In the band picture, the energy level of the additional electron lies close to the bottom of the conduction band. Such an energy level is called a donor level as it can donate an electron to the empty conduction band by thermal excitation. We may see this by assuming that the fifth electron of the donor is orbiting around a hydrogen-like nucleus consisting of the core of the donor atom with the following modifications made into the formula for the energy of an electron in the hydrogen atom.
is called a donor level as it can donate an electron to the empty conduction band by thermal excitation. We may see this by assuming that the fifth electron of the donor is orbiting around a hydrogen-like nucleus consisting of the core of the donor atom with the following modifications made into the formula for the energy of an electron in the hydrogen atom. 
