Mobility Assessment of Depletion-Mode Oxide Thin-Film Transistors Using the Comprehensive Depletion-Mode Model

In an enhancement-mode, n-channel (p-channel) oxide thin-film transistor (TFT), current arises as a consequence of electron (hole) transport within a narrow accumulation layer. The square-law model accurately describes enhancement-mode TFT behavior and establishes the equations appropriate for carrier mobility extraction. In contrast, in a depletion-mode oxide TFT, carrier transport can occur within an accumulation layer and/or within the 'bulk' portion of the channel. The comprehensive depletion-mode model accurately describes depletion-mode TFT behavior and establishes a set of equations, different from those obtained from square-law theory, which can be used for carrier mobility extraction. Simulation reveals that when square-law theory mobility extraction equations are used to assess depletion-mode TFTs, the estimated interface mobility is often overestimated.