A method of forming a microelectronic device comprises forming a microelectronic device structure comprising a base structure, a doped semiconductive material overlying the base structure, a stack structure overlying the doped semiconductive material, cell pillar structures vertically extending through the stack structure and the doped semiconductive material and into the base structure, and digit line structures vertically overlying the stack structure. An additional microelectronic device structure comprising control logic devices is formed. The microelectronic device structure is attached to the additional microelectronic device structure to form a microelectronic device structure assembly. The base structure and portions of the cell pillar structures vertically extending into the base structure are removed to expose the doped semiconductive material. The doped semiconductive material is then patterned to form at least one source structure over the stack structure and coupled to the cell pillar structures. Microelectronic devices and electronic systems are also described.