A nitride-based semiconductor device includes a buffer, a first nitride-based semiconductor layer, a shield layer, a second nitride-based semiconductor layer, a pair of S/D electrodes, and a gate electrode. The first nitride-based semiconductor layer is disposed over the buffer and forms a first interface with the buffer. The shield layer includes a first isolation compound and is interposed between the buffer and the first nitride-based semiconductor layer. The first isolation compound has a bandgap greater than a bandgap of the buffer and greater than a bandgap of the first nitride-based semiconductor layer. The second nitride-based semiconductor layer is disposed on the first nitride-based semiconductor layer and has a bandgap less than the bandgap of the first isolation compound and greater than the bandgap of the first nitride-based semiconductor layer. The S/D electrodes and a gate electrode are disposed over the second nitride-based semiconductor layer.