A semiconductor device according to an embodiment includes a first gate-all-around (GAA) transistor and a second GAA transistor. The first GAA transistor includes a first plurality of channel members, a first interfacial layer over the first plurality of channel members, a first hafnium-containing dielectric layer over the first interfacial layer, and a metal gate electrode layer over the first hafnium-containing dielectric layer. The second GAA transistor includes a second plurality of channel members, a second interfacial layer over the second plurality of channel members, a second hafnium-containing dielectric layer over the second interfacial layer, and the metal gate electrode layer over the second hafnium-containing dielectric layer. A first thickness of the first interfacial layer is greater than a second thickness of the second interfacial layer. A third thickness of the first hafnium-containing dielectric layer is smaller than a fourth thickness of the second hafnium-containing dielectric layer.