A HEMT device including a substrate structure, a channel layer, a barrier layer, a gate electrode, a drain electrode, a first source field plate, a second source field plate, and a dielectric structure is provided. The first source field plate extends from the second side of the gate electrode to the first side of the gate electrode. The second source field plate is located on the first side of the gate electrode and is located between the drain electrode and the first source field plate. There is a gap between the first source field plate and the second source field plate. The first source field plate has an end adjacent to the gap. The thickness of the dielectric structure located directly below the second source field plate is greater than the thickness of the dielectric structure located directly below the end of the first source field plate.