Semiconductor structure and method for forming the same

A method for forming a semiconductor structure, comprising:

• forming a first fin structure and a second fin structure over a substrate, wherein the second fin structure is adjacent to the first fin structure;
• forming an isolation structure over the substrate, wherein the first fin structure and the second fin structure are protruded from the isolation structure;
• forming a first hard mask layer covering the first fin structure;
• trimming the second fin structure, wherein a first width of the first fin structure is greater than a second width of the second fin structure;
• forming a Ge-containing material covering the second fin structure;
• annealing the second fin structure and the Ge-containing material to form a modified second fin structure;
• and
• trimming the modified second fin structure to a third width, wherein the first width is greater than the third width.

A semiconductor structure, comprising:

• an isolation structure formed over a substrate;
• a first fin structure protruded from the isolation structure, wherein the first fin structure is doped with Ge element, and a profile of the Ge element in the first fin structure extends below a top surface of the isolation structure, wherein a doped region is formed in the first fin structure, the doped region has a tapered portion extending below the top surface of the isolation structure;
• and
• a gate structure formed over the first fin structure.