A semiconductor device includes a transistor. The transistor includes gate trenches formed in a semiconductor substrate, extending in a first horizontal direction and patterning the semiconductor substrate into ridges. The ridges are arranged between two adjacent gate trenches, respectively. The transistor further includes a gate electrode arranged in at least one of the gate trenches, a source region of a first conductivity type, a channel region, and a drift region of the first conductivity type. The source region, channel region and a part of the drift region are arranged in the ridges. The gate electrode is insulated from the channel region and the drift region. The channel region includes a doped portion of a second conductivity type. A doping concentration of the doped portion decreases in a second horizontal direction intersecting the first horizontal direction from a region close to the gate electrode to a central portion of the ridge.