A nitride-based semiconductor device including a first and a second nitride-based semiconductor layers, a source electrode and a drain electrode, and a gate structure. The gate structure includes at least one conductive layer and two or more doped nitride-based semiconductor layers. The at least one conductive layer includes metal, and is in contact with the second nitride-based semiconductor layer to form a metal-semiconductor junction therebetween. The two or more doped nitride-based semiconductor layers are in contact with the second nitride-based semiconductor layer and abut against the conductive layer, so as to form contact interfaces abutting against the metal-semiconductor junction with the second nitride-based semiconductor.