The effect of oxygen content on current-stress-induced instability was investigated in bottom-gate amorphous InGaZnO (a-IGZO) thin-film transistors. The observed positive threshold voltage shift (ΔV_T) was dominated by electron trapping in the gate insulator (GI), whereas it was compensated by donor creation in a-IGZO active regions when both current flows and a high lateral electric field were present. Stress-induced ΔV_T increased with increasing oxygen content irrespective of the type of stress because oxygen content influenced GI quality, i.e., higher density of GI electron traps, as well as typical direct current (DC) performance like threshold voltage, mobility, and subthreshold swing. It was also found that self-heating became another important mechanism, especially when the vertical electric field and channel current were the same, independent of the oxygen content. The increased ΔV_T with oxygen content under positive gate bias stress, positive gate and drain bias stress, and target current stress was consistently explained by considering a combination of the density of GI electron traps, electric field relaxation, and self-heating-assisted electron trapping.

In this study, the oxygen flow rate (OFR) dependence of negative bias illumination stress (NBIS) and positive bias stress (PBS) in amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) devices was investigated. Unlike in previous studies, our study simultaneously considered and analyzed the bias stress-induced degradation mechanisms by charge trapping and defect creation. NBIS and PBS instability were measured according to the OFR splits of bottom-gate (BG) IGZO TFTs and the behavior of full subgap range density of states (DOS) was experimentally tracked with especial emphasis on deep-level oxygen vacancy ( ${V}_{\text {O}}$ ) peak. It was found that the observed DOS variation was consistent with the ${V}_{\text {O}}$ ionization model during NBIS. In addition, the threshold voltage shift by charge trapping ( ${\Delta } {V}_{\text {T,CT}}$ ) and the threshold voltage shift by defect creation, i.e., change of DOS, ( ${\Delta } {V}_{\text {T,DOS}}$ ) were separated and extracted through the subthreshold slope decomposition method. After NBIS, the threshold voltage shift ( ${\Delta } {V}_{\text {T}}$ ) was composed of a component by charge trapping and a component with the ionization of ${V}_{\text {O}}$ . After PBS, ${\Delta } {V}_{\text {T}}$ was dominated by a component arising due to charge trapping. Fitting through the stretched exponential function (SEF) was performed for each separately extracted ${\Delta } {V}_{\text {T}}$ , and the activation energy for each ${\Delta } {V}_{\text {T}}$ was extracted using the inverse Laplace transform method through the extracted ${\Delta } {V}_{\text {T0}}$ , $\tau $ , and $\beta $ . Activation energy by charge trapping and DOS was extracted from NBIS and PBS, respectively. As a result of simultaneously considering the two degradation mechanisms, the extracted activation energy was located between the previous values. Experimentally extracted sets of DOS-specific parameters before and after bias stress were applied to technology computer-aided design (TCAD) simulations. Accurate reproduction of TFT current–voltage ( ${I}$ – ${V}$ ) curves before and after bias stress and recovery suggests that the methodology and parameter set used in this study are reasonable and potentially useful for a more robust and systematic analysis of the bias stress-induced instability in a-IGZO BG TFTs.

We propose an experimental method to decompose the positive gate-bias stress (PBS)-induced threshold voltage shift ( $\Delta {V}_{\mathsf {th}})$ of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) into the contributions of distinct degradation mechanisms. Top-gate self-aligned coplanar structure TFTs are used for this letter. Stress-time-divided measurements, which combine the subgap density-of-states (DOS) extraction and the analysis on recovery characteristics, are performed to separate the $\Delta {V}_{\mathsf {th}}$ components. Change in excess oxygen ( $\text{O}_{\mathsf {ex}})$ -related DOS is clearly observed, and $\Delta {V}_{\mathsf {th}}$ by PBS is quantitatively decomposed into the contributions of the active $\text{O}_{\mathsf {ex}}$ , and the deep and shallow gate insulator traps. The quantitative decomposition of PBS-induced $\Delta {V}_{\mathsf {th}}$ provides physical insight and key guidelines for PBS stability optimization of a-IGZO 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 $\left[J_{\text{TFE}}\propto \text{exp}\left(V_{R,\text{Schottky}}/V_o\right)\right]$ and trap-assisted generation $\left(J_{\text{gen}}\propto \sqrt{V_{R,\text{Schottky}}}\right)$ in addition to the thermionic emission current ($J_S$ : 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.

In this study, we demonstrate both of digital and analog memory operations in InGaZnO (IGZO) memristor devices by controlling the electrode materials for neuromorphic application. The switching properties of the devices are determined by the initial energy barrier characteristics between the metal electrodes and the IGZO switching layer. Digital switching characteristics are obtained after the forming process when Schottky junction occurs at both of top and bottom electrodes. On the other hands, analog resistive switching is achieved when Schottky and Ohmic junctions exist at each side because the applied voltage modulates the Schottky barrier height through the Ohmic contact. In addition, the weight-update properties of the devices are verified depending on identical and incremental pulse schemes. The incremental pulse trains improve the linearity and variation of weight modulation, leading to the stable learning characteristics of neuromorphic system in terms of pattern recognition with MNIST hand-written digit images.

We propose analytical current and capacitance models for amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) for the simulation of a-IGZO TFT-based-circuits. The proposed models are composed of physical parameters including the subgap density of states and confirmed by comparing the calculated current-voltage and capacitance-voltage characteristics with measured ones. The proposed models are expected to be useful for the optimization of fabrication processes and for the prospective estimation of the effect of process conditions on the circuit performances. Through the circuit simulation implemented with the proposed models, we verify that the proposed analytical model and simulation methodology can be applicable to the expectation of the complicated circuit behaviors. The simulation framework with proposed analytical models is expected to be a powerful tool in the optimization process and circuit design with amorphous oxide semiconductor TFTs including a-IGZO TFTs.