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Illustrations of carrier transport (a) at room temperature (RT), 300 K, and (b) at low temperature, when Fermi level (EF) resides within tail states. At RT, the Fermi probability function f(E) shows a moderate change (see Fig. 1(a)), yielding the condition nfree > ntail, thus insignificant trapping at localized tail states. In contrast, f(E) becomes steep at a low temperature (see Fig. 1(b)), yielding nfree < ntail, thus trap-limited conduction becomes dominant. In Figs. 1(a) and 1(b), potential barriers above conduction band minima (Em) are arising from a random distribution of metal atoms (e.g., Ga and Zn) in an amorphous oxide film (e.g., a-InGaZnO) (see Ref. 7). (c) Schematic plot of free and trapped carrier densities at RT and low temperature as a function of EF from the conduction band edge (Em).
(a) Schematic profiles of the gap states in AOS TFT in comparison with a-Si:H TFT, indicating the exponential tail states (Ntail(E)) for typical values of kTt and Ntc of a-Si:H and a-InGaZnO TFTs (see Refs. 8–11). In contrast to bulk films, the gap state profile in a TFT is limited by the interface states (Nit). (b) In AOS TFTs, density of the free carrier (nfree) and carrier trapped at localized tail states (ntail) as a function of the EF from the Em for T = 300 K and T = 77 K, numerically computed using Fermi-Dirac statistics (see Ref. 13).
(a) Cross-section view of a bottom gate a-InGaZnO TFT structure, examined here, and (b) measured drain current (IDS) vs. drain voltage (VDS) of the a-InGaZnO TFT for a different temperature from 77 K to 300 K (inset: the evolution of VT and VFB with temperatures).
(a) Correspondence of VGS with EF. (b) The distribution of the localized tail states in a-InGaZnO TFTs extracted the conductance measured at 77 K.
(a) The extracted field effect mobility (μFE) as a function of temperature for a different gate voltage, showing a linear dependence on 1/kT. (b) The retrieved activation energy as a function of gate voltage, indicating the percolation threshold (VP) by a linear extrapolation. (c) The illustration of the percolation conduction through the potential barriers even without thermal excitation when EF > Em, filling out all the localized tail states.
Material parameters (Ntc, kTt, and Nit) extracted for the a-InGaZnO TFTs, examined here, in comparison with values for a-Si:H TFTs adapted from Ref. 13.
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