Near-Infrared Light Detection using CMOS Silicon Avalanche Photodiodes (SiAPDs)
Ehsan Kamrani; Frédéric Lesage; Mohamad Sawan
DOI: 10.1117/3.1002245.ch20
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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.

A minimal signal-to-noise ratio (SNR) of ~40 dB is usually needed for low-intensity light-detection application. SiAPDs with dark current in the nano-ampere range, and the generated photocurrent in the hundreds of micro-ampere range confirms SNR of much higher than 40 dB. SiAPDs have been commercially available for more than 30 years, usually built with a dedicated process, which does not allow monolithic integration with other electronic circuitry. The main characteristics of the most common photodetectors are summarized and compared in Table 20.1. As shown in this table, the SiAPD offers good characteristics for NIR light-detection applications. The silicon-based APD as an indirect-bandgap semiconductor (in contrast to the direct-bandgap semiconductors such as GaAs) has emerged as a versatile and easy-to-use detector when compared to other available detectors. Optical sensors for extremely low-level-light conditions must convert each incoming photon into a measurable electrical signal. Single-photon detectors, as these sensors are called, can be employed in vision systems with 3D imaging and ranging capability, for sensing at night or in caves, for low-data-rate intra- and inter-platform communications, and for molecular sensing in bio-analytical fluorescence imaging. Geiger-mode SiAPDs (GM-SiAPDs) have been developed in a variety of processes and for many different applications. The first generation of CMOS SiAPD devices use a large-feature-size CMOS process to maximize the silicon die area for a low-cost solution, but the use of a highly doped layer at the GM-SiAPD device surface limits the performance of the SiAPDs, particularly in optical detection efficiency and dark-noise response.

© 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)

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