A method for forming a semiconductor device is provided. The method includes providing a semiconductor substrate having a main horizontal surface, an opposite surface and a completely embedded dielectric region. A deep vertical trench is etched from the main horizontal surface into the semiconductor substrate using the dielectric region as an etch stop. A vertical transistor structure is formed in the semiconductor substrate. A first metallization in ohmic contact with the transistor structure is formed on the main horizontal surface. The semiconductor substrate is thinned at the opposite surface at least close to the dielectric region. Further, a semiconductor device is provided.