Steady-state hopping conduction in the conduction-band tail of<i>a</i>-Si:H studied in thin-film transistors

Abstract

We have measured the temperature-dependent channel conductivity of a thin-film transistor that employs hydrogenated amorphous silicon (a-Si:H) as the active layer. Two regimes of conductivity are observed. At temperatures above 100 K the conductivity is thermally activated indicating transport above the mobility edge ${\mathit{E}}_{\mathit{C}}$ that separates localized from extended states. From the activation energy we conclude that the Fermi energy is moved as close as 62 meV towards ${\mathit{E}}_{\mathit{C}}$ for gate voltages above 100 V. In the low-temperature regime (T50 K) the conductivity is no longer activated with a single activation energy indicating transport by hopping in the localized band tail states of a-Si:H. We present calculations of the conductivities in a model where the microscopic tunneling rates are averaged in order to obtain the energy-dependent mobility. The agreement of these calculations with the data is considerably improved when a localization length of the band tail states is adopted that increases inversely with the square root of their energy below ${\mathit{E}}_{\mathit{C}}$.