Patent ID: 8139388

Claim:
A nonvolatile semiconductor storage device comprising: a ferroelectric material; and a means for applying voltage pulses having a same wave height and varied pulse widths, wherein the ferroelectric material possesses multivalued ferroelectric memory cells which are fed with quantities of polarization varied in conformity with the varied pulse widths of the voltage pulses and enabled to produce states of storage varied in conformity with the variation in the polarization quantity, one of the voltage pulses applied to the multivalued ferroelectric memory cells is used for coarse adjustment capable of largely varying threshold voltages of the memory cells, the rest of the voltage pulses are used for fine adjustment capable of finely varying the threshold voltages of the memory cells, and the states of storage in the memory cells are produced by causing the threshold voltages of the memory cells to be moved bit by bit till they reach threshold voltages equivalent to the expected states of storage by the application of the pulses for the fine adjustment after the threshold voltages of the memory cells have been moved in close proximity to the threshold voltages equivalent to the expected states of storage by the application of the pulses for the coarse adjustment.