Ferroelectric Random Access Memory
Ferroelectric RAM
FRAM
(FRAM) A type of non-volatile read/write random
access semiconductor memory. FRAM combines the advantages
of SRAM - writing is roughly as fast as reading, and EPROM
- non-volatility and in-circuit programmability. Current (Feb
1997) disadvantages are high cost and low density, but that
may change in the future. Density is currently at most 32KB
on a chip, compared with 512KB for SRAM, 1MB for EPROM and 8MB
for DRAM.
A ferroelectric memory cell consists of a ferroelectric
capacitor and a MOS transistor. Its construction is
similar to the storage cell of a DRAM. The difference is in
the dielectric properties of the material between the
capacitor's electrodes. This material has a high dielectric
constant and can be polarized by an electric field. The
polarisation remains until it gets reversed by an opposite
electrical field. This makes the memory non-volatile. Note
that ferroelectric material, despite its name, does not
necessarily contain iron. The most well-known ferroelectric
substance is BaTiO3, which does not contain iron.
Data is read by applying an electric field to the capacitor.
If this switches the cell into the opposite state (flipping
over the electrical dipoles in the ferroelectric material)
then more charge is moved than if the cell was not flipped.
This can be detected and amplified by sense amplifiers.
Reading destroys the contents of a cell which must therefore
be written back after a read. This is similar to the
precharge operation in DRAM, though it only needs to be done
after a read rather than periodically as with DRAM refresh.
In fact it is most like the operation of ferrite core
memory.
FRAM has similar applications to EEPROM, but can be written
much faster. The simplicity of the memory cell promises high
density devices which can compete with DRAM.
RAMTRON is the company behind FRAM.
(1997-02-17)
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