A gate-all-around (GAA) semiconductor device can include a fin structure that includes alternatingly layered first and second semiconductor patterns. A source region can extend into the alternatingly layered first and second semiconductor patterns and a drain region can extend into the alternatingly layered first and second semiconductor patterns. A gate electrode can extend between the source region and the drain region and surround channel portions of the second semiconductor patterns between the source region and the drain region to define gaps between the source and drain regions. A semiconductor oxide can be on first side walls of the gap that face the source and drain regions and can be absent from at least one of second side walls of the gaps that face the second semiconductor patterns. A gate insulating layer can be on the first side walls of the gaps between the gate electrode and the semiconductor oxide.