[en] Highlights: • Heavy doping of Germanium nanocrystallites (NCs). • Photoconductivity measurements for effective band gap determination. • Band gap narrowing analysis. • Electrical activation of ~15% is found for the incorporated P impurities in the NCs. We investigate the photoconductivity of a n⁺-ZnO/n-Ge NCs/p⁺-GaAs junction where the active layer consists of heavily n-doped Ge NCs synthesized in the gas phase. Measurement of a significant current at energies smaller than the band gap of GaAs demonstrates the photogeneration of charge carriers by the Ge NCs. From the correlation of the NC size with the absorption threshold, a narrowing of the direct band gap in the Ge NC thin film is obtained and attributed to the heavy doping of the Ge NCs. A remarkably high electrical activation of ~15% is found for the incorporated P impurities in the NCs.