A semiconductor device with densified dielectric structures and a method of fabricating the same are disclosed. The method includes forming a fin structure, forming an isolation structure adjacent to the fin structure, forming a source/drain (S/D) region on the fin structure, depositing a flowable dielectric layer on the isolation structure, converting the flowable dielectric layer into a non-flowable dielectric layer, performing a densification process on the non-flowable dielectric layer, and repeating the depositing, converting, and performing to form a stack of densified dielectric layers surrounding the S/D region.