The invention discloses a semiconductor device and a manufacturing method thereof. The semiconductor device comprises: a substrate; a semiconductor layer which is located on the substrate, wherein two-dimensional electron gas is formed in the semiconductor layer; and a source electrode and a drain electrode which are located on the side, away from the substrate, of the semiconductor layer, and a grid electrode which is located between the source electrode and the drain electrode, wherein a buried layer is formed in the semiconductor layer on one side, close to the substrate, of the two-dimensional electron gas, the buried layer and the semiconductor layer form a pn junction, the buried layer comprises a first buried layer and / or at least one second buried layer, the edge, close to the drain electrode, of the grid electrode is overlapped with the first buried layer, one part of the first buried layer extends from the edge, close to the drain electrode, of the grid electrode to the drainelectrode, and / or the second buried layer is located between the grid electrode and the drain electrode. According to the embodiment of the invention, the buried layer is formed in the semiconductorlayer, so that the breakdown voltage of the semiconductor device is improved.