A memory device includes a conductive strip stack structure having conductive strips and insulating layers stacked in a staggered manner and a channel opening passing through the conductive strips and the insulating layer; a memory layer disposed in the channel opening and overlying the conductive strips; a channel layer overlying the memory layer; a semiconductor pad extending upwards from a bottom of the channel opening beyond an upper surface of a bottom conductive strip, in contact with the channel layer, and electrically isolated from the conductive strips; wherein the channel layer includes a first portion having a first doping concentration and a second portion having a second doping concentration disposed on the first portion.