A semiconductor device is disclosed, comprising: a substrate; a semiconductor layer disposed on the substrate; a source electrode and a drain electrode disposed on the semiconductor layer, and a gate electrode disposed between the source electrode and the drain electrode; a dielectric layer disposed on at least a part of the surface of the semiconductor layer which is between the gate electrode and the drain electrode, the dielectric layer having at least a recess therein; and a source field plate disposed on the dielectric layer and at least partially covering the recess, the source field plate being electrically connected to the source electrode through at least a conductive path, wherein a part of the source field plate above the gate electrode has a varying distance from an upper surface of the semiconductor layer. A method of manufacturing such a semiconductor device is also disclosed.