The simplest quenching circuit is commonly called passive quenching circuit and comprises a single resistor in series with the SPAD. This experimental setup has been employed since the early studies on the avalanche breakdown in junctions. The avalanche current self-quenches simply because it develops a voltage drop across a high-value ballast load RL (about 100 kΩ or more). After the quenching of the avalanche current, the SPAD bias slowly recovers to the operating bias, and therefore the detector is ready to be ignited again. This circuit mode is therefore called passive quenching passive reset (PQPR), although an active circuit element can be used for reset forming a passive quench active reset (PQAR) circuit mode. A detailed description of the quenching process is reported by Zappa et al.
