As far as I understood, when we connect P and N materials, electrons from N drift to P, thus create negative ions in P and positive ions in N.
=> electric field (barrier potential) must depend on an amount of electrons recombined with holes in P region.
=> The more electrons recombined, the more ions created, the more barrier potential.
BUT! Silicon has 14 electrons and 4 on the 3d level. Germanium has 32 electrons and 4 on the 4th level.
=> at 25C degrees, electrons in N region in Germanium must have more energy than electrons in Silicon, because less energy required to remove electron from an atom => more electrons can drift to P region, thus barrier potential for Germanium should be higher, however
Vbp Germanium = 0.3 V Vbp Silicon = 0.7 V
P.S. I am talking about P doped and N doped silicon compared to the junction between P doped and N doped germanium