A gate electrode (3), a source electrode (4), and a drain electrode (5) is provided on a surface of the semiconductor substrate (1,2). An insulating film (6) covers the surface of the semiconductor substrate (1,2) in a region between the gate electrode (3) and the drain electrode (5). A source field plate (7) is provided on the insulating film (6) and not connected with the drain electrode (5). A diode (8) has a cathode connected with the source field plate (7) and an anode having a constant potential.

A gate electrode (3), a source electrode (4), and a drain electrode (5) is provided on a surface of the semiconductor substrate (1,2). An insulating film (6) covers the surface of the semiconductor substrate (1,2) in a region between the gate electrode (3) and the drain electrode (5). A source field plate (7) is provided on the insulating film (6) and not connected with the drain electrode (5). A diode (8) has a cathode connected with the source field plate (7) and an anode having a constant potential.