A semiconductor structure includes a substrate, a source/drain region, a composite layer and a plug. The source/drain region and the composite layer are over the substrate. The composite layer includes a first sublayer having a first material, a second sublayer having a second material, and a third sublayer having the first material. A bandgap of the second material is larger than that of the first material. The second sublayer is between the first sublayer and the third sublayer. The plug is through the composite layer, and electrically connected to the source/drain region. The plug includes a first portion laterally adjoining the first sublayer, a second portion laterally adjoining the second sublayer, and a third portion laterally adjoining the third sublayer, and a first width of the first portion and a third width of the third portion is smaller than a second width of the second portion.