A memory cell of memory device, comprises an active region of a memory cell defined in a semiconductor substrate, and a conductive gate electrode in a trench of the active region. The gate electrode is isolated from the semiconductor substrate. An insulation layer is on the active region and on the conductive gate electrode. A conductive contact is in the insulation layer on the active region at a side of the gate electrode and isolated from the gate electrode. The contact has a first width at a top portion thereof and a second width at a bottom portion thereof, the first width being greater than the second width. The contact is formed of a single-crystal material.