Memristor

Under the guidance of N.S Lect. Ms. VIDYA DEVI .M

By AJAY KUMAR (1BY05EC003)

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MOORE’S LAW. FUNDAMENTAL CIRCUIT ELEMENTS. THE MISSING ELEMENT.  HP’s INVENTION. OTHER SIMILAR INVENTIONS. HYBRID CHIPS. MEMRISTENCE IN SPINTRONICS .  POTENTIAL FIELDS. CONCLUSION. 

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Silicon ICs Status And Trends

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MOORE’S LAW: The number of transistors that can be placed inexpensively on an integrated circuit has increased exponentially, doubling approximately every two years. 3

FUNDAMENTAL CIRCUIT ELEMENTS :

Leon Chua suspected that a memristor should exist based primarily on symmetry. There are four fundamental circuit variables: electric current, voltage, charge, and magnetic flux. For these variables, we have resistors to relate current to voltage, capacitors to relate voltage to charge, and inductors to relate current to magnetic flux,  We are missing one to relate charge to magnetic flux, so the concept of memristor comes in. 4

Proof :  Integral of current is charge

dq/dt = I (amperes)

 Integral of voltage is flux  Resistor

dΦ/dt = V (volts)

dv = R di ,

dv/di = R

dq = C dv ,

dq/dv = C

(ohms)  Capacitor

(farad)  Inductor

dΦ = L di , dΦ/di = L (henry) 5

The missing element : Φ~q

MEMRISTOR MEM – memory, RISTOR - resistor 6

Analogous understanding of Memristance: Pipe

Water

Water

 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.  The rate of flow of the water through the pipe is like electrical   

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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. An analogy for a memristor is an interesting kind of pipe that expands or shrinks when water flows through it.  If water flows through the pipe in one direction, the diameter of the pipe increases, thus enabling the water to flow faster. If water flows through the pipe in the opposite direction, the diameter of the pipe decreases, thus slowing down the flow of water. If the water pressure is turned off, the pipe will retain its most 7 recent diameter until the water is turned back on. Thus it

hp's answer :  The Memristor team of HP on 30th April 2008

devised a modeled which was claimed to be the missing element.  The team was lead by Stanley Williams, Director of the IQSL (information & quantum systems lab),the members include : *Dmitri B. Strukov *Gregory S. Snider *Duncan R. Stewarty

 Crossbar latch 2005

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Crossbar latches

 crossbar latches can be used to perform calculations. Crossbar latches consists of a grid of microscopic wires linked by molecules at their intersections. Are smaller, potentially cheaper to make. A crossbar latch consists of three wires a "latch" wire and two control or clock wires. *The latch wire lies under the other two. *The wires are connected by molecules which transfer electrical impulses from one wire to the next.  A series of electrical impulses will close the molecular switch between the latch wire and the first clock wire, the impulses will then open the switch between the latch wire and other clock wire. A computer interprets this action as a "0“ while opening the first switch and closing the second becomes a "1." 9

Crossbar cont..  The junction between the wires is small = 2 nanometers.  The equivalent junction in current transistors inside 90

   

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nanometer chips is about 60 nanometers. Many crossbar latches can be put into the same space that now transistors hold , traditional transistors will never hit these limits . Shrinking the electrical junctions in a chip increases performance. The wires are put into place through nano-imprint lithography. A customized mold is placed into a film later & the imprints left by the mold become the templates for the wires. The molecular switches do not have to be placed individually at the juncture of the wires , Only the wires at the junctions will carry a current & all of the other molecules are left unused. 10 Drawback

Memristor :

The wires in this image are 50 nanometers wide, which comes to about 150 atoms.. 11

Memristor cont.. The HP device is composed of a thin (50 nm) titanium dioxide film between two 5 nm thick electrodes, one Ti the other Pt. Initially, there are two layers to the titanium dioxide film, one of which has a slight depletion of oxygen atoms. The oxygen vacancies act as carriers meaning that the depleted layer has a much lower resistance than the nondepleted layer. When an electric field is applied, the oxygen vacancies drift changing the boundary between the high-resistance and low-resistance layers.  When a current is applied (through the wire) to the upper layer, the vacancies are pushed into the lower level of titanium dioxide. This changes the resistance of the lower level.  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.  It is considered a nanoionic device. Memristance is displayed only when both the doped layer and depleted layer contribute to resistance. When enough charge has passed through the memristor that the ions can no longer move, the device enters hysteresis. 12

Cont.. An atomic force microscope view of a circuit with 17 memristors.

 Ion mobility of 10−10 cm2/(V·s) the highest known drift ionic mobilities occur in advanced superionic conductors, such as rubidium silver iodide with about 2×10−4 cm2/(V·s) conducting silver ions at room temperature.  Electrons and holes in silicon have a mobility ~1000 cm2/(V·s).  A relatively low bias of 1 volt can be used

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Proof :  V = R (w) * I

w = internal state

dw/dt = f ( w, i )  Physical model R(w) = { [w(t) * Ron ]+ (1-w(t))Roff } where w = 0 , Roff w = 1 , Ron  dw/dt = (Ron / β ) * i(t) ……… …………(1) β = magnetic flux in weber v(t) = { [w(t) * Ron ]+ (1-w(t))Roff } * i(t) let r = Roff/Ron = resistance ratio  Rearranging , v(t) = β{x(t) + r[1 − x(t)]} * dw(t)/dt by integrating , ϕ = β{−[(r − 1)/2 ]*(w^2) + r *w + c] …… ……………(2) c = constant of integration ϕ = quadratic function of charge.

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Memristance eq.  Writing eq (1) in physical parameters

[dx(t)/dt]/D = [μV * Ron * i(t) ] / D^2 where x(t) = co-ordinate of the boundary D = thickness of the film μV = avg ion mobility integrating, [x(t)/D ] = { μV* Ron* q(t) } / D^2  using in eq (2) ϕ = −[(Ron * μV)/2*D^2]{Roff/Ron− 1}q^2 + Roff *q if Roff ≫ Ron,  memristance , M(q) ≡ dϕ/dq = Roff {1 −(μV *Ron *q)/D^2}

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Other Players: IBM  Racetrack memories

Intel & microelectronics  PCM chips *Phase change memory *a bit on a substrate similar to discs is heated, on cooling, bits attain variable resistances representing various data. 16

IBMrepresented cont.. *Data by blue and red segments, is collected on a nanowire. It then passes through a reader that delivers the information to a computer. * The information stream on the wire can be moved back and forth. In existing memory, data sits in a fixed spot. * The wire concept could allow memory makers to increase the density of memory chips. Data can also be erased and rerecorded on the tracks.

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SPINTRONICS  Spintronics is the use of electron spin

to store information.  Electrons can spin in two ways up or down * each spin has different energy * magnetic field affects the spins  Memristors are expressed as :

 Three examples of spintronic devices:

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(1) MTJ spintronics G0 = MTJ conductance when the free-layer magnetization direction is perpendicular to the reference-layer magnetization Direction. Tunneling magneto resistance (TMR) =ratio of the difference between high and low conductance to low conductance. High conductance corresponds to the case of free-layer magnetization parallel to pinned-layer magnetization & low conductance corresponds to the case of freelayer magnetization antiparallel to pinned-layer magnetization. 19

(a) Variation of resistance, voltage wrt time. (b) Variation of current wrt voltage. (2) Thin film

(3)Spin valve

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Hybrid chips :

Combination of transistors and memristors

Less heat generation More logic gates per unit area of chip Low power comsumption 21

Potentials :  Possible Replacement for D-RAM

* A computer memory that can supplement today's commonly used dynamic random access memory (D-RAM). Computers using conventional D-RAM lack the ability to retain information once they are turned off. When power is restored to a D-RAM-based computer, a slow, energyconsuming "boot-up" process is necessary to retrieve data stored on a magnetic disk required to run the system.  Pattern recognition * Recently, a simple electronic circuit consisting of an LC contour and a memristor was used to model experiments on adaptive behavior of unicellular organisms. It was shown that the electronic circuit subjected to a train of periodic pulses learns and anticipates the next pulse to come ,similarly to the behavior of slime molds Physarum polycephalum subjected to periodic changes of environment . Such a learning circuit may find applications in pattern recognition. 22

Conclusion  Problem addressed

*The problem of heat generation due to higher densities that also defects and affects the basic physics of the devices.

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Market addressed * Semiconductor Industries * Data Storage Devices

 Researchers believe that

memristors can one day synthesize the human brain’s pattern.

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Thanks are due to :Bmsit library for ieee papers. Friends for other info. Technical FAQ by Memristor lead scientist, Stan

Williams of HP Labs.  "Talk of the Nation" interview with co-discover Stan Williams. HP Reveals Memristor, The Fourth Passive Circuit Element.  BBC News - Electronics' 'missing link' found. Scribd.com Springer.com Google.com Cuil.com 24

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Thank you for patient listening at this hour ;-) 26