Buried guard rings and method for forming the same

A twin-well CMOS integrated circuit device includes an n-well region and a p-well region. Each of the n-well and p-well region includes spaced-apart regions which serve as drain and source regions, a channel region between the spaced-apart regions, a shallow trench isolation structure contiguous with one of the spaced-apart regions, and a doped diffused region extending from the surface of the well region, around and underneath the trench isolation structure, to a region beneath the contiguous spaced-apart region.