A semiconductor device and method of forming the same are disclosed. The method of forming a semiconductor device includes providing a substrate, an isolation structure over the substrate, and at least two fins extending from the substrate and through the isolation structure; etching the at least two fins, thereby forming at least two trenches; growing first epitaxial features in the at least two trenches; growing second epitaxial features over the first epitaxial features in a first growth condition; and after the second epitaxial features reach a target critical dimension, growing the second epitaxial features in a second growth condition different from the first growth condition.