A semiconductor device includes a semiconductor substrate, a gate electrode, a channel region, a pair of source/drain regions and 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 channel region includes a pair of first sides opposing to each other in a channel length direction, and a pair of second sides opposing to each other in a channel width direction. The source/drain regions are adjacent to the pair of first sides of the channel region in the channel length direction. The threshold voltage adjusting region covers the pair of second sides of the channel region in the channel width direction, and exposing the pair of first sides of the channel region in the channel length direction.