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Infrared sensors have been available since the 1940s to detect, measure, and monitor the thermal radiation emitted by objects. Silicon avalanche photodiodes (SiAPDs) are a potential candidate for low-level light detection, especially in the visible and near-infrared (NIR) regions due to their bias-dependent internal gain and their ability to amplify the photogenerated signal by avalanche multiplication. SiAPDs became popular for several applications including light detection and ranging (LIDAR), military, astronomy, photon counting, and fiber optic communication. They are potential candidates for applications such as quantum cryptography, profilometry of remote objects, fluorescence spectroscopy, and biomedical imaging systems such as positron emission tomography (PET), singlephoton emission computed tomography (SPECT), and NIR spectroscopy (NIRS) as a functional and noninvasive tool for brain monitoring and imaging. In all of these applications, SiAPD plays a critical role, affecting the overall performance and functionality of the device. As an example, in NIRS, the brain tissue is illuminated by NIR radiation, and the reflected signal is observed to investigate the brain's function. In the NIR range (650- 950 nm), water has relatively low absorption, while oxy- and deoxyhemoglobin have high absorption. Due to these properties, NIR light can penetrate biological tissues in the range of 0.5-3 cm, allowing investigation of relatively deep brain tissue and a potential to differentiate between healthy and diseased tissues. A critical element for NIRS front-end receivers includes a low-noise, sensitive photodetector to ensure maximum detection of the reflected NIR light that is strongly attenuated (seven to nine orders of magnitude) by the biological tissues.
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