Fabrication methods for semiconductor device structures are provided. In an exemplary embodiment, a method of fabricating an electrically-isolated FinFET semiconductor device includes the steps of forming a silicon oxide layer over a semiconductor substrate including a silicon material and forming a first hard mask layer over the silicon oxide layer. The method further includes the steps of forming a first plurality of void spaces in the first hard mask layer and forming a second hard mask layer in the first plurality of void spaces. Still further, the method includes the steps of removing the remaining portions of the first hard mask layer, thereby forming a second plurality of void spaces in the second hard mask layer, extending the second plurality of void spaces into the silicon oxide layer, and forming a plurality of fin structures in the extended second plurality of void spaces.