01-10-2010, 11:51 AM
[attachment=4716],
Ferroelectric RAM
abstract
Ferroelectric RAM (FeRAM or FRAM[1]) is a random access memory similar in construction to DRAM but uses a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non-volatile memory technologies that offer the same functionality as Flash memory. FeRAM advantages over Flash include: lower power usage, faster write performance and a much greater maximum number (exceeding 1016 for 3.3 V devices) of write-erase cycles. FeRAM disadvantages are: much lower storage densities than Flash devices, storage capacity limitations and higher cost.
Description
Conventional DRAM consists of a grid of small capacitors and their associated wiring and signaling transistors. Each storage element, a cell, consists of one capacitor and one transistor, a so-called "1T-1C" device. DRAM cells scale directly with the size of the semiconductor fabrication process being used to make it. For instance, on the 90 nm process used by most memory providers to make DDR2 DRAM, the cell size is 0.22 μm², which includes the capacitor, transistor, wiring, and some amount of "blank space" between the various parts – it appears 35% utilization is typical, leaving 65% of the space wasted.
Data in a DRAM is stored as the presence or lack of an electrical charge in the capacitor, with the lack of charge generally representing "0". Writing is accomplished by activating the associated control transistor, draining the cell to write a "0", or sending current into it from a supply line if the new value should be "1". Reading is similar in nature; the transistor is again activated, draining the charge to a sense amplifier. If a pulse of charge is noticed in the amplifier the cell held a charge and thus reads "1", the lack of such a pulse indicates a "0". Note that this process is destructive, once the cell has been read, if it did hold a "1" it must be re-charged to that value again. Since a cell loses its charge after some time due to leak currents, it needs to be actively refreshed at intervals.