A method includes forming a stack of semiconductor layers over a substrate. The stack includes a first layer including a first semiconductor material over the substrate, a second layer including a second semiconductor material over the first layer, a third layer including the first semiconductor material over the second layer, and a fourth layer including a third semiconductor material over the third layer. The method further includes patterning the stack to form a semiconductor structure, forming a sacrificial gate over the semiconductor structure, forming epitaxial regions adjacent to the sacrificial gate, removing the sacrificial gate to form a recess, selectively removing the first layer and the third layer from the semiconductor structure through the recess to form an opening, selectively removing the second layer from the semiconductor structure through the recess to expand the opening, and forming a replacement gate in the recess and the opening.