A semiconductor device structure, along with methods of forming such, are described. The structure includes a fin disposed over a semiconductor substrate, and the fin has a first width. The structure further includes an isolation region disposed around the fin, a gate electrode disposed over the fin and the isolation region, and a fill material disposed in the gate electrode. The fill material is in contact with a top surface of a portion of the semiconductor substrate, the top surface has at least a portion having a substantially flat cross-section, and the portion of the top surface has a second width substantially greater than the first width.