| 摘要 |
PURPOSE: To provide a nonvolatile semiconductor storage device having such a memory cell constitution that the variation of the shapes of memory cells, which are arranged adjacently to each other in a mirror image relation caused by the asymmetry between the sources and drains of the memory cells does not affect the apparent characteristics of the memory cells. CONSTITUTION: Only the electric current flowing through a floating gate having good characteristics is made detectable from the outside by performing writing and reading-out by using a memory cell constituted of two floating gate electrodes 131 and 132 which are arranged adjacently to each other so as to sandwich a source 16 offset from the floating gate electrodes 131 and 132 as one unit. At the time of writing, in addition, a storage density which is equivalent to that of the conventional memory cell or higher is realized by causing the above-mentioned one set of memory cells to not only store one bit, but also two ore more bits.
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