Non-volatile random-access memory

Non-volatile random-access memory (NVRAM) is random-access memory that retains its information when power is turned off (non-volatile). This is in contrast to dynamic random-access memory (DRAM) and static random-access memory (SRAM), which both maintain data only for as long as power is applied.

The best-known form of NVRAM memory today is flash memory. Some drawbacks to flash memory include the requirement to write it in larger blocks than many computers can automatically address, and the relatively limited longevity of flash memory due to its finite number of write-erase cycles (most consumer flash products at the time of writing can withstand only around 100,000 rewrites before memory begins to deteriorate). Another drawback is the performance limitations preventing flash from matching the response times and, in some cases, the random addressability offered by traditional forms of RAM. Several newer technologies are attempting to replace flash in certain roles, and some even claim to be a truly universal memory, offering the performance of the best SRAM devices with the non-volatility of flash. To date these alternatives have not yet become mainstream.

Early NVRAMs

Early computers used a variety of memory systems, some of which happened to be non-volatile, although not typically by design but simply as a side-effect of their construction. The most common form of memory through the 1960s was magnetic-core memory, which stored data in the polarity of small magnets. Since the magnets held their state even with the power removed, core memory was also non-volatile. Such memory contrasted sharply with memory based on active electronic devices, originally tube (or thermionic valve) based flip-flop devices, and later semiconductor based flip-flop (SRAM), or even charge storage systems (DRAM).

Rapid advances in semiconductor fabrication in the 1970s led to a new generation of solid state memories that core simply could not compete with. Relentless market forces have dramatically improved these devices over the years, and today the low-cost and high-performance DRAM forms the vast majority of a typical computer's main memory. However there are many roles where non-volatility is important, either in cases where the power will be removed for periods of time or where the constant power needs of DRAM conflicts with low-power devices. For many years, there was no practical RAM-like device to fill this niche, and many systems used a combination of RAM and some form of ROM for these roles.

Custom ROM was the earliest solution, but had the disadvantage of being able to be written to only once, when the chip was initially designed. ROMs consist of a series of diodes permanently wired to return the required data, the diodes being built in this configuration when they are being fabricated.

PROM improved on this design, allowing the chip to be written electrically by the end-user. PROM consists of a series of diodes that are initially all set to a single value, "1" for instance. By applying higher power than normal, a selected diode can be "burned out" (like a fuse), thereby permanently setting that bit to "0". PROM was a boon to companies who wished to update the contents with new revisions, or alternately produce a number of different products using the same chip. For instance, PROM was widely used for game console cartridges in the 1980s.

Those who required real RAM-like performance and non-volatility typically have had to use conventional RAM devices and a battery backup. This nonvolatile BIOS memory, often called CMOS RAM or parameter RAM, was a common solution in earlier computer systems like the original Apple Macintosh, which used a small amount of memory powered by a battery for storing basic setup information like the selected boot volume. Much larger battery backed memories are still used today as caches for high-speed databases, requiring a performance level newer NVRAM devices have not yet managed to meet.

The floating-gate transistor

A huge advance in NVRAM technology was the introduction of the floating-gate transistor, which led to the introduction of erasable programmable read-only memory, or EPROM. EPROM consists of a grid of transistors whose gate terminal (the "switch") is protected by a high-quality insulator. By "pushing" electrons onto the base with the application of higher-than-normal voltage, the electrons become trapped on the far side of the insulator, thereby permanently switching the transistor "on" ("1"). EPROM can be re-set to the "base state" (all "1"s or "0"s, depending on the design) by applying ultraviolet light (UV). The UV photons have enough energy to push the electrons through the insulator and return the base to a ground state. At that point the EPROM can be re-written from scratch.

An improvement on EPROM, EEPROM, soon followed. The extra "E" stands for electrically, referring to the ability to reset EEPROM using electricity instead of UV, making the devices much easier to use in practice. The bits are re-set with the application of even higher power through the other terminals of the transistor (source and drain). This high power pulse, in effect, sucks the electrons through the insulator, returning it to the ground state. This process has the disadvantage of mechanically degrading the chip, however, so memory systems based on floating-gate transistors in general have short write-lifetimes, on the order of 10⁵ writes to any particular bit.

One approach to overcoming the rewrite count limitation is to have a standard SRAM where each bit is backed up by an EEPROM bit. In normal operation the chip functions as a fast SRAM and in case of power failure the content is quickly transferred to the EEPROM part, from where it gets loaded back at the next power up. Such chips were called NOVRAMs by their manufacturers.

The basis of flash memory is identical to EEPROM, and differs largely in internal layout. Flash allows its memory to be written only in blocks, which greatly simplifies the internal wiring and allows for higher densities. Memory storage density is the main determinant of cost in most computer memory systems, and due to this flash has evolved into one of the lowest cost solid-state memory devices available. Starting around 2000, demand for ever-greater quantities of flash have driven manufacturers to use only the latest fabrication systems in order to increase density as much as possible. Although fabrication limits are starting to come into play, new "multi-bit" techniques appear to be able to double or quadruple the density even at existing linewidths.

Newer approaches

Flash and EEPROM's limited write-cycles are a serious problem for any real RAM-like role, however. In addition, the high power needed to write the cells is a problem in low-power roles, where NVRAM is often used. The power also needs time to be "built up" in a device known as a charge pump, which makes writing dramatically slower than reading, often as much as 1,000 times. A number of new memory devices have been proposed to address these shortcomings.

Ferroelectric RAM

To date, the only such system to enter widespread production is ferroelectric RAM, or F-RAM (sometimes referred to as FeRAM). F-RAM is a random-access memory similar in construction to DRAM but (instead of a dielectric layer like in DRAM) contains a thin ferroelectric film of lead zirconate titanate [Pb(Zr,Ti)O₃], commonly referred to as PZT. The Zr/Ti atoms in the PZT change polarity in an electric field, thereby producing a binary switch. Unlike RAM devices, F-RAM retains its data memory when power is shut off or interrupted, due to the PZT crystal maintaining polarity. Due to this crystal structure and how it is influenced, F-RAM offers distinct properties from other nonvolatile memory options, including extremely high endurance (exceeding 10¹⁶ for 3.3 V devices), ultra low power consumption (since F-RAM does not require a charge pump like other non-volatile memories), single-cycle write speeds, and gamma radiation tolerance. Ramtron International has developed, produced, and licensed ferroelectric RAM (F-RAM), and other companies that have licensed and produced F-RAM technology include Texas Instruments, Rohm, and Fujitsu.

Magnetoresistive RAM

Another approach to see major development effort is magnetoresistive random-access memory, or MRAM, which uses magnetic elements and in general operates in a fashion similar to core, at least for the first-generation technology. Only one MRAM chip has entered production to date: Everspin Technologies' 4 Mbit part, which is a first-generation MRAM that utilizes cross-point field induced writing. Two second-generation techniques are currently in development: Thermal Assisted Switching (TAS), which is being developed by Crocus Technology, and spin-transfer torque (STT) on which Crocus, Hynix, IBM, and several other companies are working. STT-MRAM appears to allow for much higher densities than the those of the first generation, but is lagging behind flash for the same reasons as FeRAM – enormous competitive pressures in the flash market.

Phase-change RAM

Another solid-state technology to see more than purely experimental development is Phase-change RAM, or PRAM. PRAM is based on the same storage mechanism as writable CDs and DVDs, but reads them based on their changes in electrical resistance rather than changes in their optical properties. Considered a "dark horse" for some time, in 2006 Samsung announced the availability of a 512 Mbit part, considerably higher capacity than either MRAM or FeRAM. The areal density of these parts appears to be even higher than modern flash devices, the lower overall storage being due to the lack of multi-bit encoding. This announcement was followed by one from Intel and STMicroelectronics, who demonstrated their own PRAM devices at the 2006 Intel Developer Forum in October.

Millipede memory

Perhaps one of the more innovative solutions is millipede memory, developed by IBM. Millipede is in essence a punched card rendered using nanotechnology in order to dramatically increase areal density. Although it was planned to introduce Millipede as early as 2003, unexpected problems in development delayed this until 2005, by which point it was no longer competitive with flash. In theory the technology offers storage densities on the order of 1 Tbit/in², greater than even the best hard drive technologies currently in use (perpendicular recording offers 636 Gbit/in² as of Dec. 2011), but future heat-assisted magnetic recording and patterned media together could support densities of 10 Tbit/in². However, slow read and write times for memories this large seem to limit this technology to hard drive replacements as opposed to high-speed RAM-like uses, although to a very large degree the same is true of flash as well.

Others

A number of more esoteric devices have been proposed, including Nano-RAM based on carbon nanotube technology, but these are currently far from commercialization. The advantages that nanostructures such as quantum dots, carbon nanotubes and nanowires offer over their silicon-based predecessors include their tiny size, speed and their density. Several concepts of molecular-scale memory devices have been developed recently. Also seeing renewed interest is silicon-oxide-nitride-oxide-silicon (SONOS) memory.

External links

• Supporting filesystems in persistent memory, LWN.net, September 2, 2014, by Jonathan Corbet