In recent years, along with the advancement of the digital techniques in electronic devices, demands for increasing the capacity of a solid state memory device and the data transfer speed have been growing for storing data, such as images, and the like. In order to meet such demands, for example, a solid state memory device is formed by using a material whose resistance value varies according to an applied electric pulse (e.g., Pr1-xCaxMnO3 (PCMO), LaSrMnO3 (LSMO), GdBaCoxOy (GBCO), etc.) as disclosed in U.S. Pat. No. 6,473,332. These materials (hereinafter, referred to as “variable-resistance material(s)”) have resistance values which increase or decrease according to the polarity of an electric pulse. In a nonvolatile memory device formed using such a material, the resistance value that changes according to the polarity of the electric pulse is used for memorization of different values.
As described above, the variable-resistance material has a characteristic such that the resistance value increases or decreases according to the polarity of an applied electric pulse. However, it is indefinite at the time of formation of a film of the variable-resistance material whether or not the resistance value of the film increases or decreases by a specific amount with high reproducibility when an electric pulse of a specific polarity is applied to the variable-resistance material film. Thus, a desired resistive state cannot be achieved even when an electric pulse is applied, and therefore, it is difficult to use the material for a memory device. Further, it is necessary to provide a specific structure of a memory device which uses a variable-resistance material.
In recent years, along with the advancement of the digital techniques in electronic devices, demands for nonvolatile memory devices have been increasing for storing data, such as images, and the like. Further, demands for an increase in the capacity of a memory device, a reduction in the writing power, a reduction in the write/read time, and longer life have been growing. There has been a flash memory which is presently in practical use, wherein nonvolatility is achieved using a mechanism such that a floating gate is provided at the gate part of a semiconductor transistor and electrons are injected in the floating gate. This flash memory has been used as an external memory device in a variety of digital cameras and personal computers.
However, the flash memory has numerous disadvantages, such as a high write voltage, a long write/erase time, short rewritable life, difficulty in increasing capacity (miniaturization of devices), etc. Thus, in order to overcome such disadvantages of the flash memory, development of novel nonvolatile memory devices, such as a semiconductor memory formed using ferroelectric (FeRAM), a semiconductor memory formed using a TMR (Tunnel MR) material (MRAM), a semiconductor memory formed using a phase-change material (OUM), etc., has been actively conducted. However, these memory devices also have disadvantages, such as difficulty in miniaturization of FeRAM device, a high write voltage in MRAM, short rewritable life of OUM, etc. Presently, there is no memory device which meets all the demands placed on the nonvolatile memory devices. Further, there has been developed by the researchers of the University of Houston a new recording method which overcomes the above disadvantages wherein the resistance value of a perovskite structure oxide is changed according to a pulse voltage (U.S. Pat. No. 6,204,139). However, as of now, this method causes significant problems in the stability of operation and production yield of a memory device.