The structure of a silicon avalanche photodiode (APD) has a significant impact on the probability of light creating charge carriers and on the generated noise due to the multiplication process or excess noise factor (ENF). In this paper, we will review front-illuminated and back-illuminated APD structures and their impact on ENF as a function of wavelength from 400 nm to 1000 nm for recently commercially produced silicon APDs targeting LIDAR and other applications. The experimental setup developed for characterization will be described and highlight the differences between the studied structures. APDs with different junction profiles were produced and measurement of ENF was found to match McIntyre’s theory for experimental k-factors (ratio of the hole impact ionization rate to that of electrons) ranging from approximately 0.05 to 0.008. The generated illuminated noise as a function of responsivity can be used as a guideline to select the APD achieving the best signal-to-noise ratio (SNR) for a given application. To help meeting this condition, optimizing the electrical field profile of an APD and making certain the electrons are the primary carriers initiating the avalanche is critical.
Silicon Avalanche Photodiodes (APDs) are used in NASA’s Global Ecosystem Dynamics Investigation (GEDI) that was launched in December 2018 and is currently measuring the Earth’s vegetation vertical structure from the International Space Station. The APDs were specially made for space lidar with a much lower hole-to-electron ionization coefficient ratio (k-factor ~0.008) than that of commercially available silicon APDs in order to reduce the APD excess noise. A silicon heater resistor was used under the APD chip to heat the device to 70°C to improve its quantum efficiency at the 1064-nm laser wavelength while maintaining a low dark current such that the overall signal to noise ratio is optimized. Special APD protection circuits were included to raise the overload damage threshold to prevent device damage from strong laser returns from specular surfaces, such as still water bodies, and space radiation events. The APD and a hybrid transimpedance amplifier circuit were hermetically sealed in a TO-8 type metal package with a sufficiently low leak rate to ensure a multi-year operation lifetime in space. The detector assemblies underwent a series of pre-launch tests per NASA Goddard Environmental Verification Standard for space qualification. The APDs have performed exactly as expected in space. A detailed description of the GEDI detector design, signal and test results are presented in this paper.
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