Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a 3D memory device includes a substrate, a peripheral circuit on the substrate, a memory stack including interleaved conductive layers and dielectric layers above the peripheral circuit, a P-type doped semiconductor layer above the memory stack, a plurality of channel structures each extending vertically through the memory stack into the P-type doped semiconductor layer, and a source contact above the memory stack and in contact with the P-type doped semiconductor layer. An upper end of each of the plurality of channel structures is flush with or below a top surface of the P-type doped semiconductor layer.