THREE-DIMENSIONAL SEMICONDUCTOR MEMORY DEVICE AND ELECTRONIC SYSTEM INCLUDING THE SAME

Disclosed are three-dimensional semiconductor memory devices and electronic systems. The three-dimensional semiconductor memory device comprises stack structures that include interlayer dielectric layers and gate electrodes that are alternately stacked on a substrate, vertical channel structures that penetrate the stack structures, and a separation structure that runs in a first direction across between the stack structures. The separation structure includes a first portion that extends in a vertical direction from the substrate, and a second portion on the first portion and including a material different from a material of the first portion. A top surface of the second portion is at a level the same as that of top surfaces of the vertical channel structures.