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Definition
Before the 1950's, ferromagnetic cores were the only type of random-access, nonvolatile
memories available. A core memory is a regular array of tiny magnetic cores that
can be magnetized in one of two opposite directions, making it possible to store
binary data in the form of a magnetic field. The success of the core memory was
due to a simple architecture that resulted in a relatively dense array of cells.
This approach was emulated in the semiconductor memories of today (DRAM's, EEPROM's,
and FRAM's). Ferromagnetic
cores, however, were too bulky and expensive compared to the smaller, low-power
semiconductor memories. In place of ferromagnetic cores ferroelectric memories
are a good substitute. The term "ferroelectric' indicates the similarity,
despite the lack of iron in the materials themselves. Ferroelectric
memory exhibit short programming time, low power consumption and nonvolatile memory,
making highly suitable for application like contact less smart card, digital cameras
which demanding many memory write operations. In other word FRAM has the feature
of both RAM and ROM. A ferroelectric memory technology consists of a complementry
metal-oxide-semiconductor (CMOS) technology with added layers on top for ferroelectric
capacitors. A ferroelectric memory cell has
at least one ferroelectric capacitor to store the binary data, and one or two
transistors that provide access to the capacitor or amplify its content for a
read operation.A ferroelectric capacitor is different
from a regular capacitor in that it substitutes the dielectric with a ferroelectric
material (lead zirconate titanate (PZT) is a common material used)-when an electric
field is applied and the charges displace from their original position spontaneous
polarization occurs and displacement becomes evident in the crystal structure
of the material. Importantly, the displacement
does not disappear in the absence of the electric field. Moreover, the direction
of polarization can be reversed or reoriented by applying an appropriate electric
field.A hysteresis loop for a ferroelectric capacitor
displays the total charge on the capacitor as a function of the applied voltage.
It behaves similarly to that of a magnetic core, but for the sharp transitions
around its coercive points, which implies that even a moderate voltage can disturb
the state of the capacitor.
One remedy for
this would be to modify a ferroelectric memory cell including a transistor in
series with the ferroelectric capacitor. Called an access transistor, it wo control
the access to the capacitor and eliminate the need for a square like hysteresis
loop compensating for the softness of the hysteresis loop characteristics and
blocking unwanted disturb signals from neighboring memory cells.
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