The invention relates to a vertically structured power transistor, such as a VD-MOS or an IGBT, having a cell comprising: two symmetrical source layers (308), preferably N+ doped, which extend from a front surface (312) of the semiconductor substrate; a well layer (307), preferably P doped, comprising an area having a higher doping concentration (307b) that extends from one source layer to the other; a source/well NP junction (J3) between the source layer and the well layer. According to the invention, a cathode formed on the front surface (312) of the semiconductor substrate has a trench portion (309) with a bottom (313) that extends into the area having a higher doping concentration (307b) of the well layer (307) to a certain depth away from the source/well NP junction (J3)