  The short-range defect with reduced symmetry is studied in the framework of
KP-approach taking into account a matrix structure of potential energy in the
equations for envelope functions. The case of the narrow-gap semiconductor,
with defects which are non-symmetric along the [001], [110], or [111]
directions, is considered. Resonant state at a single defect is analyzed within
the Koster-Slater approximation. The bandstructure modification of the alloy,
formed by non-symmetric impurities, is discussed and a generalized virtual
crystal approximation is introduced.
