A semiconductor device includes a channel region; a gate dielectric over the channel region; a gate electrode over the gate dielectric; and a first source/drain region adjacent the gate dielectric. The first source/drain region is of a first conductivity type. At least one of the channel region and the first source/drain region includes a superlattice structure. The semiconductor device further includes a second source/drain region on an opposite side of the channel region than the first source/drain region. The second source/drain region is of a second conductivity type opposite the first conductivity type. At most, one of the first source/drain region and the second source/drain region comprises an additional superlattice structure.