A method for forming a memory device includes: forming a first layer stack and a second layer stack successively over a substrate, wherein each of the first and the second layer stacks comprises a dielectric layer, a channel layer, and a source/drain layer formed successively over the substrate; forming openings that extends through the first layer stack and the second layer stack, where the openings includes first openings within boundaries of the first and the second layer stacks, and a second opening extending from a sidewall of the second layer stack toward the first openings; forming inner spacers by replacing portions of the source/drain layer exposed by the openings with a dielectric material; lining sidewalls of the openings with a ferroelectric material; and forming first gate electrodes in the first openings and a dummy gate electrode in the second opening by filling the openings with an electrically conductive material.