Epitaxial features of semiconductor devices

A semiconductor device, comprising:

• a first active region and a second active region;
• an isolation region interposing the first active region and the second active region;
• insulating layer disposed on the isolation region and abutting sidewalls of the first active region and the second active region extending above the isolation region;
• a source/drain feature on the first active region and the second active region, wherein the source/drain feature is a Si:P feature including three regions:
• a first region having a first concentration of phosphorus (P), wherein the first region has a bottommost surface higher than a plane extending from an uppermost surface of the isolation region;
• a second region on the first region and having a second concentration of P, the second concentration of P higher than the first concentration of P;
• and
• a third region on the second region and having a third concentration of P, the third concentration of P higher than the second concentration of P, wherein each of the first region, the second region and the third region interface a portion of the insulating layer.

A method of semiconductor device fabrication, the method comprising:

• forming a first active region and a second active region extending above a substrate;
• and
• forming an epitaxial feature on the first active region and the second active region, wherein the forming the epitaxial feature includes:
• growing a first epitaxial region having a first portion over the first active region and a second portion over the second active region;
• growing a second epitaxial region over the first and second portions of the first epitaxial region, wherein growing the second epitaxial region includes forming a merged portion between the first active region and the second active region, wherein the growing the second epitaxial region includes using hydrogen (H2) as a carrier gas;
• and
• growing a third epitaxial region over the second epitaxial region, wherein one of the growing the first epitaxial region or the growing the third epitaxial region includes using nitrogen (N2) as a carrier gas.