A method of decoding a block with a Soft Output Viterbi Algorithm using a trellis representation and a sliding window wherein each position of the sliding window has a path determination stage at one end of the sliding window and a symbol decision stage at another end of the sliding window is disclosed. The plurality of gate structures include a first select-gate and a second select-gate disposed on the cell region, the first select-gate and the second select-gate spaced apart from each other. The first portion extends from a surface of a semiconductor substrate to a first depth in the semiconductor substrate, and has a width that tapers from a first width at the surface of the semiconductor substrate to a second width at the first depth, the first width being greater than the second width.