In this work, an indirect-gap semiconductor Mg2Si has been synthesized and studied. Mg2Si was formed as a 682 nm thick polycrystalline film (the formation was established using Raman scattering and Fourier-transform IR spectroscopies). X-ray phase analysis revealed that the Mg2Si (220) crystallographic direction in the sample was predominant. Based on the Hall effect measurements, the electronic type of the film conductivity was proven. Photodiode structures with Al/Mg2Si/Si-n/Au-Sb and Au/Mg2Si/Si-n/Au-Sb p–n junctions were made from the film. Their current-voltage characteristics were measured and analyzed. The dependences of the structure photoresponse on the radiation wavelength, the sign and magnitude of the applied potential were established, and their features were identified. An analysis of the obtained experimental data showed that a double p–n junction with a barrier layer was forming in the high-temperature annealing of silicon. This junction, together with the Mg2Si/Si-p heterojunction, determined the unique photospectral characteristics of the system.