If the depletion region becomes too deep, the electric field strength may become insensitive to the reverse bias When the applied potential has to drop uniformly over a large distance, the field strength does not increase rapidly with the reverse voltage. Thick NIR-sensitive SPADs with depleted absorption volumes generally achieve a photon timing resolution between 100 ps and 300 ps, which is higher than thin SPADs. To attain an acceptable timing resolution, an electric field must be present in the absorption volume, for example, by depletion. The deep absorption region potentially results in significant variability of the transport time of photogenerated carriers. Therefore, to scale down silicon SPADs without significant impact on the PDE, further device improvements are required.Īnother consideration for NIR silicon SPADs is the photon timing resolution. The corresponding loss in the detection efficiency is more substantial for small SPADs, in which the peripheral region is proportionally larger. Carriers generated in this peripheral volume have a low probability of moving through the field peak and triggering breakdown. A guard ring and other doped regions are located near the edges of the planar junction to prevent premature breakdown, establish electrical contacts, and provide isolation. The breakdown triggering probability relates to the strength of the multiplication field, which increases gradually with the excess bias. Charge carriers generated inside and below the multiplication region can trigger discrete avalanche breakdown events. This field peak constitutes the multiplication region of the device. When the device operates at a finite excess bias above the breakdown voltage, the p-n junction contains a laterally uniform electric field peak. By virtue of these features, the device architecture is well-suited for large format ToF imaging arrays with integrated electronics.Ī typical NIR-sensitive silicon SPAD contains a planar p-n junction embedded in a thick lowly-doped or graded absorption volume. Furthermore, the detector achieves a photon detection efficiency of 27% at 905 nm, with an excess bias of 3.5 V that is controlled by integrated CMOS electronics, and a timing resolution of 240 ps. The device has a pitch of 15 µm, which has the potential to be scaled down without significant performance loss. The SPAD is integrated with a customized 130 nm CMOS technology and a dedicated BSI process. A charge-focusing electric field extends into a 10 µm deep absorption volume, whereby electrons generated in all corners of the device can move efficiently towards the multiplication region. The detector contains a 2 µm wide multiplication region with a spherically-uniform electric field peak enforced by field-line crowding. A backside-illuminated (BSI) near-infrared enhanced silicon single-photon avalanche diode (SPAD) for time-of-flight (ToF) light detection and ranging applications is presented.
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