= Hydrogenic donor state
A shallow <donor impurity> in an isotropic <semiconductor> can be approximated by an electron of <effective mass> $m^*$ in a Coulomb potential screened by relative <permittivity> $\epsilon_r$. Its ground-state binding energy and radius are
$$
E_B=13.6057\,\mathrm{eV}\,\frac{m^*/m_e}{\epsilon_r^2},\qquad
a_B^*=a_0\frac{\epsilon_r}{m^*/m_e}.
$$
The donor level lies $E_B$ below the <conduction band>. These are magnitudes: the bound electron's energy relative to the band edge is negative. Band anisotropy, short-distance impurity structure and screening by mobile carriers can invalidate the simple <Bohr model> rescaling.
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