A 3D memory array in which epitaxial source/drain regions which are horizontally merged and vertically unmerged are used as source lines and bit lines and methods of forming the same are disclosed. In an embodiment, a memory array includes a first channel region over a semiconductor substrate; a first epitaxial region electrically coupled to the first channel region; a second epitaxial region directly over the first epitaxial region in a direction perpendicular to a major surface of the semiconductor substrate; a dielectric material between the first epitaxial region and the second epitaxial region, the second epitaxial region being isolated from the first epitaxial region by the dielectric material; a gate dielectric surrounding the first channel region; and a gate electrode surrounding the gate dielectric.