SPADs in CMOS: When Physics Meets Engineering

Single-photon avalanche diodes (SPADs) are a class of photodiodes biased above breakdown; in this mode of operation, known as Geiger mode, the devices are capable of detecting one or more photons with high timing accuracy. SPADs are useful in a variety of applications where picosecond timing accuracy, >100dB dynamic range, and high readout speed are requirements [1]. Integrating SPADs in planar silicon processes is a relatively recent achievement that has propelled this relatively unknown device onto quasi-stardom [2]. But, while the physics of solid-state SPADs is well understood [3], it is only with the advent of devices integrated in conventional CMOS processes [4], that the evolution onto smaller and smaller feature sizes has rapidly advanced to the point that it has now become possible to envision large imaging systems based on SPADs.