A method is provided of manufacturing a semiconductor device comprising a first, n-type field effect transistor (1) and a second, p-type field effect transistor (2). The method comprises depositing a gate dielectric layer over a substrate; depositing a gate metal layer (22) over the gate dielectric layer, depositing a solid metal oxide layer (15) over the gate dielectric layer; removing a portion of the solid metal oxide layer (15) over an area of the substrate corresponding to the n-type transistor; and completing gate stacks for the n-type and p-type transistors and forming source and drain regions. The invention thus provides a device which is compatible with IC technology and easy to manufacture. The deposition of a solid metal oxide layer provides a simplified manufacturing process, by avoiding the complexity of gas exposure to form an oxide layer.