A ferroelectric semiconductor device includes an active region extending in one direction, a gate insulating layer crossing the active region, a ferroelectric layer disposed on the gate insulating layer and including a hafnium oxide, a gate electrode layer disposed on the ferroelectric layer, and source/drain regions disposed on the active region to be adjacent to both sides of the gate insulating layer, wherein the ferroelectric layer includes 20% or more of orthorhombic crystals, and an upper surface of the source/drain region is located at a level equal to or higher than an upper surface of the ferroelectric layer.