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BY SWATI MALIK MTECH (VLSI) ROLL NO. MVLSI-265-2K10 MEMRISTOR BY SWATI MALIK MTECH (VLSI) ROLL NO. MVLSI-265-2K10

introduction Currently known fundamental passive elements – Resistors, Capacitors & Inductors. MEMRISTOR is the 4thpassive element. Memristors are passive two terminal circuit elements. Its resistance depends on the amount of the charge passed through it. That’s why it’s called memristor.

HISTORY and discovery MEMRISTOR Theory was formulated by Leon Chua in 1971 in his paper “memristor - the missing circuit element”. He suspected that a memristor should exist to provide symmetry with resistor, capacitor & inductor. Finally the practical model was invented by a team led by Stanley Williams of HP Labs on April 30,2008. Leon Chua

Need for memristor The known three fundamental circuit elements as resistor, capacitor and inductor relates four fundamental circuit variables as electric current, voltage, charge and magnetic flux. There are six different mathematical relations connecting pairs of four fundamental circuit variables viz. current I, voltage v, charge q, and magnetic flux Φ. Thus there should be four basic circuit elements described by the remaining relation between the variables. But for more than 30 years it was written off as a mathematical dalliance.

Memristor theory DEFINITION: “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.” Φ=Mq circuit symbol where m is the memristance. MEMRISTANCE: Memristance is simply charge-dependent resistance. UNIT = Ω

Memristor theory The memristor is static if no current is applied. If I(t)=0, then V(t)=0 and M(t) is a constant. This is the essence of the memory effect.

Construction and working reduced oxidised Memristor is composed of a thin (5nm) Titanium dioxide film between two platinum electrodes. Initially there are two layers, one slightly depleted of Oxygen atoms, other non-depleted layer. The depleted layer has much lower resistance than the non-depleted layer. Applied voltage makes the oxygen vacancies(+ve) of doped portion to shift towards the –ve voltage.

Construction and working Direction of shift can be changed by reversing the polarity. Shift between the layers is permanent in nature. It exists even after the voltage has been removed. Causes the permanent change in resistance.

Analogy of memristor Consider a kind of pipe that expands or shrinks when water flows through it. Water is analogous to charge and pressure to voltage. If water flows through the pipe in one direction, the diameter of the pipe increases, thus enabling the water to flow faster and vice versa. If the water pressure is turned off, the pipe will retain it most recent diameter until the water is turned back on. Thus it remembers how much water has flowed through it.

V-I CHARACTERISTICS The most common v-i trace is a ‘figure 8’ or a ‘pinched loop’ For this current i=0, when voltage v=0. On the application of electric field, oxygen vacancies drift, changing boundary between high & low resistance layers. Memristance is only displayed when the doped layer & depleted layer both contribute to resistance. The device enters hysteresis when enough charge has passed through memristor & ions can no longer move.

PROPERTIES Retains its resistance level even after power had been shut down. Remembers (or recalls) the last resistance it had, before being shut off. Doesn’t require energy to maintain the data storage. Memristivity has an inverse square relationship with thickness of the material, so smaller = better! Nonvolatile state can be accomplished by memristors because their state is encoded by impedance (physically), not by voltage.

Types of memristors Molecular and Ionic Thin Film Memristive Systems These type of memristors primarily rely on different material properties of thin film atomic lattices that exhibit hysteresis under the application of charge. Eg., are: Titanium dioxide memristor Polymeric (ionic) memristor Spin Based and Magnetic memristive systems These rely on the property of degree of freedom in electron spin. Here, electron spin polarization is altered to achieve hysteresis. Spintronic Memristors Spin Torque Memristors

applications CROSSBAR LATCHES Crossbar latches is a technology used with memristors to potentially replace the transistors. Allows much the same functionality of transistors but can be made far smaller than any transistor. Consists of a signal line crossed by two control lines & can simulate AND, OR & NOT gates. A memristor circuit requires lower voltage, less power and less time to turn on than competitive memory and processing devices.

applications REPLACEMENT FOR DRAM Computers using conventional D-RAM lack the ability to retain information once they are turned off or simply they are volatile. With the introduction of memristor based RAMs, there will be no time wasted booting the system. With sudden power failures also, memristors retain the last information it was working on and nothing is lost. REPLACEMENT FOR FLASH MEMORY Memristor is as a powerful replacement for flash with quick writing and rewriting capabilities. Denser cells allow memristor circuits to store more data than flash memory consuming same space.

applications ARTIFICIAL BRAIN The ability to store and retrieve a vast array of intermediate values also pave the way to a completely different class of computing capabilities like an analog computer can store anywhere from 0 to 1 unlike any other digital element which can assume only these two states. Memristor technology leads to computer systems that can remember and associate patterns in a way similar to how people do. This could be used to substantially improve face recognition technology or to provide more complex biometric recognition systems that could effectively restrict access to personal information.

advantages Creating a Analog Computer that works much faster than Digital ones. Provides greater reliability when power is interrupted in data centers. Density allows for more information to be stored. It does not require power to maintain its memory. Compatible with current CMOS interfaces.

limitations Though hundreds of thousands of memristor semiconductors have already been built, there is still much more to be perfected. No design standards (rules) have been set. The most significant limitation is that the memristors function at about half the speed of today’s DRAM memory cells. The problem of heat generation due to higher densities that also defects and affects the basic physics of the devices.

New horizons The developers of memristor are now experimenting with new devices with same memristic properties. The two such elements discovered are: Memcapacitor and Meminductor Here both capacitance and inductance depend on the history and state of the system respectively. Both of them show the peculiar pinched hysteresis loop. Unlike memristor these can store energy. memcapacitor meminductor

conclusion This is the first experimental realization and physical model for a memristor – the fourth nonlinear passive circuit element that has been ‘missing’ for nearly 30 years. It takes a lot of transistors and capacitors to do the job of a single memristor. No combination of R,L,C circuit could duplicate the memristance. So the memristor qualifies as a fundamental circuit element.

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