A semiconductor structure includes a substrate, a gate structure, and two silicon-containing structures. The substrate includes two recesses defined therein and two doping regions of a first dopant type. Each of the two doping regions extends along a bottom surface and at least portion of a sidewall of a corresponding one of the two recesses. The gate structure is over the substrate and between the two recesses. The two silicon-containing structures are of a second dopant type different from the first dopant type. Each of the two silicon-containing structures fills a corresponding one of the two recesses, and an upper portion of each of the two silicon-containing structures has a dopant concentration higher than that of a lower portion of each of the two silicon-containing structures.