I thought of writing this blog on types of NVRAM (Non
Volatile RAM) to share and also, enhance my knowledge on NVRAM market. In my
knowledge, currently, there are three types of technologies on NVRAM are
available in market.
In NVRAM, each bit has its own memory address,
and can be part of the computer's addressable memory along with the volatile
static and dynamic RAM (SRAM and DRAM) chips shipped with the computer.
NVRAM are devices which are able to retain information when electrical power is
The first semiconductor NVRAM technology was
battery-backed SRAM. It was created by the simple expedient of providing a rechargeable
battery to keep power applied to the SRAM when system power was removed. This
is still in use and works well for limited time periods, but the batteries take
up useful space, and eventually discharge. Computer users who store their
computers without power for long periods of time find that the units lose their
CMOS setup information because it is typically stored in battery-backed SRAM.
Today, so-called flash memory takes the place of
battery-backed SRAM in a number of applications. Most notably, flash has made
possible compact "memory sticks," which are just flash memory chips packaged along
with a USB interface. When plugged into a USB port, they appear as a "removable
drive." They serve the same function as floppy disks. Flash memory can be (and
is) used for more reliable CMOS Setup storage and virtually any other NVRAM
application. Flash's main drawback is a limitation on the number of read/write
cycles its cells can endure.
A second type of NVRAM that is currently gaining popularity is magneto
resistive RAM (MRAM). MRAM's greatest advantage over flash is a virtually
unlimited number of read/write cycles.
A third NVRAM technology currently in production is the ferroelectric
RAM (FRAM). Like DRAM, FRAM stores information as voltage on a capacitor.
Instead of using a linear dielectric, such as silicon dioxide (basically,
glass), FRAM uses a non-linear ferroelectric dielectric, such as lead zirconate
titanate (PZT). PZT is a crystalline material whose crystal unit cells have a
permanent electric dipole moment. Applying an electric field (by putting a
charging voltage across the capacitor) causes atomic rearrangements within the
unit cells to align all the dipole moments with the impressed electric field.
Removing the supply voltage leaves the dipoles still aligned, so the potential
difference between the plates persists.
Other NVRAM technologies under development include phase-change RAM (PRAM),
Silicon-Oxide-Nitride-Oxide-Silicon (SONOS), Resistive Random Access Memory
(RRAM), Nano-RAM (NRAM), and perhaps others. All of these are under development.