A semiconductor structure includes a gate structure disposed over a substrate, and a plurality of source/drain features disposed on the substrate and interposed by the gate structure. Each of the source/drain features includes a first doped source/drain region extended away from the substrate, and a second doped source/drain region disposed on top and side surfaces of the first doped source/drain region, in which a phosphorus doping concentration of the first doped source/drain region is lower than a doping concentration of the second doped source/drain region.