A semiconductor device can include a semiconductor substrate having a memory cell region and a pad region that is adjacent to the memory cell region, the pad region can include a first pad region, a second pad region between the memory cell region and the first pad region, and a buffer region that is between the first and second pad regions. A separation source structure can include a first portion and a second portion that are parallel to each other in a plan view of the semiconductor device. A first source structure and a second source structure can be disposed between the first and second portions of the separation source structure, where the first and second source structures can have end portions that oppose each other, the first source structure being disposed in the first pad region, and the second source structure being disposed in the second pad region. A gate group can be disposed in the memory cell region and the pad region between the first and second portions of the separation source structure, where each of the end portions of the first and second source structures has a planar shape, and a width of each end portion increases and then decreases as each of the end portions extends toward the other.