Three-dimensional memory device and method for forming the same

Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a 3D memory device includes an N-type doped region of a substrate, an N-type doped semiconductor layer on the N-type doped region, a memory stack including interleaved conductive layers and dielectric layers on the N-type doped semiconductor layer, a channel structure extending vertically through the memory stack and the N-type doped semiconductor layer into the N-type doped region, and a source contact structure extending vertically through the memory stack and the N-type doped semiconductor layer into the N-type doped region. A first lateral dimension of a first portion of the source contact structure surrounded by the N-type doped region is greater than a second lateral dimension of a second portion of the source contact structure surrounded by the memory stack.