In an embodiment, a method includes forming a first gate electrode over a substrate. The method also includes forming a first gate dielectric layer over the first gate electrode. The method also includes depositing a semiconductor layer over the first gate dielectric layer. The method also includes forming source/drain regions over the first gate dielectric layer and the semiconductor layer, the source/drain regions overlapping ends of the semiconductor layer. The method also includes forming a second gate dielectric layer over the semiconductor layer and the source/drain regions. The method also includes and forming a second gate electrode over the second gate dielectric layer.