Ferroelectric Random Access Memory

storage (FRAM) A type of non-volatile read/write randomaccess 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 corememory.

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)