A 3D memory device includes a memory stack including interleaved stack conductive layers and stack dielectric layers, a semiconductor layer, and a channel structure extending vertically through the memory stack into the semiconductor layer. A first lateral dimension of a first portion of the channel structure facing the semiconductor layer is greater than a second lateral dimension of a second portion of the channel structure facing the memory stack. The channel structure includes a memory film and a semiconductor channel A first doping concentration of part of the semiconductor channel in the first portion of the channel structure is greater than a second doping concentration of part of the semiconductor channel in the second portion of the channel structure.