A low-cost field-effect transistor with a moisture-resistant gate covered by a thick moisture-resistant insulating film which suppresses an increase in gate capacitance, and a method of manufacturing the field-effect transistor. The field-effect transistor has one of a T-shaped gate electrode and Γ-shaped gate electrode, a drain electrode, and a source electrode, the source electrode and the drain electrode being electrically connected through an n-doped semiconductor region. The gate, source, and drain electrodes are located on a semiconductor layer which includes an insulating film having a thickness of 50 nm or less and covering a surface of the gate electrode and a surface of the semiconductor layer. A silicon nitride film, deposited by catalytic CVD, covers the insulating film and includes a void volume located between a portion of the gate electrode corresponding to a canopy of an open umbrella and the semiconductor layer.