Current-driven threshold switching of a small polaron semiconductor to a metastable conductor

Steady-state current flow through a nonuniform medium generally alters the local carrier density. In particular, driving conventional high-mobility charge carriers through a region in which they collapse into low-mobility small polarons propels the small-polaron density beyond its equilibrium value. There are two contributions to the local augmentation of the small-polaron density. These contributions are proportional to the ratios of the rate governing intersite motion of a conventional high-mobility carrier R-f to (i) the (relatively slow) rate governing intersite hopping of a small polaron R, and to (ii) the (even slower) rate governing conversion of a carrier between being quasifree and being a small polaron, r. As a result of the very large values of these ratios, R-f/R and R/r > 1, large increases in the small-polaron density can be obtained with accessible electric-field strengths, < 10(6) V/cm. However, upon being driven to a sufficiently high density, small polarons become unstable with respect to conversion into nonpolaronic carriers. As a result, currents beyond a threshold value can convert a small-polaron semiconductor to a high-mobility semiconductor. Reducing the current permits the material's carriers to relax back to being small polarons. This phenomenon may account for the threshold switching that is observed in materials in which equilibrated carriers appear to form small polarons (e.g., amorphous boron, transition-metal oxide glasses, and chalcogenide glasses).