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A solid-state impact-ionization multiplier (SIM) was designed to amplify signals from arbitrary current sources through impact ionization. A primary application is amplification of signals produced by photodiodes. Photodiodes made from any semiconductor can be wired directly to the SIM's injection node. Planar versions of the SIM suffer from nonideal impact ionization efficiency as a result of injected carriers drifting through the device's depletion region to the output electrode without passing through the highest electric field regions and undergoing ionization events. Low impact ionization efficiency can lead to an increased excess noise factor, higher temperature sensitivity, and higher voltage sensitivity (rate of gain change with respect to applied voltage). This paper describes increasing SIM ionization efficiencies by introducing an insulator between the SIM's injection and output electrodes, effectively directing the carriers into the highest electric field. This method has shown to greatly increase the impact ionization efficiency in simulation and experimental results. Ionization efficiency improvements are demonstrated primarily through decreases in voltage sensitivity.