Interpretation of subthreshold swing and threshold voltage in solution-processed zinc oxide TFTs

Subthreshold conduction in amorphous and nanocrystalline transparent conductive oxide (TCO) thin film transistors (TFTs) is routinely evaluated using equation frameworks based on weak inversion borrowed from those used in conventional MOSFETs [1]. Unlike traditional MOSFETs, the electrostatics of TCOs are overwhelmingly determined by the properties of localized states that predominantly consist of charged acceptors exponentially distributed in energy below the conduction band edge [2]. Furthermore, TFTs operate in accumulation, and switching occurs at the flatband voltage, where the depletion charge also vanishes. This suggests that, unlike traditional MOSFETs, no significant diffusion current is expected, since the ratio of diffusion current to the drift current is proportional to the ratio of depletion charge to the electron concentration [3]. In this work, we estimate the depletion charge and electron concentration obtained from quasi-static capacitance-voltage (QSCV) data and demonstrate that the ratio of the depletion charge to the electron concentration becomes negligibly small at and above the flatband voltage. In doing so, we provide experimental evidence demonstrating the limited contribution of diffusion current in these systems, contrary to commonly used equation frameworks, e.g. [4]. Furthermore, based on the proposed transport behavior, we propose a consistent threshold voltage definition. Recently, Qiang et al defined threshold voltage using the point of degenerate conduction, although the tail state concentrations used in their work may be substantially lower than those observed experimentally [5]. We instead offer a definition of threshold voltage that is consistent with an experimental derived density of states, namely, using the intersection between the extended and localized states, corresponding to a Mott transition (occurring at lower voltages) [6] rather than a degeneracy transition (occurring at higher voltages). This is thus more consistent - ith transport mechanisms in these systems in the on-state.