Embodiments of three-dimensional (3D) memory devices having a memory layer that confines electron transportation and methods for forming the same are disclosed. A method for forming a 3D memory device includes the following operations. An initial channel hole in a structure is formed. The structure includes a plurality first layers and a plurality of second layers alternatingly arranged over a substrate. An offset between a side surface of each one of the plurality of first layers and a side surface of each one of the plurality of second layers can be formed on a sidewall of the initial channel hole to form a channel hole. The channel hole with a channel-forming structure can be formed to form a semiconductor channel. The channel-forming structure can include a memory layer extending along a vertical direction. The plurality of second layers can then be replaced with a plurality of gate electrodes.