Design of a Multi-Level/Analog Ferroelectric Memory Device

Abstract

Increasing the memory density and utilizing the novel characteristics of ferroelectric devices is important in making ferroelectric memory devices more desirable to the consumer. This paper describes a design that allows multiple levels to be stored in a ferroelectric based memory cell. It can be used to store multiple bits or analog values in a high speed nonvolatile memory. The design utilizes the hysteresis characteristic of ferroelectric transistors to store an analog value in the memory cell. The design also compensates for the decay of the polarization of the ferroelectric material over time. This is done by utilizing a pair of ferroelectric transistors to store the data. One transistor is used a reference to determine the amount of decay that has occurred since the pair was programmed. The second transistor stores the analog value as a polarization value between zero and saturated. The design allows digital data to be stored as multiple bits in each memory cell. The number of bits per cell that can be stored will vary with the decay rate of the ferroelectric transistors and the repeatability of polarization between transistors. It is predicted that each memory cell may be able to store 8 bits or more. The design is based on data taken from actual ferroelectric transistors. Although the circuit has not been fabricated, a prototype circuit is now under construction. The design of this circuit is different than multi-level FLASH or silicon transistor circuits. The differences between these types of circuits are described in this paper. This memory design will be useful because it allows higher memory density, compensates for the environmental and ferroelectric aging processes, allows analog values to be directly stored in memory, compensates for the thermal and radiation environments associated with space operations, and relies only on existing technologies.

Tristate Memory Using Ferroelectric-Insulator-Semiconductor Heterojunctions for 50% Increased Data Storage

“…There have been many reports on the design and fabrication of multilevel or even analog-type ferroelectric memories, but no breakthrough has been reported yet. [7][8][9] Most efforts have been concentrated on using a depolarized state as the third memory state in addition to the two spontaneously polarized states with opposite signs. Raiter and Cockburn reported the possibility of using three data-signal levels to increase the array storage density from 1 bit per cell to 1.5 bits per cell by using three-level FeRAM.…”