FIG. 4A is a plan view showing a pattern of the memory cell; FIG. 4B is a cross sectional view taken along the line A--A' shown in FIG. 4A; FIG. 4C is a cross sectional view taken along the line B--B' shown in FIG. 4A; and FIG. 4D is an enlarged plan view showing a part of the pattern shown in FIG. 4A. The equivalent circuit of the memory cell is the same as that shown in FIG. 1.