In an embodiment, a Group III nitride-based transistor device, includes a first Group III nitride barrier layer arranged on a Group III nitride channel layer, the first Group III nitride barrier layer and the Group III nitride channel layer having differing bandgaps and forming a heterojunction capable of supporting a two-dimensional charge gas. A source, a gate and a drain are on an upper surface of the first Group III nitride barrier layer. A gate recess extends from the upper surface of the first Group III nitride barrier layer into the first Group III nitride barrier layer. A p-doped Group III nitride material arranged in the gate recess has a first extension extending on the upper surface of the first Group III nitride barrier layer towards the drain. The first extension has a length ld, and 0 nm≤ld≤200 nm.