A method for forming a semiconductor device is provided. The method includes: forming, over a substrate, a stacked transistor structure comprising: a bottom channel structure and a top channel structure, a gate structure extending across the bottom and top channel structures, a first and a second bottom S/D structure on the bottom channel structure, and a first and a second top S/D structure on the top channel structure; forming a first and a second bottom S/D contact on the first and the second bottom S/D structures; forming a contact isolation layer capping the first and second bottom S/D contacts, and covering the capped first and second bottom S/D contacts with an ILD layer; forming a first contact trench; forming a second contact trench; and forming a first top S/D contact.