Three-dimensional (3D) memory devices and methods for forming the same are disclosed. In certain aspects, a 3D memory device includes a stack structure including interleaved conductive layers and stack dielectric layers, a channel structure extending through the stack structure, and a doped semiconductor layer. The channel structure includes a memory film and a semiconductor channel. The semiconductor channel includes a doped portion and an undoped portion. A part of the doped portion of the semiconductor channel extends beyond the stack structure in a first direction. A part of the doped semiconductor layer is in contact with a sidewall of the part of the doped portion of the semiconductor channel that extends beyond the stack structure.