A method for preparing a semiconductor device structure includes forming a first fin structure and a second fin structure over a semiconductor substrate, forming an isolation structure over the semiconductor substrate, partially removing the first fin structure and the second fin structure to form a recessed portion of the first fin structure and a recessed portion of the second fin structure, epitaxially growing a first source/drain (S/D) structure over the recessed portion of the first fin structure and a second S/D structure over the recessed portion of the second fin structure, partially removing the isolation structure through the first opening to form a second opening, and forming a contact etch stop layer (CESL) over the first S/D structure and the second S/D structure such that an air gap is formed and sealed in the first opening and the second opening.