The typical ionization energy of a donor in silicon is ( Take ${\epsilon }_r=12(\text{silicon)}$)

When donor  impurity  (+5  valence)  added  to  a  pure  silicon (+4  valence),  the  +5  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 –1 electronic charge) can be approximated to revolve around +1 additional charge. The situation is like the hydrogen atom for which energy is given by $E=-\frac{13.6}{n^2}eV.$ For 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 $E=-\frac{13.6}{{\epsilon }_r^2{n}^2}eV$ For Si,  ${\epsilon }_r=12$ and for $n=1$ 4 1, $E=0.1\text{\hspace{0.17em}eV}$