Variable-Load Quenching Circuit for Single-Photon Avalanche Diodes

We present a compact 50 µm x 100 µm cell for single-photon detection, based on a new circuitry monolithically integrated together with a 20 µm-diameter CMOS Single-Photon Avalanche Diode (SPAD). The detector quenching relies on a novel mechanism based on starving the avalanche current till quenching through a variable-load (VLQC, Variable-Load Quenching Circuit). Fabricated in a standard 0.35 µm CMOS technology, the topology allows a SPAD bias voltage higher than the chip supply voltage to be used. Moreover it preserves the advantages of active quenching circuits, in terms of hold-off capability (from 40 ns to 2 µs) and fast reset (<2 ns), while maintaining the low avalanche charge (<1.6 pC/avalanche) and extremely small dimensions of passive quenching circuits. The cell enables the development of large-dimension dense arrays of SPADs, for two-dimensional imaging at the photon counting level with photon-timing jitter better than 40 ps.