MEMRISTOR and Memristive

Nanoarchitectures

CONTENTS Introduction Symmetry of relationships Memristor theory How memristance works Crossbar architecture Device structure An Analytical Approach for Memristive Nanoarchitectures Complementary resistive switch Conclusion Reference

INTRODUCTION The three fundamental passive circuit elements are:- • Capacitor • Resistor • Inductor

Defined by four circuit variables:-i, v, q,Ψ

L

V I

Q

q=Cv

Ψ

V I

Q

q=Cv

Resistors

v=Ri

Capacitors

Ψ

V I

Q

Capacitors q=Cv

Resistors

v=Ri

IndInductorsΨ=Li

v=dΨ/dt i=dq/dt

?

?

Ψ

INTRODUCTION

Possible combinations of two variables gives:-1. dΨ/dt=V2. dq/dt=I

Other three defines common circuit elements:1. dv/di=R Resistance2. dΨ/di=L Inductance3. dq/dv=C Capacitance4. dΨ/dq=?

INTRODUCTION MEMRISTOR which connects Ψ and q. Proposed by LEON.O.CHUA in 1971,a professor of electrical

engineering at California.

Relationship diagram.

Memristor symbol

LEON.O.CHUA

V I

Q Ψ

Capacitors q=Cv

Resistors

v=Ri

InductorsΨ=Li

v=dΨ/dt i=dq/dt

Ψ=Mq

Memristors

INTRODUCTION MEMRISTOR is a contraction of “memory resistor”.

It can remember the current passing through it even after the current has disappeared.

It is a two terminal 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.

Analogy: a pipe through which water flows.

The material: thin film of Titanium dioxide.

The ability to indefinitely store resistance values means that MEMRISTOR can be used as a nonvolatile memory.

MEMRISTOR THEORY

The memristance function describes the charge-dependent

rate of change of flux.

M(q)= d Ψ/dq

This can be written as:

M(q)= (d Ψ/dt)/(dq/dt) = v /I

Thus M(q) has the unit of resistance.

A crossbar architecture is a fully connected mesh of perpendicular wires.Any two crossing wires are connected by switch.To close the switch, a positive voltage is applied across two wires to be connectedTo open the switch the voltage is reversed.

How Memristance Works

CROSSBAR ARCHITECTURE

DEVICE STRUCTURE Made by sandwiching a thin film of titanium dioxide TiO₂

between two platinum electrodes.

Oxidize the surface of the bottom Platinum wire to make an extremely thin layer of platinum dioxide, which is highly conducting.

Next assembled a dense film, only one molecule thick, of specially designed switching molecules.

Over this, deposited a 2-3nm layer of titanium metal which bonds strongly to the molecules.

Final layer was the top Platinum electrode.

How Memristor Works A positive voltage on the switch repels the oxygen

deficiencies in the metallic upper TiO2-x layer,sending them into the insulating TiO2 layer below.

That causes the boundary between the two materials to move down,increasing the % of conducting TiO2-x and thus the conductivity of the entire switch.

The more positive voltage applied,the more conductive the cube becomes.

A negative voltage on the switch attracts the positively charged oxygen bubbles,pulling them out of the TiO2.

The amount of insulating, resistive TiO2 increases ,there by making the switch as a whole resistive.

The more negative voltage is applied,the less conductive the cube becomes.

How Memristor memorizes

When the voltage turned off,positive or negative,the oxygen bubbles do not migrate. They stay where they are,which means that the boundary between the two titanium dioxide layers is frozen.

That is how the memristor “remembers” how much voltage was last applied.

What Sets Memristor Apart?Conventional devices use only 0 and 1; Memristor can

use anything between 0 and 1.By changing the speed and strength of the current, it

is possible to change the behaviour 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.Faster than Flash memory.

Innovating nanotechnology due to the fact that it performs better the smaller it becomes.

An Analytical Approach for MemristiveNanoarchitectures

The imposed limitation was addressed by Linn et al. through adaptation of two series memristive elements connected with opposing polarities. This structure is referred to as complementary resistive switch (CRS)

The unique aspect of this device is in using a series of high resistance states (HRS), RHRS, and low resistance states (LRS), RLRS,

to introduce logic “0” and logic “1.” As an example, an LRS/HRS combination represents “1” and an HRS/LRS state represents “0.”

Complementary resistive switchThe CRS is fabricated using Ag/Tio2/Tio(2-

x).Device operating in -4 v to 4 v range

Working of CRSFor a read operation

Vth,set<Vread<Vth,resetFor a write operation Vth,reset<VwriteVoltage below Vth,set does not alter device

state

Research are going on for hardware circuit based memristors which emulates brain like function.

It should be also 1000 or million times faster than running program on digital computer.

The first commercial circuit using memristor is to arrive within next three years.

Memristor will change circuit design in the 21st century as radically as the transistor changed in the 20th .

CRS array is less affected by the programing and read error rate that limit nanocrossbar array size compared to MEMRISTIVE array

CONCLUSION

ReferencesL. O. Chua, “Memristor—The missing circuit

element,” IEEE Trans.Circuit Theory, vol. 18, no. 5, pp. 507–519, Sep. 1971.

Omid Kavehei, Said Al-Sarawi, Kyoung-Rok Cho, Kamran Eshraghian, “ An Analytical Approach for Memristive Nanoarchitectures”, IEEE Transactions on nanotechnology, vol. 11, no. 2, March 2012

L. Chua, “Resistance switching memories are memristors,” Appl. Phys.A: Mater. Sci. Process., vol. 102, pp. 765–783, 2011.

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