Methods of fabricating semiconductor devices include forming a transistor on and/or in a semiconductor substrate, wherein the transistor includes a source/drain region and a gate pattern disposed on a channel region adjacent the source/drain region. An insulating layer is formed on the transistor and patterned to expose the source/drain region. A semiconductor source layer is formed on the exposed source/drain region and on an adjacent portion of the insulating layer. A metal source layer is formed on the semiconductor source layer. Annealing, is performed to form a first metal-semiconductor compound region on the source/drain region and a second metal-semiconductor compound region on the adjacent portion of the insulating layer. The first metal-semiconductor compound region may be thicker than the second metal-semiconductor compound region. The metal source layer may include a metal layer and a metal nitride barrier layer.