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 ...