  In this paper, we investigate renormalization of charge carrier effective
masses and bandgap narrowing in n-GaAs and wurtzite-type n-GaN over a wide
range of temperatures and dopant concentrations. The calculations are based on
the Green's function formalism. Contrary to the previous works, we consider the
regions below as well as above the Mott transition. Special attention is paid
to formation of donor subband and condition for the Mott transition. We also
take into account the effects caused by optical phonons. The latter strongly
depend on the doping level because of dynamic screening. It is shown that three
specific doping levels may be set off in n-GaN. They correspond to 1) Mott
transition, 2) resonance amplification of optical phonon-plasmon, and 3) full
dynamic screening of optical phonons, respectively. Contrary to the case of
n-GaN, the effect of full dynamic screening cannot be implemented in n-GaAs
because of stronger nonparabolicity of conduction band.
