My professor told me that when an intrinsic semiconductor (ex-Silicon) is doped with a penta-valent atom (ex-Phosphorus) then the extra electron is quite far away from the nucleus of Phosphorus.I asked him about the calculation of energy of impurity levels created in the semiconductor then he told me the analogy of hydrogen atom. He said that as the extra electron is quite far away from nucleus of Phosphorus then we can assume the nucleus to have a positive charge $+e$. Now we should assume that the electron revolves in a circular orbit and then we should do the calculations as predicted by Bohr's model. He told me to use the effective mass of electron in semiconductor instead of the original mass.He told me that the energy required to go from impurity level to conduction band would be close to $kT$ where $T$ is the temperature of semiconductor.He himself did the calculations and the result was quite close to the experimental data provided.
Why the Bohr's model is so close to the real model?