The typical ionisation energy of a donor in silicon is

0.1eV

(c) 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 – 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 $ε_0$. However, if the medium has a permittivity $ε_r$, relative to $ε_0$, then $E=-\frac{13.6}{ε_r^2n^2}eV$

For Si, $ε_r = 12$ and for n = 1, E – 0.1eV