Abstract-A monolithic circuit has been designed for active-quenching and active-reset of single-photon avalanche diodes (SPADs), which operate above the breakdown voltage BD for detecting single photons. To the best of our knowledge, this is the first fully integrated circuit of this kind ever reported. It can operate with any available SPAD device, since it generates pulses high enough to quench detectors biased up to 20 V above BD . The deadtime after each photon detection is adjustable; the minimum value is 50 ns, corresponding to 20 Mcounts/s maximum saturated photon-counting rate. The power dissipation is low (20-mW standby), suitable also for portable instruments. The small size and high reliability of the circuit make it possible to develop miniaturized detector modules and SPAD-array detector instruments. The circuit opens the path to new developments in many applications of photon counting, from DNA sequencing to ultrahigh-sensitivity imaging.Index Terms-Avalanche photodiodes, Geiger mode, photon counting, photodetectors, quenching circuits, single photon. Avalanche photodiodes (APDs) operate in a linear amplification mode, biased slightly below the breakdown voltage . Their gain is moderate (a few hundred at best) and affected by strong excess noise [7]; the detection of single photons is possible, but neither practical nor very efficient. Single-photon avalanche diodes (SPADs) are instead p-n junctions that operate in Geiger mode, biased well above the breakdown voltage. At such bias, the electric field in the depletion layer is so high that a single electron-hole pair generation can trigger a selfsustaining avalanche current in the milliampere range. Current keeps flowing until the avalanche is quenched by lowering the bias voltage below ; the SPAD must then be reset to the quiescent bias level, in order to detect subsequent photons. If the primary electron-hole pair is photogenerated, the avalanche onset marks the photon arrival time. The thermal generation of carriers randomly triggers the avalanche also without illumination (dark counts) and represents the internal detector noise. Silicon SPADs can be classified in two groups. Thin silicon SPADs [8] are planar devices with depletion layer of a few micrometers, low breakdown voltage (15-40 V), good detection efficiency (about 45% at 500-nm wavelength, 10% at 830 nm, and a few 0.1% at 1064 nm) and excellent time resolution (a few tens of picoseconds). Thick SPADs [9], [15] have reachthrough structure, depletion layer some tens of micrometers thick, high breakdown voltage (250-450 V), very good detection efficiency (higher than 50% in the 540-850-nm range and still some percent at 1064 nm) and moderate time resolution (typically, 350-ps full-width-at-half-maximum).
I. SINGLE-PHOTON DETECTORSBy increasing the excess bias voltage, SPAD detection efficiency and the timing resolution are improved, but dark-counting rate and afterpulsing effects are increased [10] and a tradeoff must be established. The excess bias voltage employed thus ranges from a few ...
