  We reconsider the theory of magnetoresistance in hopping semiconductors.
First, we have shown that the random potential of the background impurities
affects significantly preexponential factor of the tunneling amplitude which
becomes to be a short-range one in contrast to the long-range one for purely
Coulomb hopping centers. This factor to some extent suppresses the negative
interference magnetoresistance and can lead to its decrease with temperature
decrease which is in agreement with earlier experimental observations. We have
also extended the theoretical models of positive spin magnetoresistance, in
particular, related to a presence of doubly occupied states (corresponding to
the upper Hubbard band) to the case of acceptor states in 2D structures. We
have shown that this mechanism can dominate over classical wave-shrinkage
magnetoresistance at low temperatures. Our results are in semi-quantitative
agreement with experimental data.
