We are working on the development of a novel amplification device called the Solid-state Impact-ionization Multiplier (SIM). The SIM employs the same gain mechanism found in avalanche photodiodes but has physical advantages that allow for lower-noise amplification. The SIM can also be connected to arbitrary current sources. This feature is especially interesting for sensitive light detection as the SIM can be integrated with a photodiode built from any semiconductor. This project was conducted with assistance from Raytheon Vision Systems and funding from DARPA and the National Science Foundation.


Figure 1 - Illustration of impact ionization mechanism in SIM device. Electrons enter an intrinsic semiconductor (low-doped) layer through a Schottky contact and drift toward a reverse biased p-n junction. A high electric-field produces ionization with electrons collected at the device’s anode and holes through the P-doped substrate.

We have demonstrated that SIMs can be cascaded together to produce very high gains. We have also investigated what happens under these high gain conditions, including space charge effects. We also measured the frequency response for SIMs connected to photodiodes (see Figure 2) and laid out a theoretical groundwork for understanding what limits the SIM’s frequency response. Future work with SIMs is focused on reducing the leakage current to allow lower signal detection, investigating mechanisms for operating SIMs at high frequencies, and integrating SIMs into communication receivers and with photodiode arrays.
Figure 2 - 3 dB frequency response vs injected input current (measured and theoretical)

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