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 mu m) 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. (C) 2011 Elsevier Ltd. All rights reserved.