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, an N-type doped semiconductor layer on the sacrificial layer, and a dielectric stack on the N-type doped semiconductor layer are subsequently formed. A channel structure extending vertically through the dielectric stack and the N-type doped semiconductor layer is formed. The dielectric stack is replaced with a memory stack, such that the channel structure extends vertically through the memory stack and the N-type doped semiconductor layer. The substrate and the sacrificial layer are removed to expose an end of the channel structure. Part of the channel structure abutting the N-type doped semiconductor layer is replaced with a semiconductor plug.