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Memristor BasedMultilevel Memory

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Memristor Based Multilevel Memory

In anticipating the end of Moores law a decade from now, many new approachesto extend the end of this law have been proposed by the memory industry. Oneapproach is to develop the Multi-Level Cell (MLC) technology which storesmultiple bits in a multilevel form of information in a memory element.

Commercially available MLC NAND memories can store four states per cell inthe current technology. Most approaches are the transistor-based PRAM (PhaseChange RAM) , except HPs resistance-based RRAM .

Recently, Stanley Williams et al. from HP had developed a remarkable memoryelement called memristor which is based on the pinched hysteresis loop exhibited

by Titanium Dioxide thin films, when sandwiched between Platinum electrodes.

The memristor was postulated by Leon O. Chua as the fourth basic element of electrical circuits in 1971. It is based on the nonlinear characteristics of the electro-magnetic mechanism in nature.

ADVANTAGES

There are several advantages of the memristor memory over conventionaltransistor-based memories. One is its strikingly small size. Though memristor isstill at its early development stage, its size is at most one tenths of its RAMcounterparts. If the fabrication technology for memristor is improved, the size andadvantage could be even more significant.

Another feature of the memristor is its incomparable potential to store analoginformation which enables the memristor to keep multiple bits of information in amemory cell.

PRACTICAL DIFFICULTIES AND IMPLEMENTATION

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A method to utilize the memristor as a multilevel memory has been proposed.There are several roadblocks in the practical use of memristors for multilevelmemory.

A difficulty comes from the nonlinearity in the vs. q curve which makes itdifficult to determine the proper pulse width for desired resistance values. Another one comes from the property of the memristor which integrates any kind of signalsincluding noise that appeared at the memristor and causes memristors to be

perturbed from their original values.

The proposed method enables the memristor to be used as multilevel memoryusing a reference resistance array by forcing the memristor to stick at a set of

predetermined fixed reference resistance values. We propose the write-in

(programming) circuit and the read-out/restoration circuit which share theinformation storing technique using the reference resistance array .

REFERENCE RESISTANCE ARRAY-BASEDMULTILEVEL MEMORY OF MEMRISTOR

The proposed method has the operating point of the memristor be maintained itsdesired location (or resistance value) utilizing a set of pre-determined multipleresistance levels. Fig. 3 shows the basic idea of the proposed method, where theResistance array to be referenced and the memristor to be programmed (tuned) areshown. The goal is to have the memristor keep any of the resistance level selectedfrom the resistance array.

If a predetermined magnitude of the current Pulse I s(t ) is applied to theresistance array, different levels of voltages Vk will appear at each node of theresistance array. The same current pulse I s(t ) is also applied to the memristor.

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The programming (tuning) of the memristor is performed byapplying additional current pulses to the memristor with the appropriate directionsuntil the voltage of the memristor equals to that of the selected node voltage in theresistance array. If the voltage of the memristor reaches that of the selected node,the resistance value of the memristor becomes the same as the partial sum of theresistance from the ground to the selected node of the resistance array.

This idea is employed in both the write-in and the read-out/restoration circuits. Detailed description of these circuits will be presented inthe following sections.