We investigate the influence of an unoccupied band on the transport properties of a strongly correlated electron system. For that purpose, additional orbitals are coupled to a Hubbard model via hybridization. The filling is one electron per site. Depending on the position of the additional band, both, a metal--to--insulator and an insulator--to--metal transition occur with increasing hybridization. The latter transition from a Mott insulator into a metal via ``self--doping'' was recently proposed to explain the low carrier concentration in $\rm Yb_4As_3$. We suggest a restrictive parameter regime for this transition making use of exact results in various limits. The predicted absence of the self--doping transition for nested Fermi surfaces is confirmed by means of an unrestricted Hartree--Fock approximation and an exact diagonalization study in one dimension. In the general case metal--insulator phase diagrams are obtained within the slave--boson mean--field and the alloy--analog approximation.

9 pages, Revtex, 6 postscript figures