Integrated circuits and fabrication methods are provided. The integrated circuit includes: a varying gate structure disposed over a substrate structure, the varying gate structure including a first gate stack in a first region of the substrate structure, and a second gate stack in a second region of the substrate structure; a first field-effect transistor in the first region, the first field-effect transistor including the first gate stack and having a first threshold voltage; and a second field-effect transistor in the second region, the second field-effect transistor including the second gate stack and having a second threshold voltage, where the first threshold voltage is different from the second threshold voltage. The methods include providing the varying gate structure, the providing including: sizing layer(s) of the varying gate structure with different thickness(es) in different region(s).