Embodiments of three-dimensional (3D) memory devices and methods for forming the 3D memory devices are disclosed. In an example, a method for forming a 3D memory device is disclosed. A first semiconductor device is formed on a first substrate. A first single-crystal silicon layer is transferred from a second substrate onto the first semiconductor device on the first substrate. A dielectric stack including interleaved sacrificial layers and dielectric layers is formed on the first single-crystal silicon layer. A channel structure extending vertically through the dielectric stack is formed. The channel structure includes a lower plug extending into the first single-crystal silicon layer and including single-crystal silicon. A memory stack including interleaved conductor layers and the dielectric layers is formed by replacing the sacrificial layers in the dielectric stack with the conductor layers. An interconnect layer above the memory stack and including a bit line electrically connected to the channel structure is formed.