Semiconductor devices with varying threshold voltage and fabrication methods thereof

Semiconductor device fabrication methods are provided which include: providing a structure with at least one region and including a dielectric layer disposed over a substrate; forming a multilayer stack structure including a threshold-voltage adjusting layer over the dielectric layer, the multilayer stack structure including a first threshold-voltage adjusting layer in a first region of the at least one region, and a second threshold-voltage adjusting layer in a second region of the at least one region; and annealing the structure to define a varying threshold voltage of the at least one region, the annealing facilitating diffusion of at least one threshold voltage adjusting species from the first threshold-voltage adjusting layer and the second threshold-voltage adjusting layer into the dielectric layer, where a threshold voltage of the first region is independent of the threshold voltage of the second region.