A semiconductor device includes a channel layer including a channel; a channel supply layer on the channel layer; a channel separation pattern on the channel supply layer; a gate electrode pattern on the channel separation pattern; and an electric-field relaxation pattern protruding from a first lateral surface of the gate electrode pattern in a first direction parallel with an upper surface of the channel layer. An interface between the channel layer and the channel supply layer is adjacent to channel. A size of the gate electrode pattern in the first direction is different from a size of the channel separation pattern in the first direction. The gate electrode pattern and the electric-field relaxation pattern form a single structure.