Fully-Integrated Active-Quenching Circuit for Single-Photon Detection

A monolithic active-quenching and active-reset circuit is presented, designed for avalanche photodiodes that detect single-photons by operating above the breakdown voltage (VB) in a digital mode, known as SPAD's. To the best of our knowledge, this is the first fully-integrated circuit of this kind ever reported. It operates with any existing SPAD, also with very high VB, since the quenching pulse is high enough to operate a SPAD up to 30V above VB. The deadtime after each photon detection is adjustable; the minimum value is 50ns, i.e. the maximum saturated counting rate is 20Mcounts/s. Free-running and gated-detector operation are provided; the minimum gate-on time is about 20ns. The small size, low power dissipation (20mW stand-by) and high reliability of the circuit make it possible to develop miniaturized detection modules, for single detectors and for detector-arrays. Various applications are interested: DNA sequencing, photoncorrelation spectroscopy, laser ranging, CMOS circuit testing by electroluminescence measurements, etc.. 1. Single-Photon Detectors and Circuits When very faint optical signals must be measured, single-photon detection [1] is the technique of choice. The intensity of steady (or slowly varying) optical signals is measured by the number of detected photons during the measurement time (photon counting); the waveform of fast optical signals is obtained by Time-Correlated Single-Photon Counting [1] (photon timing). Important applications are found in many fields: non-invasive testing of VLSI circuits [2]; luminescence microscopy [3]; fluorescent decays in physics, chemistry, biology [4]; laser diode and fiber optics characterization [5]; laser ranging in space and telemetry [6]; laser velocimetry and dynamic light scattering [7]; quantum mechanics [8]; criptography [9]; astronomy [10]; single molecule detection [11], and so on. Avalanche-Photodiodes (APDs) operating in the linear amplification mode (analog mode) are biased slightly below the breakdown voltage. Their gain is limited to a few hundred at best and is affected by strong statistical fluctuations. The detection of single photons with such APDs is possible, but not practical and not very efficient. It is much more efficiently obtained by exploiting avalanche photodiodes operating in the socalled Geiger-mode (digital mode), biased well above the breakdown voltage. Such detectors are currently called Single-Photon Avalanche Diodes (SPADs). The absorption of a photon releases a carrier pair, which starts an avalanche build-up leading to a macroscopic self-sustaining avalanche current. The transition from quiescent condition to full avalanche current denotes the detection of a photon and marks its arrival time. Then, in order to detect other subsequent photons, the avalanche current must be quenched and the SPAD reset to quiescent conditions. Simple Passive-Quenching circuits can be employed to the purpose, but in order to fully exploit the available performance of SPAD's, ActiveQuenching Circuits (AQC) are required [13]. A monolithic integrated circuit designed for activequenching and active-reset of SPAD's (iAQC) is herewith introduced. At the best of our knowledge, this is the first reported fully-integrated Active Quenching Circuit (European and US patent pending [14]). 2. Basic Features of the Circuit The circuit is designed to operate with any known SPAD, including types with high breakdown voltage (VB), exceeding 400V, which operate more than 20V above VB and generate high avalanche currents, exceeding 100mA. In a previous work, we reported an AQC design where the basic block of the circuit was an ASIC, and some external discrete DMOS transistors [15] were added for providing the capability of biasing the detector tens of Volts above VB. Complete monolithic integration of the AQC has now been attained by designing the circuit in a standard high-voltage 0.8μm CMOS technology of AMS, Austria Microsystems [16], with two layers of metal and a high resistive poly. A basic requirement is to minimize the duration of the avalanche current through the SPAD, because three detrimental effects increase proportionally to the total charge in the avalanche current pulse. First, a small percentage of the avalanche current carriers are trapped in deep levels within the junction and are then released with a significant delay. They can thus re-trigger the SPAD [13] and cause an afterpulsing effect, which very