Semiconductor devices and methods of fabricating the same

A semiconductor structure, comprising:

• a semiconductor layer;
• a first source/drain feature extending into the semiconductor layer;
• a first barrier layer below the first source/drain feature, the first barrier layer disposed at a first distance away from a top surface of the semiconductor layer and including a dopant at a first concentration;
• a second source/drain feature extending into the semiconductor layer;
• and
• a second barrier layer below the second source/drain feature, the second barrier layer disposed at a second distance away from the top surface of the semiconductor layer and including the dopant at a second concentration, wherein the first distance is less than the second distance.

A method, comprising:

• performing a first etching process to form a first recess in a semiconductor layer adjacent a first gate structure;
• forming a first doped layer in the first recess, wherein the first doped layer includes a dopant at a first concentration;
• performing a second etching process to laterally expand a portion of the first recess;
• forming a source/drain feature in the first recess, wherein a bottom surface of the source/drain feature contacts the first doped layer;
• performing a third etching process to form a second recess adjacent a second gate structure;
• and
• forming a second doped layer in the second recess, the second doped layer including the dopant at a second concentration that is less than the first concentration.