A method of forming a semiconductor device includes following steps. Firstly, a first transistor is formed on a first surface of a substrate. Next, a thinning process is performed on the second surface of the substrate which is opposite to the first surface, to form a third surface. Then, a second transistor is formed on the third surface, in which the second transistor and the first transistor are electrically connected to each other through a through-silicon via penetrating through the first surface and the third surface.