The 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. Previous versions of the SIM suffer from non-ideal impact ionization efficiency as a result of injected carriers drifting through the device's depletion region without passing through the highest electric field regions and undergoing ionization events. Low impact ionization efficiency can lead to an increased excess noise factor, temperature sensitivity, and voltage sensitivity. This manuscript 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.