Flash memory integrated circuit devices include an integrated circuit substrate. A cell array on the integrated circuit substrate includes a plurality of cell transistors. A bit line is coupled to ones of the plurality of cell transistors and a first pass transistor is coupled to the bit line. The first pass transistor has a first diffusion structure configured to provide a breakdown voltage higher than that of a second diffusion structure. One or more second pass transistor(s) are coupled to the first pass transistor. The second pass transistor(s) have the second diffusion structure. The second diffusion structure may have a resistance smaller than a resistance of the first diffusion structure.