Planar Indium Gallium Arsenide (In0.57Ga0.47As) Gunn diode was fabricated on a semi-insulating indium phosphide substrate with on chip matching circuit to enable the extraction of second harmonic in millimeter and terahertz frequencies. The In0.57Ga0.47As planar Gunn diodes were designed with an active length of 4 mu m, channel width of 120 lm and integrated with a novel diamond resonator to suppress the fundamental and extract the second harmonic. The experimental results gave good fundamental suppression and extraction of second harmonic (121.68 GHz) with an RF output power of -14.1 dBm. This is highest recorded power at the second harmonic from a planar Gunn diode. (C) 2015 The Authors. Published by Elsevier Ltd.