An analysis of the mechanism of formation of composite nanoparticles of Ag/Si shows that it depends on their stoichiometric composition, conditions of condensation and crystallisation. During the crystallisation of the nanoparticle core, eutectics consisting of silver and silicon layers are formed. Diffraction fringes in the nanoparticle core are caused by the formation of eutectic platelets during crystallisation of phase components. Layers of the conductor Ag and semiconductor Si form, creating an electric charge at the boundaries. We demonstrate the dependence of the sizes of these Ag/Si nanoparticles on the rates of heating and cooling of the materials during synthesis.