Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a 3D memory device includes a P-type doped region of a substrate, an N-type doped semiconductor layer on the P-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 P-type doped region, an N-type doped semiconductor plug extending vertically into the P-type doped region, and a source contact structure extending vertically through the memory stack to be in contact with the N-type doped semiconductor plug.