A semiconductor device includes a second semiconductor substrate vertically stacked on a first semiconductor substrate. The first semiconductor substrate includes a first diffusion barrier layer covering a first surface of a first semiconductor substrate body, and a first through via having a third surface exposed to a second surface of the first diffusion barrier layer. The second semiconductor substrate includes a second semiconductor substrate body, a second diffusion barrier layer directly bonded to a surface of the first diffusion barrier layer, and a front pad having a smaller surface area than the third surface of the first through via and directly bonded to the third surface of the first through via.