Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a method for forming a 3D memory device is disclosed. A sacrificial layer on a substrate, a first stop layer on the sacrificial layer, an N-type doped semiconductor layer on the first stop layer, and a dielectric stack on the N-type doped semiconductor layer are sequentially formed. A plurality of channel structures each extending vertically through the dielectric stack and the N-type doped semiconductor layer are formed, stopping at the first stop layer. The dielectric stack is replaced with a memory stack, such that each of the plurality of channel structures extends vertically through the memory stack and the N-type doped semiconductor layer. The substrate, the sacrificial layer, and the first stop layer are sequentially removed to expose an end of each of the plurality of channel structures. A conductive layer is formed in contact with the ends of the plurality of channel structures.