array including an array of memory cells each of which is similar the memory cell shown in FIGS. 2A and 2B; FIG. 4A is a plan view, FIG. 4B is a cross-sectional view, and FIG. 4C is another cross-sectional view, showing a memory cell array according to an embodiment of the present invention; FIGS. 5A to 5F show processing steps of producing a memory cell array according to an embodiment of the pr