Semiconductor device

A semiconductor device comprising:

• a substrate;
• a ground selection transistor stacked on the substrate;
• a plurality of memory cell transistors stacked on the ground selection transistor,
• wherein the plurality of memory cell transistors is stacked on each other in a first direction,
• wherein the first direction is substantially perpendicular to the substrate, wherein a lowermost memory cell transistor of the plurality of memory cell transistors is stacked on the ground selection transistor,
• wherein the plurality of memory cell transistors comprises a first channel layer doped with a first impurity type dopant and extended in the first direction;
• a string selection transistor stacked on an uppermost memory cell transistor of the plurality of memory cell transistors;
• a drain electrode stacked on the string selection transistor in the first direction;
• and
• a source region formed on the substrate, wherein the source region is spaced apart from the plurality of memory cell transistors in a second direction substantially perpendicular to the first direction,
• wherein the string selection transistor comprises a second channel layer doped with a second conductivity type dopant and extended in the first direction.