  It is shown that pinning of an electron Wigner crystal by a small
concentration of charged impurities creates the finite density of charged
localized states near the Fermi level. In the case of residual impurities in
the spacer this density of states is related to nonlinear screening of a close
acceptor by a Wigner crystal vacancy. On the other hand, intentional doping by
a remote layer of donors is a source of a long range potential, which generates
dislocations in Wigner crystal. Dislocations in turn create charged localized
states near the Fermi level. In both cases Coulomb interaction of localized
states leads to the soft Coulomb gap and ES variable range hopping at low
enough temperatures.
