Variable-load quenching circuit for single-photon avalanche diodes.

We present a compact 50 microm x 100 microm cell for single-photon detection, based on a new circuitry monolithically integrated together with a 20 microm-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 microm 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 micros) 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.