We present detailed design, fabrication and characterization of In0.23Ga0.77As-based planar Gunn diodes in this paper. The devices have AlGaAs/InGaAs/AlGaAs heterojunctions that were grown on a semi-insulating GaAs wafer using molecular beam epitaxy technology. Electron beam lithography was used to define anode and cathode terminal patterns. Devices with various anode–cathode separations (e.g. 4–1.4 µm) were fabricated on the same chip. Spectrum measurements showed oscillation frequencies between 36 GHz and 118 GHz in the fundamental transit-time mode of operation. These devices show great potential as millimeter wave and sub-millimeter wave signal sources for their small size, MMIC compatibility and lithographically controlled oscillation frequencies.