A gate controlled semiconductor device comprising a collector region of a first conductivity type; a drift region of a second conductivity type located over the collector region; a body region of a first conductivity type located over the drift region; at least one first contact region of a second conductivity type located above the body region and having a higher doping concentration compared to the body region. The device further comprises at least one second contact region of a first conductivity type located laterally adjacent to the at least one first contact region, the at least one second contact region having a higher doping concentration than the body region. The device further comprises at least one active trench extending from a surface into the drift region, in which the at least one first contact region adjoins the at least one active trench so that, in use, a channel region is formed along said at least one active trench and within the body region. The at least one active trench comprises: two vertical sidewalls and a bottom surface between the two vertical sidewalls; and an insulation layer along the vertical side walls and the bottom surface, wherein the insulation layer along at least one vertical side wall comprises different thicknesses; at least one auxiliary trench extending from the surface into the drift region. The at least one auxiliary trench comprises: two vertical sidewalls and a bottom surface between the two vertical sidewalls; and an insulation layer along the vertical side walls and the bottom surface, wherein the insulation layer along at least one vertical side wall comprises a constant thickness.