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FERROELECTRIC RAM [FRAM]

FEATURES OF FRAM
1.FRAM allows systems to retain information even when power is lost i.e.; non-volatile.
2.The number of write cycles supported by the FRAM components is
nearly unlimited”up to 10 billion read/writes.
3.Low power requirements.
4.When an electric field is applied to a ferroelectric crystal,
the central atom moves in the direction of the field.
5.As the atom moves within the crystal, it passes through an energy barrier, causing a charge spike.
6.Internal circuits sense the charge spike and set the memory
If the electric field is removed from the crystal, the central atom stays in position, preserving the state of the memory. This
makes FRAM non-volatile, without any periodic refresh.
7.Once a cell is accessed for a read operation, its data are presented in the form of an anal
Signal to sense amplifier, where they are compared against a reference voltage to find
the logic level.
A ferroelectric memory cell, known as IT- IC (one transistor, one capacitor) structure which is similar to that of DRAM.
The difference is that ferroelectric film is used as its storage capacitor rather than paraelectric material as in DRAM.
Figure above shows memory cell structure, consists of a single ferroelectric capacitor that is connected to a Plateline(PL) at one end and, via an access transistor, to a Bitline(BL) at the other end. Raising the wordline (WL) and hence turning ON the access transistor accesses the cell.

FRAM TECHONOLOGY

¢ When an electric field is applied to a ferroelectric crystal, the central atom moves in the direction of the field.
¢As the atom moves within the crystal, it passes through an energy barrier,causing a charge spike.
¢ Internal circuits sense the charge spike and set the memory. If the electric field is removed from the crystal, the central atom stays in position, preserving the state of the memory.
¢ This makes FRAM non-volatile, without any periodic refresh

FRAM READ OPERATION


¢ An electric field is applied.

¢ If the atoms are near the cube "floors" and the electric field pushes them to the top, the cell gives off a current pulse.
¢ This pulse, representing a stored 1 or 0, is detected by a sense amplifier. If the atoms are already near their cubes' "ceilings," they don't budge when the field is applied and the cell gives off a smaller pulse.
¢ Reading an FRAM cell destroys the data stored in its capacitor. So after the bit is read, the sense amplifier writes the data back into the cell, just as in a DRAM.

FRAM WRITE OPERATION

To write a "1" into the memory cell,
the BL is raised to Vdd-
Then the WL is raised to Vdd + Vt.
This allows a full Vdd to appear across the ferroelectric capacitor
At this time the state of ferroelectric is independent of its initial state.
Next, the PL is pulsed, WL stays activated until
the PL is pulled down completely and the BL is driven back to zero.
The final state of the capacitor is a negative charge state S1.


To write a "0" into the cell


the BL is driven to 0V prior to activating the WL.
The rest of the operation is similar to that of writing a "1
The written data is held in the cell even though the selection of the wordline is changed to non selected state (i.e. transistor is OFF), so it is nonvolatile.

FRAM AS RAM AND ROM

¢ The key advantage to FRAM over DRAM is what happens between the read and write cycles. In DRAM, every cell must be periodically read and then re-written, a process known as refresh..
¢ In contrast, FRAM only requires power when actually reading or writing a cell. The vast majority of power used in DRAM is used for refresh power usage about 99% lower than DRAM.

ADVANTAGES
* FRAM allows systems to retain information even when power is lost, without resorting to batteries, EEPROM, or flash.
* Access times are the same as for standard SRAM, so there's no delay-at-write access as there is for EEPROM or flash.
* Low power consumption, low voltage operation and high write endurance make it superior than other non-volatile memories like EEPROM & FLASH.
* It is less expensive than magnetic memories.

DISADVANTAGES
Present high cost.
Low density compared to DRAM & SRAM.
FUTURE OF FRAM
Increased memory capacity
High density, to operate under very high temperatures.
Combine FRAM with other logic technologies to offer more enhanced devices.
APPLICATIONS
Personal digital assistants (PDAs), handheld phones, power meters, and smart card, and in security systems
SMART CARDS USING FRAM
¢ Dial a connection on a mobile telephone and be charged on a per-call basis
¢ Establish your identity when logging on to an Internet access provider or to an online bank
¢ Pay for parking at parking meters or to get on subways, trains, or buses
¢ Give hospitals or doctors personal data without filling out a form
¢ Make small purchases at electronic stores on the Web (a kind of cybercash)
¢ Buy gasoline at a gasoline station

CONCLUSION
Ferroelectric memories are superior to EPROMâ„¢s & Flash memories
in terms of write access time & overall power consumption.Two eg: of
such applications are contactless smart cards & digital cameras.
Future personal wireless connectivity applications that are battery
driven will demand large amounts of non volatile storage to retain
accessed internet webpages, contain compressed video, voice and
data. The density and energy efficiency of writing data to memory
would seem to indicate that ferroelectric memory will play a major role
in these types of consumer products.