A method includes forming a gate structure over a semiconductor substrate. The gate structure defines a channel region in the semiconductor substrate. Trenches are formed in the semiconductor substrate, and the trenches are interposed by the channel region. A first semiconductor layer is epitaxially grown in the trenches, and the first semiconductor layer has a first dopant with a first dopant concentration. A second semiconductor layer is epitaxially grown over the first semiconductor layer, and the second semiconductor layer has a second dopant with a second dopant concentration. The second dopant has an electrical carrier type opposite to an electrical carrier type of the first dopant.