A three-dimensional (3D) memory device includes a doped semiconductor layer, a stack structure, and a channel structure. The stack structure includes interleaved conductive layers and dielectric layers formed on the doped semiconductor layer. The conductive layers include a plurality of word lines, and a drain select gate line. The channel structure extends through the stack structure along a first direction and is in contact with the doped semiconductor layer. The drain select gate line includes a first dielectric layer in contact with the channel structure, and a first polysilicon layer in contact with the first dielectric layer.