Mixer circuitry having a semiconductor body formed therein mixer circuitry having an oscillator having a heterojunction bipolar transistor and a mixer having a Schottky diode. The heterojunction transistor has a collector region formed in one portion of doped layer of the semiconductor body and the diode has a metal electrode is Schottky contact with another portion of such doped layer. The mixer is includes a diode and a DC biasing circuit, comprising a constant current, for biasing such diode to predetermined operating point substantially invariant with power of an input signal fed to such mixer.