A method of manufacturing a semiconductor device includes partially removing an upper portion of an active fin of a substrate loaded in a chamber to form a trench; and forming a source/drain layer in the trench, which includes providing a silicon source gas, a germanium source gas, an etching gas and a carrier gas into the chamber to perform a selective epitaxial growth (SEG) process using a top surface of the active fin exposed by the trench as a seed so that a silicon-germanium layer is grown; and purging the chamber by providing the carrier gas into the chamber to etch the silicon-germanium layer.