An integrated circuit device includes a source/drain region having a recess in its top, a contact plug extending on the source/drain region from within the recess, and a metal silicide layer lining the recess and having a first portion covering a bottom of the contact plug and a second portion that is integral with the first portion and covers a lower part of sides of the contact plug. The second portion of the silicide layer may have a thickness different from a thickness of the first portion of the silicide layer. The silicide layer is formed at a relatively low temperature to offer an improved resistance characteristic as between the source/drain region and the contact plug.