Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a 3D memory device includes a memory stack comprising interleaved conductive layers and dielectric layers, a plurality of semiconductor layers contacted with each other and located adjacent to the memory stack, a plurality of channel structures each extending vertically through the memory stack and at least one of the semiconductor layers, a source contact in contact with at least one of the semiconductor layers, and a contact pad located on one side of the semiconductor layers that are away from the memory stack.