A method of manufacturing a semiconductor device includes providing a substrate structure including a semiconductor substrate, an interlayer dielectric layer on the semiconductor substrate, multiple trenches extending through the interlayer dielectric layer to the semiconductor substrate and including a first trench of a first NMOS device and a second trench of a second NMOS device, and a dielectric layer on sidewalls and a bottom of the trenches, forming an NMOS work function adjustment layer on the dielectric layer, performing a first oxidation treatment on the NMOS work function adjustment layer in the first trench to form a first oxide layer, and a second oxidation treatment on the NMOS work function adjustment layer in the second trench to form a second oxide layer, and forming a metal electrode layer in the trenches. The first oxide layer has an oxygen content lower than that of the second oxide layer.