A semiconductor device includes a semiconductor substrate, a gate electrode, a pair of source/drain regions and a a threshold voltage adjusting region. The gate electrode is over the semiconductor substrate. The channel region is between the semiconductor substrate and the gate electrode. The source/drain regions are adjacent to two opposing sides of the channel region in a channel length direction. The threshold voltage adjusting region is adjacent to two opposing sides of the channel region in a channel width direction, wherein the threshold voltage adjusting region and the channel region have the same doping type.