Flash memory devices with memory cells strings including dummy transistors with selective threshold voltages

Flash memory devices include a first memory cell string including a plurality of serially-connected memory cells and first and second serially-connected dummy transistors configured to couple the serially-connected memory cells to a bit line and a second memory cell string including a plurality of serially-connected memory cells and first and second serially-connected dummy transistors configured to couple the serially-connected memory cells to the bit line. The first dummy memory cells of the first and second memory cell strings have gates connected in common to a first dummy word line and have different threshold voltages and the second dummy memory cells of the first and second memory cell strings have gates connected in common to a second dummy bit line and have different threshold voltages. In some embodiments, the first dummy memory cell of the first memory cell string and the second dummy memory cell of the second memory cell string may have threshold voltages greater than a predetermined voltage and the second dummy memory cell of the first memory cell string and the first dummy memory cell of the second memory cell string may have threshold voltages less than the predetermined voltage.