Ferroelectric random access memory



storage
(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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