Gated IR imaging with 128x128 HgCdTe electron avalanche photodiode FPA

The next generation of IR sensor systems will include active imaging capabilities. One example of such a system is a gated-active/passive system. The gated-active/passive system promises long-range target detection and identification. A detector that is capable of both active and passive modes of operation opens up the possibility of a self-aligned system that uses a single focal plane. The detector would need to be sensitive in the 3-5 mu m band for passive mode operation. In the active mode, the detector would need to be sensitive in eye-safe range, e.g. 1.55 mu m, and have internal gain to achieve the required system sensitivity. The MWIR HgCdTe electron injection avalanche photodiode (e-APD) not only provides state-of-the-art 3-5 mu m spectral sensitivity, but also high avalanche photodiode gain without minimal excess noise. Gains of greater than 1000 have been measured in MWIR e-APDs with a gain independent excess noise factor of 1.3. This paper reports the application of the mid-wave HgCdTe e-APD for near-IR gated-active/passive imaging. Specifically a 128x128 FPA composed of 40 mu m pitch, 4.2 mu m to 5 mu m cutoff, APD detectors with a custom readout integrated circuit was designed, fabricated, and tested. Median gains as high as 946 at I I V bias with noise equivalent inputs as low as 0.4 photon were measured at 80 K. A gated imaging demonstration system was designed and built using commercially available parts. High resolution gated imagery out to 9 km was obtained with this system that demonstrated predicted MTF, precision gating, and sub 10 photon sensitivity.