A semiconductor transistor device operable as a RF mixer is described. The device includes a field effect transistor with a field plate electrode. The field plate electrode is structured and located to be electromagnetically coupled to the gate electrode of the field effect transistor when carrying a radio frequency oscillation signal, to cause the field effect transistor to produce an output signal that is a mixed signal at the gate and the field plate. A semiconductor device for a radio receiver is also described. The device includes a field effect transistor with a field plate. A transient voltage signal is applied to the field plate responsive to detection of an interfering signal.