The invention relates to a normally-off high-electron-mobility field-effect transistor having a superposition of a first layer of semiconductor material and a second layer of semiconductor material so as form an electron gas layer at the interface between the first and second layers. A trench separates the superposition into first and second domains. An insulating element is positioned in the trench in order to electrically insulate the first and second domains. A p-doped semiconductor element is in contact with the first or the second layer of semiconductor material of the first and second domains, and extends continuously between the first and second domains. A gate insulator is positioned on the semiconductor element and a gate electrode is positioned on the gate insulator.