  To explain the main features of the metal-insulator transition (MIT) in 2D we
suggest a simple model taking into account strongly localized states in the
band tail of 2D conductivity band with a specific emphasize of a role of
doubly-occupied states (upper Hubbard band). The metallic behavior of
resistance is explained as result of activation of localized electrons to
conductance band leading to a suppression of non-linear screening of the
disorder potential. The magnetoresistance (MR) in the critical region is
related to depopulation of double occupied localized states also leading to
partial suppression of the nonlinear screening. The most informative data are
related to nearly activated temperature dependence of MR in strongly insulating
limit (which can be in particular reached from the metallic state in high
enough fields). According to our model this behavior originates due to a
lowering of a position of chemical potential in the upper Hubbard band due to
Zeeman splitting. We compare the theoretical predictions to the existing
experimental data and demonstrate that the model explains such features of the
2D MIT as scaling behavior in the critical region, saturation of MR and H/T
scaling of MR in the insulating limit. The quantitative analysis of MR in
strongly insulating limit based on the model suggested leads to the values of
g-factors being in good agreement with known values for localized states in
corresponding materials.
