Variable-Range Hopping in Polycrystalline Silicon Thin-Film Transistor

Low-temperature transport measurements on a polycrystalline silicon thin-film transistor have been made in order to clarify the role of bandtail states in the grain boundaries. We find a Mott variable-range hopping (VRH) law of the resistance in the barely insulating regime of the metal-insulator transition. At lower temperatures, the resistance crosses over to Efros-Shklovskii VRH with the opening of a soft Coulomb gap. The magnetoresistance (MR) is negative, and its features are qualitatively explained by existing theories of quantum interference effects in VRH. Furthermore, the Hall coefficient remains essentially constant even in the range of thermally activated conduction and Mott VRH.