  We argue that the dominant charge carrier in glassy semiconducting alloys is
a compound particle in the form of an electron or hole bound to an intimate
pair of topological lattice defects; the particle is similar to the polaron
solution of the Su-Schrieffer-Heeger Hamiltonian. The spatial component of the
density of states for these special polarons is determined by the length scale
of spatial modulation of electronegativity caused by a separate set of
standalone topological defects. The latter length scale is fixed by the
cooperativity size for structural relaxation; the size is largely independent
of temperature in the glass but above melting, it decreases with temperature.
Thus we predict that the temperature dependence of the electrical conductivity
should exhibit a jump in the slope near the glass transition; the size of the
jump is predicted to increase with the fragility of the melt. The predicted
values of the jump and of the conductivity itself are consistent with
experiment.
