A gate structure is formed across a single crystalline semiconductor fin. An amorphizing ion implantation is performed employing the gate structure as an implantation mask to amorphize surface portions of the semiconductor fin into inverted U-shaped amorphous semiconductor portions. A gate spacer is formed around the gate structure, and the inverted U-shaped amorphous semiconductor portions are etched selective to a single crystalline portion of the semiconductor fin and the gate structure. A pair of inverted U-shaped cavities is formed underneath the gate spacer and above the remaining portion of the semiconductor fin. A doped epitaxial semiconductor material is deposited by a selective epitaxy process to form doped epitaxial active regions that include self-aligned extension portions underlying the gate spacer.