Geiger-Mode Avalanche Photodiodes in Standard CMOS Technologies

Photodiodes are the simplest but most versatile semiconductor optoelectronic devices. They \ncan be used for direct detection of light, of soft X and gamma rays, and of particles such as \nelectrons or neutrons. For many years, the sensors of choice for most research and industrial \napplications needing photon counting or timing have been vacuum-based devices such as \nPhoto-Multiplier Tubes, PMT, and Micro-Channel Plates, MCP (Renker, 2004). Although \nthese photodetectors provide good sensitivity, noise and timing characteristics, they still \nsuffer from limitations owing to their large power consumption, high operation voltages \nand sensitivity to magnetic fields, as well as they are still bulky, fragile and expensive. New \napproaches to high-sensitivity imagers tend to use CCD cameras coupled with either MCP \nImage Intensifiers, I-CCDs, or Electron Multipliers, EM-CCDs (Dussault & Hoess, 2004), but \nthey still have limited performances in extreme time-resolved measurements. \nA fully solid-state solution can improve design flexibility, cost, miniaturization, integration \ndensity, reliability and signal processing capabilities in photodetectors. In particular, Single- \nPhoton Avalanche Diodes, SPADs, fabricated by conventional planar technology on silicon \ncan be used as particle (Stapels et al., 2007) and photon (Ghioni et al., 2007) detectors with \nhigh intrinsic gain and speed. These SPAD are silicon Avalanche PhotoDiodes biased above \nbreakdown. This operation regime, known as Geiger mode, gives excellent single-photon \nsensitivity thanks to the avalanche caused by impact ionization of the photogenerated \ncarriers (Cova et al., 1996). The number of carriers generated as a result of the absorption of \na single photon determines the optical gain of the device, which in the case of SPADs may \nbe virtually infinite. \nThe basic concepts concerning the behaviour of G-APDs and the physical processes taking \nplace during their operation will be reviewed next, as well as the main performance \nparameters and noise sources.