A semiconductor device includes a group III-semiconductor-nitride-based channel layer, a group III-semiconductor-nitride-based barrier layer formed on the channel layer, a two-dimensional electron gas channel formed in the channel layer, a first current electrode and a second current electrode formed on the barrier layer and laterally spaced from each other, and a gate structure formed on the barrier layer between the first and second current electrodes. The barrier layer has a symmetrically shaped recess between the first and second current electrodes, the symmetrically shaped recess including a first recess portion formed in a part of an upper surface of the barrier layer and a second recess portion formed within the first recess portion. The gate structure includes a group III-semiconductor-nitride-based doped layer that fills the symmetrically shaped recess and an electrically conductive gate electrode formed on an upper side of the doped layer that is opposite from the barrier layer.