Compact SPAD pixels with fast and accurate photon counting in the analog domain

A compact pixel for single-photon detection in the analog domain is presented. The pixel integrates a single-photon avalanche diode(SPAD), a passive quenching & active recharging circuit(PQARC), and an analog counter for fast and accurate sensing and counting of photons. Fabricated in a standard 0.18 μm CMOS technology, the simulated and experimental results reveal that the dead time of the PQARC is about 8 ns and the maximum photon-counting rate can reach 125 Mcps(counting per second). The analog counter can achieve an 8-bit counting range with a voltage step of 6.9 mV. The differential nonlinearity(DNL) and integral nonlinearity(INL) of the analog counter are within the ± 0.6 and ± 1.2 LSB, respectively, indicating high linearity of photon counting. Due to its simple circuit structure and compact layout configuration, the total area occupation of the presented pixel is about 1500 μm^(2), leading to a high fill factor of 9.2%. The presented in-pixel front-end circuit is very suitable for the high-density array integration of SPAD sensors.