A transistor structure of a semiconductor memory device comprises: an active area having a plurality of trenches and a substrate surface, the trenches having openings oriented toward the substrate surface; a plurality of gate structures embedded in the trenches, wherein the substrate surface comprises source regions located on outer sides of the gate structures and a drain region located between the gate structures; node contacts each disposed on one of the source regions; a bit line contact disposed on the drain region and connectable to a bit line, the node contacts sharing the bit line contact through adjacent gate structures, wherein the drain region comprises a first ion implantation layer extending inwardly from the bit line contact, each of the source regions comprising a second ion implantation layer extending inwardly from a corresponding node contact, the first ion implantation layer being deeper than the second ion implantation layer.