A semiconductor device includes a body including a first junction region; a pillar positioned over the body, and including a vertical channel region and a second junction region over the vertical channel region; a gate trench exposing side surfaces of the pillar; a gate dielectric layer covering the gate trench; and a gate electrode embedded in the gate trench, with the gate dielectric layer interposed therebetween. The gate electrode includes a first work function liner overlapping with the vertical channel region, and including an aluminum-containing metal nitride; a second work function liner overlapping with the second junction region, and including a silicon-containing non-metal material; and an air gap positioned between the second work function liner and the second junction region.