A method includes providing a substrate; forming a first structure over the substrate, the first structure including a first gate trench and a first channel exposed in the first gate trench; forming a second structure over the substrate, the second structure including a second gate trench and a second channel exposed in the second gate trench; depositing a gate dielectric layer covering surfaces of the first and second channels exposed in the respective first and second gate trenches; recessing the gate dielectric layer in the second gate trench to be lower than the gate dielectric layer in the first gate trench; and forming a gate electrode layer over the gate dielectric layer in the first and second gate trenches.