Ferroelectric RAM(FRAM/FeRAM)

Ferroelectric RAM ( also called FRAM or FeRAM) is a non-volatile Random Access Memory that uses a ferroelectric film capacitor to store data. Possessing the characteristics of both ROM and RAM devices, FRAM features high speed acess, high endurance in write mode, low power conception, non-volatility and excellent tamper resistance. FRAM is one of the growing number of ailernative non-volatile memory technologies.

HISTORY

The developement of FRAM started in late 1980's. Much of the current FRAM technology was developed by Ramtron a fabless semiconductor company. One major licensee is Fujistu, who operates the largest semi conductor foundry production line with FRAM capability. Fujistu produces devices for Ramtron. Texas instruments has collobrated with ramtron to produce FeRAM test chips in a modified 130 nm process. By the end of 2005 Ramtron reported that they were evaluating prototype samples of an 8 megabit FeRAM chip manufactured using Texas Instruments FeRAM process. In the same year Fujistu collaborated with Sieko-Epson in the developement of a 180 nm FeRAM process. Reaserches in this field have also been conducting at Institutions like Cambridge university, Totento university, IMEC-Belgium, Samsung, Thoshiba, Oki, Matsushita, Infineon, Hynix and Symetrix.

WORKING

DRAM consists of a large number of capacitors and thier signaling transistors and thier associated wirings. Each storage unit -called a cell- consists of one capacitor and one transistor so it is called a 1t-1c device. Data is stored as the presence or absence of charge in the capacitor. While writing the signaling transister is activated , the cell is drained to write a 0-bit,or send current to it from supply line to write a 1-bit. While reading the charge is drained to an amplifier if a pulse of charge is noticed in the amplifier it reads a 1bit in the cell, and no pulse reads a 0-bit in the cell.

stucture of a FeRAM cell

In a FeRAM cell capacitor ferroelectric material (lead zirconate titanate-pzt) is used. The dielectric constant of a ferroelectric material is much higher than that of a linear dielectric because of the semi-pemanant dipoles formed by the structure of the ferroelectric material. When an external electric field is applied the dipoles will align in the direction of the field and even after the charge is removed the dipoles retain their polarization. This feature used to store data in FeRAM cell. Two possible electric polarizations are used to store binary 0's and 1's in each cell. If the positive remnant polarization(+Pr) is used to store the 0'bit, then negative remnant polarization(-Pr) is used to store a 1'bit.

structure of a 1transister FeRAM cell and it's working machanism

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Writing is done by appying a field across the ferroelectric by charging the plates of the capacitor. This will force the atoms inside the ferroelectric to 'up' or 'down' orientation depending on the polarity of the applied charge and by thus to store '1' or '0'.

To read data from FeRAM cell the transister will force the cell into a perticular state, for eg. consider it is '0'. If the cell already contained a '0' nothing will happen in the output. If it already held a '1' then it wiil cause the reorientation of the atoms in the ferroelectric and will produce a current pulse in the output. Reading FeRAM is a distructive process (because it overwrits the data) and it requires the cell to be rewritten if it is changed.

ADVANTAGES

1.Density- The cost of a memory system is mainly depends upon the density of its components.While comparing with other memory systems FeRAM can have higher coponent density.

2.Power Consumption-Power consumed by FeRAM is very law when compared with others. In the case of a DRAM cell charge applied to the matalic plates leaks across the insulating material so continuous power supply is needed to refresh the cells and for a flash memory high voltage is required for pushing the electrons so the power usage will be high.

3.perfomance- Since read- write operations in FeRAM is associated with the movements of atoms, which is very faster FeRAM could be much faster than other existing mechanisms.