A semiconductor structure is provided. The semiconductor structure includes: a substrate; a source structure and a drain structure disposed on the substrate; a gate structure disposed on the substrate and between the source structure and the drain structure; a first field plate disposed on the substrate; a first oxide layer disposed between the substrate and the first field plate; a second field plate disposed on the first field plate, wherein the second field plate is closer to the drain structure than the first field plate; a planarized second oxide layer disposed between the first oxide layer and the second field plate; and a third field plate disposed on the second field plate, wherein the third field plate is closer to the drain structure than the second field plate.