30 Page Seminar Report

8/14/2019 30 Page Seminar Report

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CHAPTER 1

INTRODUCTION

A memristor is a two-terminal element whose resistance depends on themagnitude, direction, and duration of the applied voltage. A memristor remembers its

most recent memristance when voltage was turned off until the next time the voltage is

turned on, and it can provide dynamical-negative resistance. These promisingcharacteristics may potentially revolutionize nanoelectronics. It can find applications in

analog and digital circuits, which are part of everyday use systems such as sensors and

mobile phones. This article discusses different aspects of the memristor including basic

characteristics, models, fabrications, and circuit designs to provide a complete picture ofthis state-of the art technology.

Memristor: What is it?

Three fundamental passive elements such as resistors, capacitors, and inductors

are currently used to build electronic circuits. The fourth fundamental element called amemristor has recently emerged. The memristor was originally proposed by Chua in

!"# however, it remained largely a theoretical concept until the demonstration of actual

fabricated devices exhibiting the characteristics of a memristor by $illiams in %&&'. Thenew two-terminal passive element is named memristor as it combines the behavior of a

memory and a resistor. (ne of the basic properties of a memristor, resistance, depends on

the magnitude, direction, and duration of the voltage applied across its terminals.

)emristor remembers its most recent resistance value when applied voltage was turnedoff and until the next time applied voltage is turned on. A memristor has several

interesting properties, including pinched hystersis and dynamical- negative resistance thatcan have a significant impact on nanoelectronics. All four fundamental elements alongwith the memristor are presented in *ig. fora comparative perspective.


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voltage across the resistor multiplied by the current through the resistor.

Capaitor

A capacitor is an electrical8electronic device that can store energy in the electric

field between a pair of conductors. The process of storing energy in the capacitor isnown as 9charging9, and involves electric charges of eual magnitude, but opposite

polarity, building up on each plate. $hen a capacitor is connected to a current source,

charge is transferred between its plates at a rate4i1t27d1t28dt.

In!"tor

An 9ideal inductor9 has inductance, but no resistance or capacitance, and does not

dissipate energy. Inductance is an effect which results from the magnetic field that forms

around a current-carrying conductor. :lectric current through the conductor creates amagnetic flux proportional to the current. A change in this current creates a change in

magnetic flux that, in turn, generates an electromotive force 1:)*2 that acts to opposethis change in current. Inductance is a measure of the amount of :)* generated for a unit

change in current. *or an inductor,

v1t27;di8dt

There are four fundamental circuit variables4 electric current, voltage, charge, and

magnetic flux. *or these variables, we have resistors to relate current to voltage,

capacitors to relate voltage to charge, and inductors to relate current to magnetic flux, butwe were missing one to relate charge to magnetic flux. That is where the )emristor

comes in.


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CHAPTER #

HI$TOR%

*or nearly


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In the case of linear elements, in which ) is a constant, memristance is identicalto resistance and, thus, is of no special interest. owever, if ) is itself a function of ,

yielding a nonlinear circuit element, then the situation is more interesting. The iGv

characteristic of such a nonlinear relation between and D for a sinusoidal input isgenerally a freuency-dependent ;issaHous figure, and no combination of nonlinear

resistive, capacitive and inductive components can duplicate the circuit properties of a

nonlinear memristor 1although including active circuit elements such as amplifiers can doso2. @ecause most valuable circuit functions are attributable to nonlinear device

characteristics, memristors compatible with integrated circuits could provide new circuit

functions such as electronic resistance switching at extremely high two-terminal devicedensities. owever, until now there has not been a material realization of a memristor.

$e are aware of over && published papers going bac to at least the early !&s

in which researchers observed and reported unusual hysteresis in their current-voltageplots of various devices and circuits based on many different types of materials and

structures. In retrospect, we can understand that those researchers were actually seeing

memristance, but they were apparently not aware of it. )emristor postulated in a seminal!" paper in the I::: Transactions on Circuit Theory by an :lectrical :ngineer

3rofessor ;eon Chua at the ?niversity of California, @ereley. The hold-up over the last

>" years, according to professor Chua, has been a misconception that has pervaded

electronic circuit theory. That misconception is that the fundamental relationship inpassive circuitry is between voltage and charge.

Anyone familiar with electronics nows the trinity of fundamental components4the resistor, the capacitor, and the inductor. 3rofessor Chua predicted that there should be

a fourth element4 a memory resistor, or )emristor. Juch a device, he figured, would

provide a similar relationship between magnetic flux and charge that a resistor givesbetween voltage and current. In practice, it will act lie a resistor whose value could vary


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according to the current passing through it and which would remember that value even

after the current disappeared. As, 3rofessor ;eon Chua pointed out in !", for the sae

of the logical completeness of circuit theory# a fourth passive element should in fact beadded to the list. e named this hypothetical element, lining flux and charge, the

E)emristor5. @ut no one new how to build one.

@uilding on their groundbreaing research in nanoelectronics, Jtanley $illiams

1Jenior *ellow, Information and Kuantum Jystems lab, 3 ;abs2, and team are the first

to prove the existence of the )emristor. They were the first to understand that thehysteresis that was being observed in the I-6 curves of a wide variety of materials and

structures was actually the result of memristance and something more general that can be

called memristive behavior. Then they went on to create an elementary circuit model

that was defined by exactly the same mathematical euations as those predicted by3rofessor Chua for the )emristor, with the exception that this model had an upper bound

to the resistance 1which means that at large bias or long times, it is a memristive device2.

Low, >" years later, electronics have finally gotten small enough to reveal the secrets of

that fourth element within the electrical characteristics of certain nanoscale devices.


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CHAPTER &

MEMRI$TOR THEOR%

The schematic diagram of a memristor for the purpose of characteristic analysis is

presented in *ig. . The figure depicts that the bias voltage causes a drift of the dopants

and electrically divides the memristor to doped and undoped regions. A small dopedregion 1which means a large undoped region2 provides higher resistance and a large

doped region 1which means a small undoped region2 provides lower resistance. Thus,transforming resistance between low and high values. *or the purpose of analysis, the

following parameters are assumed for the memristor4

MLactive4 the total active length of the memristor. This remains fixed once a memristor is

manufactured.

M ldoped1t24 the doped active length of the memristor. This change with the voltage

applied across the two terminals.MRdoped4 resistance of the doped layer of length Lactive. This is euivalent to (L-state

resistance of the memristorR(L as used in some literature.MRundoped4 resistance of the undoped layer of lengthLactive. This is euivalent to (**-state resistance of the memristorR(** as used in some literature.

M v1t24 the applied biasing voltage across the memristor.

M q1t24 the resulting charge in the memristor.M i1t24 the resulting current through the memristor.

M 4 the average carrier mobility.


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@y applying Nirchhoff5s voltage law 1N6;2 on the euivalent circuit of the

memristor, the following expression is obtained4

*or linear drifting with uniform field, the doped active length ldoped1t2 is the

product of carrier velocity and carrier drifting time. The carrier velocity is obtained as

product of and applied electric field of the following form417v1t28Lactive2.The drifting time can be calculated from the ratio of charge and current of

1q1t28i1t22. Thus, the following expression can be deduced4

Assuming that Rdoped is very small compared to Rundoped, the following is derived

from 124

@y substituting 1%2 in 1>2, the following expression is obtained for the

memristance 1M24


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CHAPTER '

T%PE$ O( MEMRI$TOR

)emristors can be of various types depending on how they are built. A briefoverview of different memristors is presented in *ig. shown below. In addition, there are

systems that, as a whole, exhibit properties of memristors and are called Bmemristive

systems. Titanium dioxide 1Ti(%2 thin film memristors were the first to be built andwidely explored for modeling and design. 3olymeric or ionic memristors utilized

dynamic doping of polymer and inorganic dielectric-type 1some form of dioxide2

materials. In this type of memristors, solid-state ionics 1either cationic or anionic2 movethroughout the structure as the charge carriers. The resonant-tunneling diode memristors

use specially doped uantum-well diodes. The manganite memristors use a substrate of

bilayer oxide films based on manganite as opposed to Ti(%. In spintronic memristors, the

direction of the spin of electrons change the magnetization state of the device, which

conseuently changes its resistance. In the spin-transfer torue memristors, the relativemagnetization alignment of the two electrodes affect the magnetic state of a magnetic

tunnel Hunction, which in turn changes its resistance.

There are uite a few vectors of inuiry researching various types of memristors.

The material implementation of a memristor is important to how they behave in a

memristive system. It is important to understand the difference between a memristor, anda memristive system, because the specific type of memristor can highlight different

strengths and weanesses, and they can be used in a memristive system for different

applications of scale or purpose. There are currently no memristor datasheets available, asmuch of the material implementations are experimental and in development. In general,

though, for any material, ysterisis, an accelerating rate of change of a property as

obHects move from one state to another, is an indicator of memristive properties.


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Currently ewlett 3acard5s version of the Titanium /ioxide susbtrate memristor

is the most generally pursued type of memristor, but the list of different memristor types

below shows there are a wide variety of systems that exhibit memristive behavior, andmore are being discovered as industries begin to build out their research, prototyping, and

manufacturing infrastructures.

1) Mole"lar an! Ioni Thin (ilm Memristive $*stems

These type of memristors primarily rely on different material properties of thin

film atomic lattices that exhibit hysterisis under the application of charge.

a2. Titanium dioxide memristors

The Titanium /ioxide memristor first developed at 3 ;abs is based on a two-layer thin Bsandwich of titanium dioxide films, composed of symmetrical lattices of

titanium and oxygen atoms. 1Titanium dioxide changes its resistance in the presence of

oxygen, which is why it5s used in oxygen sensors.2 The motion of atoms in the films aretied to the movement of electrons in the material, which allows a state change in the

atomic structure of the memristor. The bottom layer acts as an insulator, and the top film

layer acts as a conductor via oxygen vacancies in the titanium dioxide. The oxygen

vacancies in the top layer are moved to the bottom layer, changing the resistance, andmaintaining the state. To access the memristive properties, crossbars of nanowires are

placed above and below the top and bottom layers, so that a charge can be passed

through.

It5s interesting to note that Jtan $illiams at 3 came to the material property oftitanium dioxide memristive effects in part through his interests in the miniaturization of

sensor technology for distributed sensing.

b. 3olymeric 1ionic2 memristors

?tilizing the properties of various solid-state ionics, one component of the

material structure, the cationic or anionic, is free to move throughout the structure as a

charge carrier. 3olymeric memristors explore dynamic doping of polymer and inorganic

dielectric-type material to attempt and provoe hysterisis type behaviors. ?sually, asingle passive layer between an electrode and an active thin film attempt to exaggerate

the extraction of ions from the electrode. The terms polymeric and ionic are often used

somewhat loosely and generically.

c. )anganite memristive systems

A substrate of bilayer oxide films based on manganite, as opposed to titanium

dioxide, were exhibited as describing memristive properties at the ?niversity of ouston

in %&&.


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d. 0esonant-tunneling diode memristors

Certain types of uantum-well diodes with special doping designs of the spacer

layers between the source and drain regions have been shown to exhibit memristiveproperties.

e. Jilicon (xide memristors

0esearchers have developed silicon oxide memristive substrates that showpromise for transitioning much of the worlds current fab and production infrastructure to

memristor production.

#) $pin +ase! an! Ma,neti memristive s*stems

Jpin-based memristive systems, as opposed to molecular and ionic nanostructure

based systems, rely on the property of degree of freedom in electron spin. In these typesof system, electron spin polarization is altered, usually through the movement of a

magnetic Bdomain wall separating polarities, allowing for hysteresis lie behaviors tooccur.

a2. Jpintronic )emristors

A type of magnetic memristor under development by several labs, notably

Jeagate, is called a spintronic memristor. In same way that the titanium dixoidememristor changes state by altering oxygen vacanccies between two seperate layers,

changing a spintronic memristors resistance state uses magnetization to alter the spin

direction of electrons in two different sections of a device. Two sections of differentelectron spin directions are ept separate based on a moving Bwall, controlled by

magnetization, and the relation of the wall dividing the electron spins is what controls the

devices overall resistance state.

b2. Jpin Torue Transfer 1JTT2 )0A)

Jince the !!&s, the development of )0A) has shown, in certain cases,

memristive properties. The configuration nown as a spin valve, the simplest structure for

a )0A) bit, allows for state change. The resistance in a memristive effective spin-

torue transfer is controlled by a spin torue induced by a current flowing through amagnetic Hunction, and is dependent on the difference in spin orientation between the two

sides of the Hunction. /epending on the material used to construct some )0A) bits,

these spin torue constructions can exhibit both ionic and magnetic properties, and aresometimes referred to as Bsecond-order memristive systems.

&) &-terminal memistors


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As an early outlier from the !&s, the advanced technology of :lectroplating,

was used to demonstrate the viability of a non solid state, three terminal memristor by

@ernard $idrow at Jtanford. The conductance was described as being controlled by thetime integral of current. Interesting to note here is the research was part of a larger

research proHect into the mathematics of early neural networ modeling. The Adaptive

;inear :lement of $idrow 1and his then-student Ted off, of Intel fame2 is a single layerneural networ based on the )cCulloch-3itts neuron, and shows that even in the early

days, the modeling of memristive systems was closely related to neuronal learning

algorithms.


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CHAPTER .

TITANIUM DIO/IDE MEMRI$TOR

Titanium dioxide 1Ti(%2 thin film memristors were the first to be built and widely

explored for modeling and design. The Ti(% thin film memristor consists of the

following distinct layers4

M ;ayer 4 the bottom titanium8platinum 1Ti83t2 bilayer electrode

M ;ayer %4 the active Ti(%layerM ;ayer >4 the active Ti(%with excess oxygen 1 Ti(%Ox 2 layer

M ;ayer =4 the top titanium8platinum 1Ti83t2 bilayer electrode.

The top and bottom Ti83t electrodes are metal connections. The Ti(%Ox with

excess oxygen provides charge carriers when voltage is applied across the top8bottomelectrodes. The charge carriers then flow toward the active Ti(%layer, thus changing the

resistance of the Ti(%layer and that of the memristor 1this decreases the resistance2. (nthe other hand, if the current direction is reversed through the memristor electrodes, then

the excess charge carriers from the Ti(% layer moves toward the Ti(%Ox layer 1this

increases the resistance2.


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CHAPTER 0

MODE O( MEMRI$TOR (ROM HP A+$

In %&&', thirty-seven years after Chua proposed the memristor, Jtanley $illiamsand his group at 3 ;abs realized the memristor in device form. The 3 model of the

memristor is described below. To realize a memristor, they used a very thin film of

titanium dioxide 1Ti(%2. The thin film is sandwiched between two platinum 13t2 contactsand one side of Ti(% is doped with oxygen vacancies. The oxygen vacancies are

positively charged ions. Thus, there is a Ti(%Hunction where one side is doped and the

other side is undoped. The device established by 3 is shown in *ig.

*ig. Jchematic of 3 memristor

In *ig., / is the device length and w is the length of the doped region. 3ure Ti(%is a semiconductor and has high resistivity. The doped oxygen vacancies mae the Ti(%-

x material conductive. The woring of the memristor established by 3 is described in.

$hen a positive voltage is applied, the positively charged oxygen vacancies in the Ti( %-x layer are repelled, moving them towards the undoped Ti(% layer. As a result, the

boundary between the two materials moves, causing an increase in the percentage of the

conducting Ti(%-x layer. This increases the conductivity of the whole device. $hen anegative voltage is applied, the positively charged oxygen vacancies are attracted, pulling

them out of Ti(% layer. This increases the amount of insulating Ti(%, thus increasing the

resistivity of the whole device. $hen the voltage is turned off, the oxygen vacancies donot move. The boundary between the two titanium dioxide layers is frozen. This is how

the memristor remembers the voltage last applied. *igure below explains the behavior of

the memristor model when positive and negative voltage is applied. *ig. 1a2 shows the

thin film of titanium-dioxide where one side is doped with positive oxygen vacancies andthe other side is the undoped. *ig. 1b2 shows the behavior when a positive voltage is

applied. The positive oxygen vacancies are repelled and they move towards the undoped


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Ti(%layer, reducing the percentage of the insulating Ti(%, thus decreasing the resistivity.

*ig. 1c2 shows the behavior when a negative voltage is applied. The positive oxygen

vacancies are attracted and they move towards the doped Ti(%-x layer, increasing thepercentage of the insulating Ti(%, thus increasing the resistivity.

*ig. @ehavior of 3 memristor when positive and negative voltages are applied.


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CHAPTER 2

(A+RICATION O( MEMRI$TOR

The above memristor was built using standard photolithography processes on anon-silicon substrate. The fabrication steps are depicted in *ig. A few-nanometers-thic

Ti83t bilayer is deposited on the silicon substrate by electron beam evaporation. This is

followed by deposition of a layer of Ti(%by radio freuency magnetron sputtering atroom temperature. The Ti(%with an excess oxygen 1 Ti(%Ox 2 layer is then formed using

the same process. The Ti(%Ox layer is made nonstoichiometric with the addition of

excess oxygen atoms by flowing oxygen gas during the deposition. These Ti( %layers arethe active layers of the memristor device. An additional layer of Ti83t bilayer is deposited

for the top electrode, thus resulting in a complete memristor.


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CHAPTER 3

4-I CHARACTERI$TIC$

The current versus voltage 1I-62 characteristic of the memristor is presented in*ig., which shows its pinched hysteresis effect. The change in the slope of the I-6

characteristic demonstrates a switching between different resistance states# where the

resistance is positive when the applied voltage increases and negative when it decreases.The symmetrical voltage bias results in double-loop I-6 hysteresis, which can collapse to

a straight line for high freuencies.

This new circuit element shares many of the properties of resistors and shares the

same unit of measurement 1ohms2. owever, in contrast to ordinary resistors, in whichthe 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 phenomenon can be understood graphically in terms of the relationship

between the current flowing through a 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. owever, for

memristors a similar graph is a little more complicated as shown in *ig.1a2 illustrates the

current vs. voltage behavior of memristance.In contrast to the straight line expected from

most resistors the behavior of a memristor appear closer to that found in hysteresis curves

associated with magnetic materials. It is notable from *ig. 1a2 that two straight line

segments are formed within the curve. These two straight line curves may be interpreted

as two distinct resistance states with the remainder of the curve as transition regions

between these two states.

*ig.1a2Current vs. 6oltage curve demonstrating hysteretic effects of

memristance.*ig.(a) illustrates an idealized resistance behavior demonstrated in

accordance with *ig.1b2 wherein the linear regions correspond to a relatively high


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resistance 10H2 and low resistance 10L2 and the transition regions are represented by

straight lines.


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CHAPTER 5

AD4ANTA6E$

ave properties which cannot be duplicated by the other circuit elements1resistors, capacitors, and inductors2.

The important property that memristor have, which cannot be duplicatedby other circuit elements is Bmemory. )emristor remembers its most recent

resistance value when applied voltage was turned off and until the next time

applied voltage is turned on. A memristor has several interesting properties,including pinched hystersis and dynamical- negative resistance, which can have a

significant impact on nanoelectronics.

Capable of replacing both /0A) and hard drives.

)emristors can be combined into devices called crossbar latches, which could

replace transistors in future computers, taing up a much smaller area. They can

also be fashioned into non-volatile solid-state memory, which would allow greaterdata density than hard drives with access times potentially similar to /0A),

replacing both components.

Jmaller than transistors while generating less heat.

)emristors are smaller than transistors, hence reducing the buliness of thecircuits. At the same time, the power loss in memristor is also very less

considering transistors.

$ors better as it gets smaller which is the opposite of transistors.

Jome scientists say that, as memristor size reduces, it wors more accurately.

@ut, this is not the case of transistors.

/evices storing && gigabytes in a suare centimeter have been created using

memristors.

3 prototyped a crossbar latch memory using the devices that can fit &&

gigabits in a suare centimeter.

Kuicer boot-ups.

The memristors memory has conseuences4 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. @ut because a memristor can


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remember voltages, a memristor-driven computer would arguably never need a

reboot. Pou could leave all your $ord files and spreadsheets open, turn off your

computer, and go get a cup of coffee or go on vacation for two wees, says$illiams. $hen you come bac, you turn on your computer and everything is

instantly on the screen exactly the way you left it, that eeps memory powered.

3 says memristor-based 0A) could on day replace /0A) altogether.

0euires less voltage 1and thus less overall power reuired2.

)emristors reuire only very less amount of voltage for its operation. This in

turn reduces the power consumption of the circuits. This maes it more useful.


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CHAPTER 17

DI$AD4ANTA6E$

Lot currently commercially available.

0ight now, 9the biggest impediment to getting memristors in the maretplaceis having so few people who can actually design circuits using memristors,9

$illiams says. Jtill, he predicts that memristors will arrive in commercial circuits

within the next three years. 0esearchers say that no real barrier preventsimplementing the memristor in circuitry immediately. @ut its up to the business

side to push products through to commercial reality.

Current versions only at 8&th the speed of /0A).

3 has reported that its version of the memristor is about one-tenth the speed

of /0A).

as the ability to learn but can also learn the wrong patterns in the beginning.

Jenior lecturer /r. Andy Thomas and his colleagues from @ielefeld?niversity construct a memristor that is capable of learning. Juch a learning

circuit may find applications, e.g., in pattern recognition. @ut ,there is a possibility

that it may learn wrong patterns in the beginning.

Jince all data on the computer becomes non-volatile, rebooting will not solve any

issues as it often times can with /0A).

0ebooting can solve many problems relating to the operation of computer.

@ut, since all the data on the computer becomes non-volatile, some times

rebooting even cannot solve these problems.

Juspected by some that the performance and speed will never match /0A) and

transistors.

Jome experiments done with memristors prove that the performance andspeed will never match with transistors and /0A).


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CHAPTER 11

POTENTIA APPICATION$

Non-volatile memor* appliations

)emristors can retain memory states, and data, in power-off modes. Lon-volatilerandom access memory, or L60A), is pretty much the first to-maret memristor

application we5ll be seeing. There are already >nm )emristors in fabrication now.Crossbar latch memory developed by ewlett 3acard is reportedly currently about one-

tenth the speed of /0A). The fab prototypes resistance is read with alternating current,

so that the stored value remains unaffected. 0osy colored industry analysts state there is

industry concurrence that these flash memory or solid state drives 1JJ/2 competitorscould start showing up in the consumer maret within % years.

o8-po8er an! remote sensin, appliations

Coupled with memcapacitors and meminductors, the complementary circuits tothe memristor which allow for the storage of charge, memristors can possibly allow for

nano-scale low power memory and distributed state storage, as a further extension of

L60A) capabilities. These are currently all hypothetical in terms of time to maret.

Cross9ar athes as Transistor Replaements

The hungry power consumption of transistors has been a barrier to bothminiaturization and microprocessor controller development. Jolid-state memristors can

be combined into devices called crossbar latches, which could replace transistors infuture computers, taing up a much smaller area. There are difficulties in this area

though, although the benefits these could bring are focusing a lot of money in their

development. Jo perhaps the Bwhere there is a will, or a dollar, there is a way adage will

get these developed. ?nless a competition war amongst industry giants becomes one ofthose patent showdowns, where companies buy out technological advances to bury them.

0emember >QR $ell, someone bought out =Q bac in %&&=, before >Q even came to

maret, and has been sitting on it ever since. And have profited greatly.

Analo, omp"tation an! ir"it Appliations

There was a trac of electrical8mathematic engineering which was largely

abandoned to stasis in the !&s, as digital mathematics and computers rose to

dominance. Analog computations embodied a whole area of research which,unfortunately, were not as scalable, reproducible, or dependable 1or politically expedient

in some cases2 as digital solutions. owever, there still exist some very important areas


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of engineering and modeling problems which reuire extremely complex and difficult

worarounds to synthesize digitally4 in part, because they map economically onto analog

models. The early wor of Lorbert $iener has already started to be revisited, after theanalog8digital split between him and Sohn vonLeumann. Analog was great, but reuired

management for scalability beyond what even the extremely complex initial digital

vaccum tube computers could provide. )emristor applications will now allow us torevisit a lot of the analog science that was abandoned in the mid !&s.

Cir"its 8hih mimi Ne"romorphi an! 9iolo,ial s*stems earnin, Cir"its;

This is a very large area of research, in part because a large part of the analog

science detailed above has to do with advances in cognitive psychology, artificialintelligence modeling, machine learning and recent neurology advances. The ability to

map peoples brain activities under )0I, CAT, and ::Q scans is leading to a treasure

trove of information about how our brains wor. @ut modeling a brain using ratiocinatedmathematics is lie using linear algebra to model calculus. Jimple electronic circuits

based on an ;C networ and memristors have been built, and used recently to modelexperiments on adaptive behavior of unicellular organisms. The experiments show thatthe electronic circuit, subHected to a train of periodic pulses, learns and anticipates the

next pulse to come, similar to the behavior of the slime mold 3hysarum polycephalum

periodic timing as it is subHected to periodic changes of environment. The recent

memristor cat brain is also getting a lot of mention. These types of learning circuits findapplications anywhere from pattern recognition to Leural Letwors. Lo more neural

pattern algorithm training on stoc maret data for the pop-sci investor4 now, you can

grow your own neural networU Sust add two drops of memristor. Lot anywhere close toreality, *PI, even in the >& years range, but very realistic in terms of helping advance the

science itself, if not the consumer maret for intelligent brains-in-a-Har.

Pro,ramma9le o,i an! $i,nal Proessin,

3rogrammable ;ogic and Jignal 3rocessing, and a variety of Control Jystemmemristor patents are out there, waiting for the microchips to fall where they may. The

memristive applications in these areas will remain relatively the same, because it will

only be a change in the underlying physical architecture, allowing their capabilities toexpand, however, to the point where their applications will most liely be unrecognizable

as related.
http://www.memristor.org/artificial-intelligence/124/slime-mold-periodic-timing-with-memristor-modelinghttp://www.memristor.org/artificial-intelligence/124/slime-mold-periodic-timing-with-memristor-modelinghttp://www.memristor.org/news/320/cat-brain-memristor-blue-gene-synaptronic-ai-chiphttp://www.memristor.org/news/320/cat-brain-memristor-blue-gene-synaptronic-ai-chiphttp://www.memristor.org/artificial-intelligence/124/slime-mold-periodic-timing-with-memristor-modelinghttp://www.memristor.org/artificial-intelligence/124/slime-mold-periodic-timing-with-memristor-modeling


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CHAPTER 1#

DE4EOPMENT$

Researhers !evelop transparent memristor tehnolo,*)

0esearchers at (regon Jtate ?niversity claim to have made a breathrough in

memristor development, creating the first memristors from cheap and readily-availablezinc tin oxide.

/ubbed the fifth circuit element, memristors - a portmanteau of memory and

resistor - was original theorised bac in !" in a paper proposing a mathematical model

for an electronic component which combines features of resistors, capacitors andinductors. Jadly, the technology of the time prevented the papers author, 3rofessor ;eon

Chua of the :lectrical :ngineering and Computer Jciences /epartment at the ?niversityof California at @ereley, from creating a prototype to prove his theory.

*ast forward a few years, and ewlett 3acards research and development labssuccessfully create the first nanoscale memristor devices, teaming up with memory

specialist ynix to commercialise memristor technology in the form of resistive 0A), or

0e0A).

The problem with 3s memristor devices is that they reuire titanium dioxide -

an expensive material. The version created at (regon Jtate, by contrast, use the farcheaper and more readily available zinc tin oxide - a material which is also, incidentally,transparent.

*lash memory has taen us a long way with its very small size and low price,explained Sohn Conley, a professor in the (J? Jchool of :lectrical :ngineering and

Computer Jcience, of his teams wor, but its nearing the end of its potential, and

memristors are a leading candidate to continue performance improvements.

As Conleys comments suggest memristors are believed to be the next big thing in

solid-state storage products, combing the benefits of non-volatile flash storage with

performance approaching that of volatile dynamic 0A). Their low power draw and highperformance mae them tempting for mobile devices, but the technology could also spell

a sea change in the way computers operate by allowing the same device to be used as

dynamic 0A) and long-term storage without sacrificing performance.

Conleys research, while concentrating on memristor technology, has also

suggested that zinc tin oxide could prove a usable alternative to the far more expensive


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indium gallium zinc oxide compound used in the construction of thin-film transistors for

displays.

The research, which was supported by funding from the ?J (ffice of Laval

0esearch, the Lational Jcience *oundation and the (regon Lanoscience and

)icrotechnologies Institute, is published in the Hournal Jolid-Jtate :lectronics.

The zinc tin oxide material used by Oregon State University means cheaper, transparent

memristors or uture electronics.

$tan


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The bioengineering approach adds another type of materials science to

computationally digital analogues of standard silicon and electricity architectures, Hoining

Jlime )olds and advances in uantum computing bits.

$ilion 9ase! Memristors: Ne8 (in!in,s

A paper detailing new findings in silicon based 0:0A) memristors, particularly

showing improved methods of substrate deposition, is out titled B0esistive switching in

silicon suboxide films.A team at ?niversity College ;ondon has developed a silicon based 0:0A) chip

that demonstrates memristive behavior, and can be fabricated only from n- and p- type

silicon and silicon oxide, and operates in ambient conditions. 0esistance contrast is up to

< orders of magnitude, switching time !&ns or shorter, and switching energy is pS8bit orlower. Jcanning Tunneling )icroscopy suggests that the individual switching elements

may be as small as &nm the table below shows the specifications achieved to date,

alongside those of conventional *lash memory, and target specifications for our next

generation of devices4

R!R"M perormance

$hile other teams in the past, such as a partnership between 0ice university and3rivatran, Inc. have been woring on silicon based substrates for awhile, the current

research utilized a novel method for both deposition and display of the pinched hysteresis

loop characteristics for memristive systems. The team has achieved on-off resistanceratios of &=4 and higher.

Cheap-an!-heer


27/32

circuits and supporting a process that is the basis for memory and learning in biological

systems. ;u hopes to build on this initial success by eventually creating an artificial cat

brain.

9$e are building a computer in the same way that nature builds a brain,9 ;u told

the Hournal#ano Letters. 9The idea is to use a completely different paradigm compared toconventional computers. The cat brain sets a realistic goal because it is much simpler

than a human brain but still extremely difficult to replicate in complexity and efficiency.9

In a mammals brain, neurons are connected to each other by synapses, which act

as reconfigurable switches that can form pathways lining thousands of neurons. )ost

importantly, synapses remember these pathways based on the strength and timing ofelectrical signals generated by the neurons.

&-D memristor hip !e9"ts

)emristors technology got a boost recently from ewlett-3acard ;abs, whichdescribed the first >-/ memristor chip at a conference in @ereley, Calif.

The )emristor and )emristive Jystems Jymposium was co-sponsored by the?niversity of California, the Jemiconductor Industry Association and the Lational

Jcience *oundation. 3 ;abs 13alo Alto, Calif.2 provided details of a prototype chip

designed by 3 researcher Kiangfei Via that staced memristor crossbar memory cells on

top of a C)(J logic chip.

9Via used imprint lithography to add a memristor crossbar on top of a C)(J

logic circuit,9 said 3 ;abs *ellow Jtan $illiams, inventor of 3s memristive memory

technology. 9e has built an integrated hybrid circuit with both transistors andmemristors.9 $illiams and 3 colleague Qreg Jnider previously proposed an *3QA inwhich configuration bits were located above C)(J transistors in a memristor crossbar.

)emristor crossbars include two titanium dioxide layers between two

perpendicular arrays of metal lines. (ne layer of titanium oxide is doped with oxygenvacancies, maing it a semiconductor. The adHacent layer is undoped, leaving it in its

natural state as an insulator.

$hen a crossbar Hunction is addressed by simultaneously applying a voltage to

one crossbar line on the top and bottom layers, oxygen vacancies drift from the doped to

the undoped layer. This causes it to begin conducting, turning 9on9 the memory bit. Thebit can again be turned 9off9 by changing the current direction, whereupon oxygen

vacancies migrate bac into the doped layer.

According to $illiams, 3 ;abs memristor-based *3QA demonstrates that aC)(J fab can mae integrated memristor8transistor circuits in three dimensions.


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Also at the symposium, Jnider unveiled a design that used memristors in their

analog mode as synapses in a neural computing architecture. )emristor crossbars are the

only technology that is dense enough to simulate the human brain, Jnider claimed,adding that the 3 ;abs crossbars are ten times denser than synapses in the human

cortex. @y stacing crossbars on a C)(J logic chip, variable resistance could mimic the

learning functions of synapses in neural networs.

3 ;abs and @oston ?niversity were recently awarded a contract by the /efenseAdvanced 0esearch 3roHects Agency to build the first artificial neural networ based on

memristors.

Also at the conference, )assimiliano /i 6entraof the ?niversity of California atJan /iego described how memristors can explain biological learning in amoebas.

Amoebas learn to change their behavior in a manner that can be explained by an ;C

circuit and a memristor.

/i 6entra also presented evidence that microscopic memristive elements arepresent in unicellular as well as multicellular organisms.
http://physics.ucsd.edu/~diventra/http://physics.ucsd.edu/~diventra/


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CHAPTER 1&

(UTURE $COPE

Although memristor research is still in its infancy, 3 ;abs is woring on a

handful of practical memristor proHects. And now $illiamss team has demonstrated a

woring memristor-transistor hybrid chip. 9@ecause memristors are made of the same

materials used in normal integrated circuits,9 says $illiams, 9it turns out to be very easyto integrate them with transistors.9 is team, which includes 3 researcher Kiangfei Via,

built a field-programmable gate array 1*3QA2 using a new design that includes

memristors made of the semiconductor titanium dioxide and far fewer transistors than

normal. :ngineers commonly use *3QAs to test prototype chip designs because they canbe reconfigured to perform a wide variety of different tass. In order to be so flexible,

however, *3QAs are large and expensive. And once the design is done, engineersgenerally abandon *3QAs for leaner 9application-specific integrated circuits.9 9$hen

you decide what logic operation you want to do, you actually flip a bunch of switches and

configuration bits in the circuit,9 says $illiams. In the new chip, these tass areperformed by memristors. 9$hat were looing at is essentially pulling out all of the

configuration bits and all of the transistor switches,9 he says. According to $illiams,

using memristors in *3QAs could help significantly lower costs. 9If our ideas wor out,

this type of *3QA will completely change the balance,9 he says. ?ltimately, the next fewyears could be very important for memristor research.

0ight now, 9the biggest impediment to getting memristors in the maretplace is

having Wso fewX people who can actually design circuits Wusing memristorsX,9 $illiamssays. Jtill, he predicts that memristors will arrive in commercial circuits within the next

three years.

When is it omin,?

0esearchers say that no real barrier prevents implementing the memristor in

circuitry immediately. @ut its up to the business side to push products through tocommercial reality. )emristors made to replace flash memory 1at a lower cost and lower

= power consumption2 will liely appear first# 3s goal is to offer them by %&>.

@eyond that, memristors will liely replace both /0A) and hard diss in the %&=-to-%& time frame. As for memristor-based analog computers, that step may tae %&-plus

years.

("t"re Researh

0ecently, researchers have defined two new memdevices- memcapacitors and

meminductor, thus realizing the concept of memory devices to capacitors and inductors.


30/32

These devices also show Epinched5 hysteresis loops in two constitutive variables- charge-

voltage for the memcapacitors and current- flux for meminductors.

)emristors are not lossless devices. As non-volatile memories, memristors do notconsume power when idle but they do dissipate energy when they are being read or

written. ence, there is a need to invent lossless non-volatile device. )emcapacitors and

meminductors are good contenders as they are lossless devices.


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CHAPTER 1'

CONCU$ION

@y redesigning certain types of circuits to include memristors, it is possible to

obtain the same function with fewer components, maing the circuit itself less expensive

and significantly decreasing its power consumption. In fact, it can be hoped to combinememristors with traditional circuit-design elements to produce a device that does

computation. The ewlett-3acard 132 group is looing at developing a memristor-

based nonvolatile memory that could be &&& times faster than magnetic diss and use

much less power. As rightly said by ;eon Chua and 0.Jtanley $illiams 1originators of

memristor2, memristors are so significant that it would be mandatory to re-write theexisting electronics engineering textboos. )emristor is the fourth fundamentalcomponent. Thus the discovery of a brand new fundamental circuit element is somethingnot to be taen lightly and has the potential to open the door to a brand new type of

electronics. 3 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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RE(ERENCE

M;. (. Chua, B)emristorFThe missing circuit element,$!!! Trans% &ircuitTheory,vol. ', no. >G>.

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