RRAM, also known as ReRAM (resistive random access memory), is a form of nonvolatile storage that operates by changing the resistance of a specially formulated solid dielectric material. An RRAM device contains a component called a memristor -- a 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 because of the materials used.
In one form of 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.
Another memristor form uses the voltage to cause a change in the state of an amorphous solid called a chalcogenide glass, flipping it rapidly from a hardened state to a more fluid-like state, which changes the resistance of the material.
ReRAM specifically works by using the method of creating physical defects in a layer of oxide material. These defects are called oxygen vacancies, and the ReRAM works like a semiconductor but with oxygen ions; these vacancies represent two values in a binary system, instead of a semiconductors' electrons and holes.
General pros and cons
Higher switching speed constitutes a principal advantage of RRAM over other nonvolatile storage technologies such as NAND flash. 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.
ReRAM and other memristor technologies also draw much less power than NAND flash. That makes them currently best suited for memory in sensor devices for industrial, automotive and internet of things (IoT) applications. As the cost of manufacturing for ReRAM and other memristors drops, they become competitive with NAND flash. The higher memory density, faster read and write speeds, and lower power draw are reasons why memristor-based memory technologies are often cited as the logical replacement in applications like solid-state drives (SSDs) and nonvolatile dual in-line memory modules (NVDIMMs).
Other memristor technologies
In addition to ReRAM, other memristor technologies in development or on the market include conductive-bridging RAM (CBRAM) and phase-change memory (PCM). CBRAM uses a layer of electrolytic material through which the conductive filaments are created or destroyed. The resistance in the dielectric material decreases or increases based on if the filament exists or not. This technology is currently on the market in ultra-low power memory for IoT applications from Adesto Technologies.
In contrast, PCM applies a current to the dielectric material to change the state of an amorphous solid -- essentially, a type of glass known as chalcogenide -- from more solid to less solid. Initially, the development was focused on finding the best material for achieving these two states, but more recent work is on allowing for the material to hold multiple states of varying levels of crystalline or amorphous states. The 3D XPoint technology developed jointly by Intel and Micron Technology is based on a lattice structure of a chalcogenide glass material that can hold four states.
In November 2016, Fujitsu Semiconductor started sales of a 4 megabyte (MB) ReRAM chip it co-developed with Panasonic Semiconductor Solutions. At the Flash Memory Summit that same August, Western Digital announced it would use 3D ReRAM in upcoming specialized SSDs, replacing NAND flash memory. No date for the release of these SSDs has been announced, however.
In October 2016, 4DS Memory Limited announced that it had developed ReRAM chips small enough to stack like 3D NAND flash, but with greater memory density.
Israeli firm Weebit Nano is developing ReRAM products based on silicon oxide, which means the ReRAM chips can be manufactured in existing fabs without having to retool the equipment.
In January 2017, Crossbar Inc. began production of ReRAM in the facilities of its partner, Semiconductor Manufacturing International Corp.
Other vendors are producing nonvolatile memory based on similar memristor technologies. Intel released its first 3D XPoint products in March 2017 under the brand name Optane. Aerospace and defense contractor BAE Systems also makes a PCM-based chip it uses in a radiation-hardened form.