Field-effect conductance in amorphous silicon thin-film transistors with a defect pool density of states

Abstract

A new computer program to analyze field-effect conductance measurements has been developed. In this program a defect pool model, where the equilibrium density of state is determined by the Fermi level, has been incorporated. Transistors with finite band bending, due to fixed charge in the insulator, will therefore have a density of states that is spatially inhomogeneous. The inhomogeneous density of states means that the subthreshold slope of a device is not always controlled by the density of states near the interface, but can become dominated by the bulk density of states, contrary to simpler models. Both electron and hole branches are modeled simultaneously and self-consistently with no assumptions made about the flatband voltage. Indeed, it is demonstrated that there is no flatband voltage in a transistor with an inhomogeneous density of state; however, a true flatband voltage can be achieved by a process of thermal bias annealing. Finite thickness effects and defect correlation energies are taken into account. The program is used to model the characteristics of thin-film transistors both before and after thermal bias annealing. The model fits the experimental results well, even though there are few free parameters. A key result is that the defect pool parameter Δ is determined, which is the energy separation of the D0/- transition observed in n-type material and the D+/0 transition in p-type material. It is found that Δ=0.44 eV for all devices fitted, which suggests this a fundamental parameter of a-Si:H. This value of Δ implies that there are about four times as many charged as neutral devices in bulk intrinsic amorphous silicon.