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01-03-2009, 01:32 PM
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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