Semiconductor device and method of fabricating the same

A semiconductor device, comprising:

• a substrate;
• an electrode structure including electrodes stacked on the substrate;
• an upper semiconductor pattern penetrating at least a portion of the electrode structure;
• and
• a lower semiconductor pattern between the substrate and the upper semiconductor pattern,
• wherein the upper semiconductor pattern includes a gap-filling portion and a sidewall portion, the sidewall portion extending from the gap-filling portion in a direction away from the substrate,
• the lower semiconductor pattern includes a concave top surface,
• the gap-filling portion fills a region enclosed by the concave top surface,
• a top surface of the gap-filling portion has a rounded shape that is deformed toward the substrate, and
• a thickness of the sidewall portion is less than a thickness of the gap-filling portion, wherein
• the electrodes include a ground selection line, cell electrodes, and a string selection line stacked on the substrate, and
• the top surface of the gap-filling portion is between the ground selection line and a lowermost one of the cell electrodes.