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A storage device includes a memory cell having a storage element having a characteristic of changing from a state of a high resistance value to a state of a low resistance value by being supplied with a voltage equal to or higher than a first threshold voltage, and changing from a state of a low res

A storage device includes a memory cell having a storage element having a characteristic of changing from a state of a high resistance value to a state of a low resistance value by being supplied with a voltage equal to or higher than a first threshold voltage, and changing from a state of a low resistance value to a state of a high resistance value by being supplied with a voltage equal to or higher than a second threshold voltage different in polarity from the first threshold voltage, and a circuit element connected in series with the storage element, wherein letting R be a resistance value of the storage element after writing, V be the second threshold voltage, and I be a current that can be passed through the storage element at a time of erasure, R≧V/I.

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What is claimed is: 1. A storage device comprising: a memory cell having a storage element; and a transistor element in saturation connected in series with said storage element, wherein, the memory cell resistance changes from a state of high resistance to a state of low resistance when a voltage e

What is claimed is: 1. A storage device comprising: a memory cell having a storage element; and a transistor element in saturation connected in series with said storage element, wherein, the memory cell resistance changes from a state of high resistance to a state of low resistance when a voltage equal to or higher than a first threshold voltage is applied in a writing operation, the memory cell resistance changes from a state of low resistance to a state of high resistance when a voltage equal to or higher than a second threshold voltage with a different polarity than the first threshold value is applied in an erasure operation, and the resistance value of said storage element after said writing operation is set to a value at least equal to the ratio between said second threshold voltage and a maximum current that can be passed through said transistor element in saturation in said erasure operation. 2. The storage device as claimed in claim 1, wherein: said transistor element in saturation is a unipolar transistor; and the current that can be passed through said storage element at a time of erasure is a saturation current of said unipolar transistor. 3. The storage device as claimed in claim 1, wherein: said transistor element in saturation is a unipolar transistor; and the current that can be passed through said storage element at a time of erasure is a maximum value of a drain current of said unipolar transistor when said second threshold voltage is applied to said storage element. 4. A storage device comprising: a plurality of memory cells each having a storage element formed by interposing a storage layer between a first electrode layer and a second electrode layer; and a transistor element in saturation connected in series with said storage element, wherein, the storage element changes from a state of high resistance to a state of low resistance when a voltage equal to or higher than a first threshold voltage is applied between the fist and second electrode layers during a writing operation, the storage element changes from a state of low resistance to a state of high resistance when a voltage equal to or higher than a second threshold voltage with a polarity different than the first threshold voltage is applied during an erasure operation, and the resistance of said storage element after writing is set to a value at least equal to the ratio between said second threshold voltage and a maximum current that can be passed through said transistor element in saturation at a time of erasure. 5. The storage device as claimed in claim 4, wherein: said transistor element in saturation is a unipolar transistor; and the current that can be passed through said storage element at a time of erasure is a saturation current of said unipolar transistor. 6. The storage device as claimed in claim 4, wherein: said transistor element in saturation is a unipolar transistor; and the current that can be passed through said storage element at a time of erasure is a maximum value of a drain current of said unipolar transistor when said second threshold voltage is applied to said storage element. 7. The storage device as claimed in claims 4, wherein a common electrode layer forms at least one of the electrode layers of said memory cell. 8. A semiconductor device comprising: a storage device including a memory cell; a storage element included in the memory cell; and a transistor element in saturation connected in series with said storage element, wherein, the storage element changes from a state of a high resistance to a state of a low resistance when supplied with a voltage equal to or higher than a first threshold voltage during a writing operation, the storage element changes from a state of a low resistance to a state of a high resistance when supplied with a voltage equal to or higher than a second threshold voltage and different in polarity from said first threshold voltage during an erasure operation, and a resistance value of said storage element after writing is set to a value at least equal to the ratio between said second threshold voltage and a maximum current that can be passed through said transistor element in saturation at a time of erasure. 9. The semiconductor device as claimed in claim 8, wherein a word line voltage setting circuit for limiting word line voltage forms a writing control circuit. 10. The semiconductor device as claimed in claim 8, wherein a current limiting circuit for limiting current flowing through one of a bit line or a source line forms a writing control circuit. 11. The semiconductor device as claimed in claim 8, wherein a writing control circuit is a pulse driving circuit for limiting pulse widths of pulse signals applied to a word line and one of a bit line or a source line. 12. A semiconductor device comprising: a storage device including a plurality of memory cells; each memory cell having a storage element formed by interposing a storage layer between a first electrode layer and a second electrode layer; and a transistor element in saturation connected in series with said storage element, wherein, said storage element changes from a state of a high resistance to a state of a low resistance when a voltage equal to or higher than a first threshold voltage is applied between the first electrode layer and the second electrode layer during a writing operation, said storage element changes from a state of a low resistance to a state of a high resistance when a voltage equal to or higher than a second threshold voltage and different in polarity from said first threshold voltage is applied during an erasure operation, and a resistance value of said storage element after writing is set to a value at least equal to the ratio between said second threshold voltage and a maximum current that can be passed through said transistor element in saturation at a time of erasure. 13. The semiconductor device as claimed in claim 12, wherein a word line voltage setting circuit for limiting word line voltage forms a writing control circuit. 14. The semiconductor device as claimed in claim 12, wherein a current limiting circuit for limiting current flowing through one of a bit line or a source line forms a writing control circuit. 15. The semiconductor device as claimed in claim 12, wherein a writing control circuit is a pulse driving circuit for limiting pulse widths of pulse signals applied to a word line and one of a bit line or a source line. 16. The semiconductor device as claimed in claims 12, wherein a common electrode layer forms at least one of the electrode layers of said memory cell.