The typical ionisation energy of donor level in silicon is

When donor impurity (+5 valence) added to a pure silicon (+4 valence), the +5 valence donor atom sits in the place of + 4 valence silicon atom. So it has a net additional + 1 electronic charge. The four valence electron form covalent bond and get fixed in the lattice. The fifth electron (with net - I electronic charge) can be approximated to revolve around + 1 additional charge. The situation is like the hydrogen atom for which energy is given by $\mathrm{E}=\frac{13.6}{{\mathrm{n}}^2}\mathrm{eV}.$ the case of hydrogen, the permittivity was taken as ${\epsilon }_0$.
However, if the medium has a permittivity ${\epsilon }_r$, relative to ${\epsilon }_0$, then $\mathrm{E}=\frac{13.6}{{\mathrm{\epsilon }}_{\mathrm{r}}^2{\mathrm{n}}^2}\mathrm{eV}.$
For Si, ${\epsilon }_r$= 12 and for n = I, E = 0. 1 eV