A semiconductor device includes: a first fin and a second fin extending from a substrate and an epitaxial source/drain region. The epitaxial source/drain region includes a first portion grown on the first fin and a second portion grown on the second fin, and the first portion and the second portion are joined at a merging boundary. The epitaxial source/drain region further includes a first subregion extending from a location level with a highest point of the epitaxial source/drain region to a location level with a highest point of the merging boundary, a second subregion extending from the location level with the highest point of the merging boundary to a location level with a lowest point of the merging boundary, and a third subregion extending from the location level with the lowest point of the merging boundary to a location level with a top surface of an STI region.