Published on Sep 20, 2019
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.
Description of FRAM
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.
The basic building block of FRAM is the Ferroelectric capacitor. A ferroelectric capacitor physically distinguished from a regular capacitor by substituting the dielectric with a ferroelectric material. In a regular dielectric, upon the application of an electric field, positive and negative charges will be displaced from their original positions-a concept that is characterized polarization. This polarization, or displacement, will vanish, however, when the electric field return back to zero. In, a ferroelectric material, on the other hand, there is a spontaneous polarization displacement that is inherent to the crystal structure of the material and does not disappear in the absence of electric field. The direction of this polarization can be reversed or reoriented by applying an appropriate electric field.
Fig.shows an array architecture in which the PL is run parallel to the BL, hence the name BL//PL for the architecture. Unlike the previous architecture, only a single memory cell can be selected by a simultaneous activation of a WL and a PL. This is the memory cell that is located at the iIntersection of the WL and the PL. It is possible to select more than one memory cell in a row by activating their corresponding platelines.