A III-nitride based semiconductor structure includes a substrate; a buffer layer disposed above the substrate; a first gallium nitrite (GaN) layer disposed above the buffer layer and including p-type GaN; a second GaN layer disposed on the first GaN layer and including at least a first region and a second region; a channel layer disposed above the second GaN layer; a barrier layer disposed above the channel layer; and a gate electrode disposed above the barrier layer. The first region of the second GaN layer is positioned correspondingly to the gate electrode and includes n-type GaN having a first doping concentration. The second region of the second GaN layer (such as the lateral portion of the second GaN layer) is positioned correspondingly to the areas outsides the gate electrode and includes n-type GaN having a second doping concentration larger than the first doping concentration.