A semiconductor device structure and a formation method are provided. The semiconductor device structure includes a stack of channel structures and includes a first epitaxial structure and a second epitaxial structure adjacent to opposite sides of the channel structures. The semiconductor device structure also includes a gate stack wrapped around each of the channel structures and a backside conductive contact connected to the second epitaxial structure. The second epitaxial structure is between a top of the backside conductive contact and a top of the gate stack. The semiconductor device structure further includes a dielectric fin stacked over an isolation structure. The dielectric fin is adjacent to the second epitaxial structure, and the isolation structure is adjacent to the backside conductive contact. The isolation structure has a first height, the dielectric fin has a second height, and the second height is greater than the first height.