Gate structures and methods of manufacture

A method, comprising:

• forming dummy gate structures on a substrate;
• forming sidewall structures on sidewalls of the dummy gate structures;
• removing the dummy gate structures to form a plurality of trenches, defined by the sidewall structures;
• forming a channel material in a first trench and a second trench;
• forming a thick oxide layer on the channel material in the first trench and the second trench, and on the substrate in a third trench and a fourth trench;
• and
• removing the thick oxide layer from the second trench to expose the channel material and the fourth trench to expose the substrate;
• forming a thin oxide layer on the channel material in the second trench and the substrate in the fourth trench;
• and
• filling remaining portions of the first trench, the second trench, the third trench and the fourth trench with gate material, wherein:
• the filling comprises:
• forming an insulating material in the first trench, the second trench, the third trench and the fourth trench;
• depositing a high-k dielectric material on the insulating material in the first trench, the second trench, the third trench and the fourth trench;
• and
• depositing metal materials on the high-k dielectric material in the he first trench, the second trench, the third trench and the fourth trench.