An extraction technique for subgap density of states (DOS) in an n-channel amorphous InGaZnO thin-film transistor (TFT) by using multifrequency capacitance-voltage (C-V ) characteristics is proposed and verified by comparing the measured I-V characteristics with the technology computeraided design simulation results incorporating the extracted DOS as parameters. It takes on the superposition of exponential tail states and exponential deep states with characteristic parameters for N TA = 1.1×10 17 cm −3 ·eV −1 , N DA = 4×10 15 cm −3 ·eV −1 , kT TA = 0.09 eV, and kT DA = 0.4 eV. The proposed technique allows obtaining the frequency-independent C-V curve, which is very useful for oxide semiconductor TFT modeling and characterization, and considers the nonlinear relation between the energy level of DOS and the gate voltage V GS . In addition, it is a simple, fast, and accurate extraction method for DOS in amorphous InGaZnO TFTs without optical illumination, temperature dependence, and numerical iteration.Index Terms-Amorphous, density of states (DOS), indiumgallium-zinc-oxide (InGaZnO), multifrequency capacitancevoltage (C-V ) characteristics, technology computer-aided design (TCAD), thin-film transistors (TFTs).

Due to the inherent property of large contact and parasitic resistances in amorphous InGaZnO (a-IGZO) thin film transistors (TFTs), a metal-semiconductor-metal (MSM) structure is a key element in a-IGZO TFTs. Therefore, voltage drops across resistances and MSM structure should be fully considered in the modeling and characterization of a-IGZO TFTs. A physics-based semiempirical model for the current-voltage characteristics of the MSM structure for the source-channel-drain contact in a-IGZO TFTs is proposed and verified with experimental results. The proposed model for the current in a-IGZO MSM structures includes a thermionic field emission [JTFE∝exp(VR,Schottky/Vo)] and trap-assisted generation (Jgen∝VR,Schottky) in addition to the thermionic emission current (JS: Independent of the bias) under reverse bias. Experimental result suggests that electrical characteristics of the MSM structure depend not only on the Schottky barrier but also on the bulk property of the a-IGZO active layer.

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ZnO-based photo-thin film transistors with enhanced photoresponse were developed using transparent conductive oxide contacts. Changing the electrode from opaque Mo to transparent In-Zn-O increases the photocurrent by five orders of magnitude. By changing the opacity of each source and drain electrode, we could observe how the photoresponse is affected. We deduce that the photocurrent generation mechanism is based on an energy band change due to the photon irradiation. More importantly, we reveal that the photocurrent is determined by the energy barrier of injected electrons at the interface between the source electrode and the active layer.