In an embodiment, a Group III nitride-based transistor device includes a source electrode, a drain electrode and a gate electrode positioned on a first major surface of a Group III nitride based-based layer, wherein the gate electrode is laterally arranged between the source electrode and the drain electrode, a passivation layer arranged on the first major surface and a field plate coupled to the source electrode, the field plate having a lower surface arranged on the passivation layer. The field plate is laterally arranged between and laterally spaced apart from the gate electrode and the drain electrode.