Avalanche Photodiode Doping Structure for Low-Voltage SPAD Pixels

The patent applies local quality by creating a guard ring structure with specific impurity concentration gradients around the SPAD pixel. The guard ring has a first region with higher impurity concentration and a second region with lower impurity concentration, forming a localized electric field distribution that suppresses surface leakage current without affecting the central detection region. This local modification of electric field characteristics reduces dark current while maintaining the small pixel size for high detection sensitivity.

The guard ring structure acts as an intermediary element between the high-impurity cathode region and the low-impurity active region. By introducing this intermediate structure with controlled impurity gradients, the patent mediates the electric field distribution to prevent direct lateral field effects that would otherwise cause increased dark current in miniaturized pixels. The guard ring serves as a buffer zone that maintains electrical isolation while allowing the small pixel dimensions to be maintained.

The patent applies local quality by differentiating the impurity concentration distribution in different spatial regions. The cathode region maintains high impurity concentration for good contact properties, while the guard ring transitions to lower impurity concentration in its second region to reduce lateral electric fields. This localized variation in impurity quality allows simultaneous optimization of contact properties and dark current suppression.

The guard ring is segmented into multiple regions with different impurity concentrations. The first region has higher impurity concentration to maintain electrical connection, while the second region has lower impurity concentration to reduce lateral electric field effects. This segmentation of the guard ring structure allows it to fulfill multiple functions: providing electrical continuity while simultaneously suppressing dark current through reduced lateral fields in the lower-impurity region.

The patent applies parameter changes by modifying the impurity concentration distribution in the guard ring structure. By creating a gradient from high to low impurity concentration, the electric field distribution is altered to achieve more efficient avalanche multiplication. This parameter modification allows the SPAD to operate at lower drive voltages while maintaining multiplication capability, thereby reducing heat generation and element deterioration.

The guard ring structure converts the potentially harmful high lateral electric fields into a beneficial vertical electric field distribution. By using the impurity gradient to shape the electric field, the patent transforms what would be a source of dark current (lateral fields) into a mechanism for improving avalanche multiplication efficiency. The same structural feature that suppresses dark current also enhances the useful avalanche effect at lower voltages.