A field effect semiconductor device includes a semiconductor body having a main horizontal surface and a first semiconductor region of a first conductivity type, a second semiconductor region of a second conductivity type arranged between the first semiconductor region and the main horizontal surface, an insulating layer arranged on the main horizontal surface, and a first metallization arranged on the insulating layer. The first and second semiconductor regions form a pn-junction. The semiconductor body further has a deep trench extending from the main horizontal surface vertically below the pn-junction and including a conductive region insulated from the first semiconductor region and the second semiconductor region, and a narrow trench including a polycrystalline semiconductor region extending from the first metallization, through the insulating layer and at least to the conductive region. A vertical poly-diode structure including a horizontally extending pn-junction is arranged at least partly in the narrow trench.