An integrated circuit (IC) device includes a semiconductor substrate having a first plurality of stacked semiconductor layers in a p-type transistor region and a second plurality of stacked semiconductor layers in a n-type transistor region. A gate dielectric layer wraps around each of the first and second plurality of stacked semiconductor layers. A first metal gate in the p-type transistor region has a work function metal layer and a first fill metal layer, where the work function metal layer wraps around and is in direct contact with the gate dielectric layer and the first fill metal layer is in direct contact with the work function metal layer. A second metal gate in the n-type transistor region has a second fill metal layer that is in direct contact with the gate dielectric layer, where the second fill metal layer has a work function about equal to or lower than 4.3 eV.