  Usually microscopic electrostatic field around charged impurity ions is
neglected when the ionization energy is concerned. The ionization energy is
considered to be equal to that of a lonely impurity atom. Here the energy of
the electrostatic field around charged impurity ions in semiconductor is taken
into account. It is shown that the energy of this field contributes to decrease
in the effective ionization energy. At high enough current carriers
concentration the effective ionization energy becomes zero.
