Methods of forming a T-shaped SBD using a single-mask process flow are disclosed. Embodiments include providing a substrate having STI regions; forming a hard mask layer over the substrate and the STI regions, the hard mask having an opening laterally separated from the STI regions; forming a recess in the substrate through the opening, the recess having a first width; forming spacers on sidewalls of the recess, with a gap therebetween; forming a trench in the substrate through the gap, the trench having a second width less than the first; removing the spacers; removing the hard mask layer; filling the trench and the recess with an oxide layer, forming a T-shaped STI region; forming another hard mask layer on a portion of the T-shaped STI region; and revealing a Fin by removing portions of the STI regions and the T-shaped STI region.