A semiconductor device, including a silicon on insulator (SOI) substrate is disclosed. The device may include gate structures formed on the SOI substrate and being spaced apart from each other in a horizontal direction, and a plurality of channels spaced apart from each other in a vertical direction. Each of the channels may extend through each of the gate structures in the horizontal direction. The device may include a seed layer and a source/drain region. The source/drain region may be connected to the channels, and each sidewall of the source/drain region in the horizontal direction may have a concave-convex shape. The device may include a protruding portion of the source/drain region formed between the gate structures that protrudes in the horizontal direction compared to a non-protruding portion of the source/drain region formed between the channels.