Methods of fabricating semiconductor integrated circuit devices are provided. A substrate is provided with gate patterns formed on first and second regions. Spaces between gate patterns on the first region are narrower than spaces between gate patterns on the second region. Source/drain trenches are formed in the substrate on opposite sides of the gate patterns on the first and second regions. A first silicon-germanium (SiGe) epitaxial layer is formed that partially fills the source/drain trenches using a first silicon source gas. A second SiGe epitaxial layer is formed directly on the first SiGe epitaxial layer to further fill the source/drain trenches using a second silicon source gas that is different from the first silicon source gas.