  We study density of states and conductivity of the doped double-exchange
system, treating interaction of charge carriers both with the localized spins
and with the impurities in the coherent potential approximation. It is shown
that under appropriate conditions there is a gap between the conduction band
and the impurity band in paramagnetic phase, while the density of states is
gapless in ferromagnetic phase. This can explain metal-insulator transition
frequently observed in manganites and magnetic semiconductors. Activated
conductivity in the insulator phase is numerically calculated.

Condensed Matter