A semiconductor device includes a first nitride-based semiconductor layer, a second nitride-based semiconductor layer, a first gate electrode, a first S/D electrode, and a first field plate. The second nitride-based semiconductor layer is disposed on the first nitride-based semiconductor layer. The first and second nitride-based semiconductor layers collectively have an active portion and an electrically isolating portion that is non-semi-conducting and surrounds the active portion to form at least two interfaces extending along a first direction and spaced apart from each other by the active portion. The first gate electrode and the first S/D electrode are disposed above the second nitride-based semiconductor layer. The first field plate is disposed above the second nitride-based semiconductor layer and extends along the second direction and across the two interfaces such that the field plate extends to the electrically isolating portion, and overlaps with the first gate electrode near the interfaces.