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Based on the physical model of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) and the extracted density of states described in Part I, a quantitative investigation of mechanisms on the negative bias illumination stress (NBIS)-induced threshold voltage VT instability of a-IGZO TFTs is presented. It is found that the shallow donor state-creation model explains the NBIS time evolution of the electrical characteristics very well. Furthermore, the semi-empirical rule of the NBIS-induced \Delta VT is proposed and demonstrated based on the shallow donor state-creation model. The proposed approach can be used to optimize the fabrication process and to explore high-performance thin-film materials for mass-production-level amorphous oxide semiconductor TFTs to be innovatively used in the near future. [ABSTRACT FROM PUBLISHER]

A combination of the multifrequency C– V and the generation–recombination current spectroscopy is proposed for a complete extraction of density of states (DOS) in amorphous InGaZnO thin-film transistors (a-IGZO TFTs) over the full subband-gap energy range (EV\leq E\leq EC) including the interface trap density between the gate oxide and the a-IGZO active layer. In particular, our result on the separate extraction of acceptor- and donor-like DOS is noticeable for a systematic design of amorphous oxide semiconductor TFTs because the former determines their dc characteristics and the latter does their threshold voltage (VT) instability under practical operation conditions. The proposed approach can be used to optimize the fabrication process of thin-film materials with high mobility and stability for mass-production-level amorphous oxide semiconductor TFTs. [ABSTRACT FROM PUBLISHER]

We propose a differential ideality factor technique (DIFT) for extraction of subgap density of states (DOS) over the bandgap in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) by using the differential ideality factor d\eta/dVGS on behalf of the ideality factor itself. Contrary to the subthreshold current method which requires an accurate threshold voltage (VT), the DIFT is free from VT itself and considerably useful to TFTs with a nonuniform distribution of DOS over the bandgap. Through the DIFT applied to an a-IGZO TFT with W/L = \200\ \mu \m/\30\ \mu\m, the subgap DOS is extracted to be a superposition of exponential deep and tail states with NDA = \7.1 \times \10^15\ \cm^-3 \cdot \eV^-1, kTDA = \0.6\ \eV, NTA = \1.5 \times \10^16\ \cm^-3 \cdot \eV^-1, and kTTA = \0.024\ \eV. [ABSTRACT FROM AUTHOR]

The effect of \O2 flow rate (OFR) during channel deposition is investigated on the electrical instability of the amorphous indium–gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) under positive gate bias stresses. From the transfer curves measured before and after bias stresses, we can observe that the high OFR degrades the electrical stability and causes the large threshold voltage shift (\Delta VT) in a-IGZO TFTs. To elucidate the origin of the observed phenomenon, we extract and compare the subgap density of states (DOS) in devices with various OFRs. The extracted DOS shows that the subgap states become higher with the increase of OFR in a wide range of bandgap, and the enhanced electron trapping due to the increased number of trap states is considered as the cause of larger \Delta VT in higher OFR devices. [ABSTRACT FROM PUBLISHER]

Analytical drain current and gate capacitance models for amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) over sub- and above-threshold regions are proposed by adopting an effective carrier density for the dominant carrier density. The effective carrier density fully considers the free carriers in the conduction band, the localized subgap deep states, and tail states over the bandgap for analytical I–V and C–V characteristics. The proposed analytical models are verified by comparing the measured I –V and C–V characteristics. The proposed models make a time-efficient simulation of a-IGZO TFT-based circuits possible due to their analytical form. [ABSTRACT FROM PUBLISHER]

The effect of the active layer thickness (TIGZO) on the negative bias stress (NBS)-induced threshold voltage shift (\Delta VT) in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) is investigated and explained by using a subgap density-of-states (DOS) model. The NBS-induced negative \Delta VT in a-IGZO TFT with a thinner TIGZO is larger than that with a thicker TIGZO. Based on the simulation result with the subgap DOS model, it is concluded that the TIGZO-dependent \Delta VT is originated from the accelerated creation of shallow donor states due to a higher surface electric field in a-IGZO TFTs with a thinner TIGZO. [ABSTRACT FROM AUTHOR]

A physics-based generation–recombination current (IG-R) spectroscopy is proposed for the extraction of the subgap donorlike density of states (DOS) of amorphous InGaZnO thin-film transistors. Physics-based Shockley–Read–Hall recombination through the subgap DOS over the bandgap is fully considered, and the potential for the carrier concentration is calculated through the DeAOTS model. The extracted parameters for the exponential deep donorlike states are NDD = \5.5 \times\break \10^21\ [\cm^-3 \cdot \eV^-1] and kTDD = \0.115\ [\eV]. [ABSTRACT FROM PUBLISHER]