Multi-stack semiconductor device and manufacturing method thereof

Provided is a multi-stack semiconductor device that includes: a lower field-effect transistor in which a lower channel structure is surrounded by a lower gate structure including a lower gate dielectric layer, a lower work-function metal layer and a lower gate metal pattern; and an upper field-effect transistor in which an upper channel structure is surrounded by an upper gate structure including an upper gate dielectric layer, an upper work-function metal layer and an upper gate metal pattern, wherein a channel width of the upper channel structure is smaller than a channel width of the lower channel structure, and wherein a replacement metal gate (RMG) inner spacer is formed between the lower work-function metal layer and the upper work-function metal layer at regions where the lower channel structure is not vertically overlapped by the upper channel structure.