A three dimensional (3D) semiconductor device includes; a vertical channel extending from a lower end proximate a substrate to an upper end and connecting a plurality of memory cells, and a cell array comprising the plurality of cells, wherein the cell array is arranged in a gate stack of layers having a stair-stepped structure disposed on the substrate. The gate stack includes a lower layer including a lower select line coupled to a lower non-memory transistor proximate the lower end, upper layers including conductive lines respectively coupled to an upper non-memory transistor proximate the upper end and connected as a single conductive piece to form an upper select line, and intermediate layers respectively including a word line and coupled to a cell transistor, wherein the intermediate layers are disposed between the lower select line and the upper select line.