A transistor is provided. The transistor includes a first source/drain epitaxial feature, a second source/drain epitaxial feature, and two or more semiconductor layers disposed between the first source/drain epitaxial feature and the second source/drain epitaxial feature. The two or more semiconductor layers comprise different materials. The transistor further includes a gate electrode layer surrounding at least a portion of the two or more semiconductor layers, wherein the transistor has two or more threshold voltages.