US Patent 11189743 Single photon avalanche diode

A photodetector includes: a substrate; a first semiconductor region, the first semiconductor region extending into the substrate from a front side of the substrate; and a second semiconductor region, the second semiconductor region further extending into the substrate from a bottom boundary of the first semiconductor region, wherein when the photodetector operates under a Geiger mode, the second semiconductor region is fully depleted to absorb a radiation source received from a back side of the substrate.