The high-gain photodetector is formed in a semiconductor-material body which
houses a PN junction and a sensitive region that is doped with rare earths, for
example erbium. The PN junction forms an acceleration and gain region separate
from the sensitive region. The PN junction is reverse-biased and generates an extensive
depletion region accommodating the sensitive region. Thereby, the incident photon
having a frequency equal to the absorption frequency of the used rare earth crosses
the PN junction, which is transparent to light, can be captured by an erbium ion
in the sensitive region, so as to generate a primary electron, which is accelerated
towards the PN junction by the electric field present, and can, in turn, generate
secondary electrons by impact, according to an avalanche process. Thereby, a single
photon can give rise to a cascade of electrons, thus considerably increasing detection efficiency.