  In the framework of the two-band model of a doped semiconductor the
self-consistent equations describing the transition into the excitonic
insulator state are obtained for the 2D case. It is found that due to the
exciton-electron interactions the excitonic phase may arise with doping in a
semiconductor stable initially with respect to excitonic transition in the
absence of doping. The effects of the strong interactions between electron
(hole) Fermi-liquid (FL) and excitonic subsystems can lead to the appearance of
the states lying in the middle of the insulating gap.
