A method of manufacturing a high-electron-mobility transistor device is provided. The method includes sequentially forming a transition layer and a semiconductor layer on a substrate, etching a portion of a surface of the semiconductor layer to form a barrier layer region having a certain depth and forming a barrier layer in the barrier layer region, forming a source electrode and a drain electrode on a 2-dimensional electron gas (2-DEG) layer upward exposed at a surface of the semiconductor layer, in defining the 2-DEG layer formed along an interface between the semiconductor layer and the barrier layer, forming a passivation layer on the semiconductor layer, the barrier layer, the source electrode, and the drain electrode and etching a portion of the passivation layer to upward expose the source electrode, the drain electrode, and the barrier layer, and forming a gate electrode on the upward exposed barrier layer.