figure 6, in order to visualize the physical set up of the crossbar design of
the memristive circuit described in this section.

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Fig.3.3 : Showing crossbar architecture and magnified memristive switch.

3.2.2. Operation – The memristor’s operation as a switch can be explained in three steps. These
first of these steps is the application of power or more importantly current to the memristor. The
second step consists of the amount of time that the current flows across the crossbar gap and how
the titanium cube converts from a semi-conductor to a conductor. The final step is the actual
memory of the cube that can be read as data.

Step 1 – As explained above, each gap that connects two platinum wires contains a mixture of
two titanium oxide layers. The initial state of the mixture is halfway between conductance and
semi-conductance. Two wires are selected to apply power to in either a positive or negative
direction. A positive direction will attempt to close the switch and a negative direction will
attempt to open the switch. The application of this power will be able to completely open the
circuit between the wires but it will not be able to completely close the circuit since the material
is still a semi-conductor by nature. Power can be selectively placed on certain wires to open and
close the switches in the memristor.

Step 2 – the second step involves a process that takes place at the atom level and is not visible by
any means. It involves the atomic process that the gap material, made from titanium dioxide,
goes through that opens and closes the switch. The initial state of the gap is neutral meaning that

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it consists of one half of pure titanium dioxide TiO2 and one half of oxygen starved titanium
dioxide TiO2-x where x in the initial state is 0.05. As positive current is applied, the positively
charged oxygen vacancies push their way into the pure TiO2 causing the resistance in the gap
material to drop, becoming more conductive, and the current to rise. Inversely, as a negative
current is applied the oxygen vacancies withdraw from the pure TiO2 and condense in the TiO2-x
half of the gap material causing the pure and more resistive TiO2 to have a greater ratio slowing
the current in the circuit. When the current is raised the switch is considered open (HI) and for
data purposes a binary 1. As current is reversed and the current is dropped the switch is
considered closed (LOW) or a binary 0 for data purposes.

Fig.3.4 : Diffusing of Oxygen molecules.
(a) TiO2-x layer having oxygen deficiencies over insulating TiO2 layer.
(b) Positive voltage applied to top layer repels oxygen deficiencies in to the insulating TiO2
layer below.
(c) Negative voltage on the switch attracts the positively charged oxygen bubbles pulling them
out of the TiO2.

.Step 3 – Step three explains the final step of memristance and is the actual step that makes the
circuit memristive in nature. As explained previously, the concept of memristance is a resistor

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that can remember what current passed through it. When power is no longer applied to the circuit
switches, the oxygen vacancies remain in the position that they were last before the power was
shut down. This means that the value of the resistance of the material gap will remain until
indefinitely until power is applied again. This is the true meaning of memristance. With an
insignificant test voltage, one that won’t affect the movement of molecules in the material gap
will allow the state of the switches to be read as data. This means that the memristor circuits are
in fact storing data physically.

If we want a positive voltage to turn the memristor off, then we want the titanium oxide
layer with vacancies on the top layer. But if you want a positive voltage to turn the memristor on,
then you need the layers reversed. In its initial state, a crossbar memory has only open switches,
and no information is stored. But once you start closing switches, you can store vast amounts of
information compactly and efficiently.

3.3. Transistor versus Memristor

The first transistor was a couple of inches across which was developed about 60
years ago. Today, a typical laptop computer uses a processor chip that contains over a billion
transistors, each one with electrodes separated by less than 50 nm of silicon. This is more than a
1000 times smaller than the diameter of a human hair. These billions of transistors are made by
“top down” methods that involve depositing thin layers of materials, patterning nano-scale
stencils and effectively carving away the unwanted bits. This approach has become overly
successful. The end result is billions of individual components on a single chip, essentially all
working perfectly and continuously for years on end. No other manufactured technology comes
close in reliability or cost.

Still, miniaturization cannot go on forever, because of the basic properties of matter. We
are already beginning to run into the problem that the silicon semiconductor, copper wiring and
oxide insulating layers in these devices are all made out of atoms. Each atom is about 0.3 nm
across.

The entire body of the transistor is being doped less consistently throughout as its sizes
are reduced below the nanometres which make the transistor more unpredictable in nature. It will
be more difficult and costly to press forward additional research and equipment involving these

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unpredictable behaviours as they occur. Therefore the electronic designs will have to replace
their transistors to the memristors which are not steadily infinitesimal, but increasingly capable.

Table: Difference between Transistor and Memristor:

Transistor Memristor
 3-terminal switching device with an  2-terminal device with one of the electrodes
input electrode (e.g. source), an acting either as a control electrode or a
output electrode (e.g. drain), and a source electrode depending on the voltage
control electrode (e.g. gate). magnitude.

 Requires a power source to retain a  Does not require a power source to retain a
data state. data state.

 Stores data by electron charge.  Stores data by resistance state.

 Scalable by reducing the lateral  Scalable by reducing the thickness of the
length and width dimensions memristor materials.
between the input and output
electrodes.

 Capable of performing analog or  Capable of performing analog or digital
digital electronic functions electronic functions depending on particular
depending on applied bias voltages. material used for memristor.

 Fabrication requires optical
lithography.  Fabrication by optical lithography but
alternative (potentially cheaper) mass
production techniques such as nanoimprint
lithography and self assembly have also
been implemented

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The memristor is very likely to follow the similar steps of how the transistor was
implemented in our electronic systems. They may argue that the transistor took approximately
sixty years to reach the extent of today’s research and capabilities. Therefore, the memristors
may take approximately just as long to actually create some of its promising potentials such as
artificial intelligence. This new advancement means more jobs for research and development and
more potential for inventions and designs. Also, the dependency on getting the transistors to
work efficiently in atom sized is lessened.

Another reason for incorporating memristors is the materials used to make each element.
Transistors are usually made of silicon, a non-metal. While this has proven to be a very reliable
source, it returns to the problem of transistors needing to become smaller. Because they are
made of a non-metal it is much harder to make them much smaller. Memristors, on the other
hand, are made of titanium oxide. Titanium is a metal which is much easier to make into smaller
size. Since memristors have twelve times the power of transistors, however, products can be
made smaller and more powerful without reducing the size of the product that powers them.

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CHAPTER 4

APPLICATIONS AND BENEFITS OF MEMRISTOR

4.1. APPLICATIONS:

The three main areas of application currently under development for memristor
electronics are

(i) Non-volatile memory

(ii) Logic/computation, and

(iii) Neuromorphics.

4.1.1. Non-volatile Memory:
Non-volatile memory is the dominant area being pursued for memristor technology. Of
course most of the companies listed (with the exception of Hewlett Packard) do not refer to their
memory in terms of the memristor and rather use a variety of acronyms (i.e. RRAM, CBRAM,
PRAM, etc.) to distinguish their particular memory design. While these acronyms do represent
real distinctions in terms of the materials used or the mechanism of resistance switching
employed, the materials are still all memristors because they all share the same characteristic
voltage-induced resistance switching behavior covered by the mathematical memristor model of
Chua. Flash memory currently dominates the semiconductor memory market.

However, each memory cell of flash requires at least one transistor meaning that flash
design is highly susceptible to an end to Moore’s law. On the other hand, memristor memory
design is often based on a crossbar architecture which does not require transistors in the memory
cells. Although transistors are still necessary for the read/write circuitry, the total number of
transistors for a million memory cells can be on the order of thousands instead of millions and
the potential for addressing trillions of memory cells exists using only millions (instead of
trillions) of transistors. Another fundamental limitation to conventional memory architectures is
Von Neumann’s bottleneck which makes it more difficult to locate information as memory

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density increases. Memristors offer a way to overcome this hurdle since they can integrate
memory and processing functions in a common circuit architecture providing a de-segregation
between processing circuitry and data storage circuitry.

4.1.2. Logic/Computation:
The uses of memristor technology for logic and computational electronics is less well
developed than for memory architectures but the seeds of innovation in this area are currently
being sown. Memristors appear particularly important to the areas of reconfigurable computing
architectures such as FPGAs in which the arrangement between arrays of basic logic gates can be
altered by reprogramming the wiring interconnections. Memristors may be ideal to improve the
integration density and reconfigurability of such systems. In addition, since some memristor
materials are capable of tunablity in their resistance state they can provide new types of analog
computational systems which may find uses in modeling probabilistic systems (e.g. weather,
stock market, bio systems) more efficiently than purely binary logic-based processors.

4.1.3. Neuromorphic Electronics :
Neuromorphics has been defined in terms of electronic analog circuits that mimic neuro-
biological architectures. Since the early papers of Leon Chua it was noted that the equations of
the memristor were closely related to behavior of neural cells. Since memristors integrate aspects
of both memory storage and signal processing in a similar manner to neural synapses they may
be ideal to create a synthetic electronic system similar to the human brain capable of handling
applications such as pattern recognition and adaptive control of robotics better than what is
achievable with modern computer architectures.

4.2. Other applications:

Signal processing with memristors, Arithmetic processing with memristors, Pattern
comparison with memristors, Memristors and artificial intelligence, Memristors and robotics.

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4.2.1. Materials:

Although the different memristor materials have their respective merits and possess
differences in terms of their underlying physics each material share the same resistance
switching properties possessed by memristors.

Variety of binary oxides such as WO3, Ir2O3, MoO3, ZrO2, and RhO2 adjusted
to have memristive properties. A variety of other memristor variations based on TiO, CuO, NiO,
ZrO, and HfO materials have been under experimental investigation for the past several years.

4.2.2. Metallization Cell:
The memristive effect is due to the formation of metallic filaments which interconnect
two electrodes separated by an electrolytic material. The metallic filaments can be broken or
reformed depending on the polarity of an applied voltage.

4.2.3. Perovskite:

Perovskite materials are based on a variety of ternary oxides including PCMO, SrTiO3,
SrZrO3, and BaTiO3. These types of materials appear to have variable resistances which
are more easily tunable via pulse number modulation which may make these materials more
attractive for analog memristor electronics than the metallization cell or binary oxide materials.

4.2.4. Molecular/Polymer:
Molecular and polymer materials have been investigated by Hewlett-Packard and
Advanced Micro Devices as the basis for new types of non-volatile memory. HP has been
working with molecular systems called rotaxane which are thought to exhibit a resistance
switching effect based on a mechanical reconfiguration of the molecule. AMD has been focusing
on ionic molecular and polymer materials which also produce resistance switching behavior and
may have superior analog memristive properties than other materials.

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4.3. BENEFITS OF MEMRISTOR:

 Provides greater resiliency and reliability when power is interrupted in data centers.

 Have great data density.

 Combines the jobs of working memory and hard drives into one tiny device.

 Faster and less expensive than MRAM.

 Uses less energy and produces less heat.

 Would allow for a quicker boot up since information is not lost when the device is turned
off.

 Operating outside of 0s and 1s allows it to imitate brain functions.

 Does not lose information when turned off.

 Has the capacity to remember the charge that flows through it at a given point in time.

 Conventional devices use only 0 and 1; Memristor can use anything between 0 and 1
(0.3, 0.8, 0.5, etc.)

 Faster than Flash memory.

 By changing the speed and strength of the current, it is possible to change the behavior of
the device.

 A fast and hard current causes it to act as a digital device.

 A soft and slow current causes it to act as an analog device.

 100 GBs of memory made from memristors on same area of 16 GBs of flash memory.
 High Defect Tolerance allows high defects to still produce high yields as opposed to one
bad transistor which can kill a CPU.

 Compatible with current CMOS interfaces.

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 As non-volatile memory, memristors do not consume power when idle.
 3 Memristors to make a NAND gate, 27 NAND gates to make a Memristor!!!
 More magnetic than magnetic disks.

4.4. Major Challenges

 The memristor’s major challenges are its relatively low speeds and the need for
designers to learn how to build circuits with the new element.
 Though hundreds of thousands of memristor semiconductors have already been built,
there is still much more to be perfected.
 Dissipates heat when being written to or read.
 No design standards (rules).

 Needs more defect engineering.

23

CONCLUSION AND FUTURE SCOPE

CONCLUSION:

By redesigning certain types of circuits to include memristors, it is possible to obtain the
same function with fewer components, making the circuit itself less expensive and significantly
decreasing its power consumption. In fact, it can be hoped to combine memristors with
traditional circuit-design elements to produce a device that does computation. The Hewlett-
Packard (HP) group is looking at developing a memristor-based nonvolatile memory that could
be 1000 times faster than magnetic disks and use much less power.
As rightly said by Leon Chua and R.Stanley Williams (originators of memristor),
memrisrors are so significant that it would be mandatory to re-write the existing electronics
engineering textbooks.

FUTURE SCOPE:

Memristor bridges the capability gaps that electronics will face in the near future
according to Moore’s Law and will replace the transistor as the main component on integrated
circuit (IC) chips. The possibilities are endless since the memristor provides the gap to
miniaturizing functional computer memory past the physical limit currently being approached
upon by transistor technology.

When is it coming? 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 lower cost and lower 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.

24

REFERENCES:

 IEEE Spectrum: The Mysterious Memristor By Sally Adee
http://www.spectrum.ieee.org/may08/6207
 Memristors Ready For Prime Time R. Colin Johnson URL:
http://www.eetimes.com/showArticle.jhtml?articleID=208803176
 Flexible memristor: Memory with a twist Vol. 453, May 1, 2008. PHYSorg.com
 L. O. Chua, Memristor The missing circuit element, IEEE Trans. Circuit Theory, vol.
CT-18, pp. 507–519, 1971.
 Memristor - Wikipedia, the free encyclopedia
 http://www.hpl.hp.com/
 “How We Found the Missing Memristor” By R. Stanley Williams, December 2008 •
IEEE Spectrum, www.spectrum.ieee.org
 http://avsonline.blogspot.com/
 http://memristor.pbworks.com/
 http://4engr.com/
 http://knol.google.com/k
 http://newsvote.bbc.co.uk/mpapps/pagetools/email/news.bbc.co.uk/2/hi/technology/7
377063.stm
 http://hubpages.com/topics/technology/5338
 http://totallyexplained.com/
 A hybrid nanomemristor/transistor logic circuit capable of self-programming Julien
Borghetti, Zhiyong Li, Joseph Straznicky, Xuema Li, Douglas A. A. Ohlberg, Wei Wu,
Duncan R. Stewart,and R. Stanley.

25

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