Frequency response of solid-state impact ionization multipliers

A study of the frequency response of solid-state impact ionization multipliers (SIMs) is presented that emphasizes the role of resistive and capacitive elements of the device to establish response limitations. SIMs are designed to amplify input currents from an external source through the impact ionization mechanism. An equivalent circuit model for the SIM is developed based on its current versus voltage characteristics, which is used to derive a frequency response model. Theoretical frequency response matches very closely to measured responses for first generation SIM devices constructed on $p$-type silicon epitaxial layers with nickel silicide Schottky contact injection points. Devices were measured using a photodiode as a current source under light intensities between $74nA$ and $7.4μA$⁠. These SIMs were shown to have a low frequency response that follows a $KT∕I$ relationship. Using an external photodiode with an effective capacitance of $6.8pF$⁠, frequency response for a $1.8μA$ input current was limited to $100kHz$⁠. A large effective barrier resistance due to the Schottky contact and $12kΩ$ space charge resistance dependent on device geometry dominate the response. Future SIM designs with higher frequency response will have to significantly lower both the input barrier resistance and space charge resistance.