Methods of fabricating semiconductor devices are provided. A substrate having active patterns and isolating layer patterns is prepared. Each of the isolating layer patterns has an upper surface higher than that of each of the active patterns. A spacer layer having a uniform thickness is formed on the substrate. The spacer layer is etched to form a spacer on a sidewall of each of the isolating layer patterns. A gate structure is formed on each of the active patterns. A selective epitaxial growth (SEG) process is performed on the active patterns having the gate structure to form isolated epitaxial layers that have upper surfaces higher than those of the isolating layer patterns, on the active patterns. Related semiconductor devices are also provided.