A semiconductor device and a manufacturing method thereof includes a source contact structure, a gate stack structure including a side region adjacent to the source contact structure, and a center region extending from the side region. The semiconductor device further includes a source gate pattern disposed under the side region of the first gate stack structure. The source gate pattern has an inclined surface facing the source contact structure. The semiconductor device also includes a channel pattern penetrating the center region of the gate stack structure, the channel pattern extending toward and contacting the source contact structure.