Ultrafast Single Photon Avalanche Diodes without slow tails in the pulse response

The time response of conventional single-photon avalanche diodes is plagued by a wavelength-dependent tail, arising from carriers photogenerated in the neutral regions beneath the photodiode junction, reaching the depletion layer by diffusion. In order to completely suppress the diffusion tail, the neutral region beneath the photodiode junction should be eliminated, while maintaining a conductive path for the avalanche current. The approach discussed is to design a photodiode in a p epilayer grown on an n substrate. The photodiode junction was obtained with a shallow n+ phosphorus deposition. The sensitive area of the detector is defined as a boron p implantation which lowers the junction breakdown voltage to 21 V. The metal plate of the n+ contact acts as optical diaphragm which avoids photon absorption in regions outside the sensitive volume. When the avalanche is triggered, the current flows to a side ohmic contact on the p epilayer through a boron-implanted buried layer, interrupted just beneath the sensitive area. At the operating conditions both the photodiode junctions and the substrate-epilayer junction are reverse biased and their depleted regions come into contact in correspondence to the buried layer interruption. This structure definitely avoids slow diffusion effects in the photodiode response, since only carriers photogenerated in the fully depleted volume and in the 0.3 μm thick upper n+ layer contribute to the photodiode signal