Method of manufacturing semiconductor devices and semiconductor devices

A semiconductor device, comprising:

• a first field effect transistor (FET) including a first gate structure disposed over a first channel region;
• and
• a second FET including a second gate structure disposed over a second channel region, wherein:
• a threshold voltage of the first FET is different from a threshold voltage of the second FET,
• each of the first and second gate structures includes:
• a gate dielectric layer;
• a first work function adjustment layer over the gate dielectric layer;
• a metal gate electrode layer disposed over the first work function adjustment layer,
• wherein the second gate structure further includes a first barrier layer below the first work function adjustment layer, and the first gate structure includes no first barrier layer.

A semiconductor device, comprising:

• a first field effect transistor (FET) including a first gate structure disposed over a first channel region;
• a second FET including a second gate structure disposed over a second channel region;
• and
• a third FET including a third gate structure disposed over a third channel region, wherein:
• a threshold voltage of the first FET is smaller, in an absolute value, than a threshold voltage of the second FET, and the threshold voltage of the second FET is smaller, in an absolute value, than a threshold voltage of the third FET,
• each of the first, second and third gate structures includes:
• a gate dielectric layer;
• a first work function adjustment layer over the gate dielectric layer;
• a metal gate electrode layer disposed over the first work function adjustment layer,
• wherein the second and third gate structures further include a first barrier layer below the first work function adjustment layer, and the first gate structure includes no first barrier layer.