A semiconductor device and a method of forming the same, the semiconductor device include a substrate, and a first transistor, a second transistor and a third transistor all disposed on the substrate. The first transistor includes a first channel, and a first barrier layer and a first work function layer stacked with each other on the first channel. The second transistor includes a second channel, and a second barrier layer and a second work function layer stacked with each other. The third transistor includes a third channel and a third barrier layer and a third work function layer stacked with each other on the third channel, wherein the first barrier layer, the second barrier layer and the third barrier layer have different nitrogen ratio. The first, the second and the third transistors have different threshold voltages, respectively.