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Impact ionization in GaAs-based planar Gunn diodes is studied through electroluminescence (EL) analysis with the aim of reducing its magnitude by means of contact design and shaping, and thus enhance device performance and reliability. Designs in which the diode ohmic anode has an overhanging Schottky extension (composite anode contact) are shown to result in a significantly reduced amount of impact ionization, as compared with a simple ohmic contact design. The EL results are consistent with Monte Carlo simulations, which show a reduced impact ionization in composite anode contact devices due to a reduced electron density beneath the anode Schottky extension that, on the one hand, weakens the Gunn domain electric field and softens its variations near the anode edge, and, on the other hand, reduces the number of electrons capable of generating holes by impact ionization. A comparison between standard and composite anode contact approaches in terms of radio-frequency operation of the devices is made showing oscillations up to 109 GHz with an output power of -5 dBm in devices featuring the composite anode contact and no oscillations from all-ohmic contact devices. The findings reported in this paper may be useful not only for the design and the fabrication of planar Gunn diodes but also for other devices such as high-electron-mobility transistors where impact ionization can result in reliability limitations.