Resistive random access memory (RRAM) is a form of nonvolatile storage that operates by changing the resistance of a specially formulated solid dielectric material. A RRAM device contains a component called a memristor (contraction of "memory resistor"), whose resistance varies when different voltages are imposed across it.
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Normally, a dielectric material does not conduct electric current. In fact, dielectric substances are employed in capacitors for the specific purpose of preventing the flow of current and maintaining separation of electric charge poles. If a sample of dielectric material is subjected to a high enough voltage, it will suddenly conduct because of a phenomenon called dielectric breakdown. In a conventional dielectric material, breakdown causes permanent damage and failure of the associated component. In a memristor, the dielectric breakdown is temporary and reversible.
In the memristor, a deliberately applied voltage causes the medium to acquire microscopic conductive paths called filaments. The filaments appear as a result of various phenomena such as metal migration or physical defects. Once a filament appears, it can be broken or reversed by the application of a different external voltage. The controlled formation and destruction of filaments in large numbers allows for storage of digital data. Numerous substances have been tested for memristor characteristics, including nickel oxide, titanium dioxide, various electrolytes, semiconductor materials, and even some organic compounds.
High switching speed constitutes a principal advantage of RRAM over other nonvolatile storage technologies. Timescales as short as 10 nanoseconds have been observed. Memristor filaments can occur in dimensions as small as a few nanometers, a tiny fraction of the wavelength of visible light in free space, offering the promise of high storage density. The occasional formation of unintended filaments (called "sneak paths") presents a challenge for engineers intent on the large-scale development of memristor technology and RRAM devices.