We discuss the ultraviolet (UV) photo-field effects in amorphous InGaZnO 4 thin-film transistors (a-IGZO TFTs) compared with those in hydrogenated amorphous silicon (a-Si:H) TFTs. It is shown that the UV illumination induces a much more significant threshold voltage (V t ) decrease and OFF-current increase for the a-IGZO TFTs than for the a-Si:H TFTs. The significant V t decrease is found to take several tens of min to return to the initial state after switching off the UV light. A qualitative model is introduced to explain the photoresponse unique to the a-IGZO TFTs.

The transfer characteristics of amorphous InGaZnO 4 thin-film transistors (a-IGZO TFTs) were measured at temperatures ranging from 298 to 523 K in order to analyze the behavior of the above-threshold (ON state) and subthreshold regions. For comparison, the transfer characteristics of a hydrogenated amorphous silicon TFT (a-Si:H TFT) were measured in the same temperature range. We developed a simple analytical model that relates the threshold voltage (V t ) decrease due to increasing temperature to the formation of point defects in a-IGZO. It is well known that the formation of point defects results in the generation of free carriers in oxide semiconductors. Incorporating the analytical model with the experimental transfer characteristics data taken at high temperatures over 423 K, we estimated the formation energy to be approximately 1.05 eV. The V t decrease because of the generation of point defects is peculiar to a-IGZO TFTs, which is not observed in a-Si:H TFTs. The results for the ON-current activation energy suggested that the density of tail states for a-IGZO is much lower than that for a-Si:H. #

We investigated the threshold voltage shifts (ΔV T) of inverted-staggered hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) induced by steady-state (dc) and pulsed (ac) gate bias-temperature-stress (BTS) conditions. Our study showed that, for an equivalent effective-stress-time, ΔV T has an apparent pulse-width dependence under negative BTS conditions–the narrower the pulse width, the smaller the ΔV T. This gate-bias pulse-width dependence is explained by an effective-carrier-concentration model, which relates ΔV T for negative pulsed gate-bias stress to the concentration of mobile carriers accumulated in the conduction channel along the a-Si:H/gate insulator interface. In addition, our investigation of the methodology of a-Si:H TFT electrical reliability evaluation indicates that, instead of steady-state BTS, pulsed BTS should be used to build the database needed to extrapolate ΔV T induced by a long-term display operation. Using these experimental results, we have shown that a-Si:H TFTs have a satisfactory electrical reliability for a long-term active-matrix liquid-crystal display (AMLCD) operation.

We report on the effect of Ar addition to an O 2 plasma on photoresist etching in an inductively coupled, traveling wave driven, large area plasma source ͑LAPS͒. We also develop a simplified spatially varying O 2 /Ar mixture discharge model corresponding to the LAPS in a two-dimensional geometry in order to account for the effect of Ar addition. A photoresist etch kinetics model and spatially varying O 2 /Ar mixture discharge model are used to explain the experimental data. We find that the addition of 50% Ar increases the plasma density and etch rate approximately by a factor of 2. From the simulation we find that argon metastables ͑Ar*͒ play an important role in the mixture plasma. The simulation predicts an enhancement in O-atom density due to Ar addition, even in the presence of dilution of the feed gas. The experimental data and predicted etch rates from the simulation are generally in good agreement, indicating that the increase in the etch rate with Ar addition is due to both the increase in the plasma density and the enhancement in O-atom density attributable to the dissociation of O 2 by Ar*.

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We discuss the sensitivity enhancement of bottom-gate type amorphous InGaZnO 4 thin-film transistor (a-InGaZnO TFT) pH sensors from the viewpoint of top-gate effects. Comparing the top-gate effects in a-InGaZnO TFTs having TaO x and SiO x ion-sensitive insulators, we draw an analogy between the operations of dual-gate TFTs and TFT pH sensors. Our new concept for enhancing pH sensitivity is characterized by a high capacitance ratio of the ion-sensitive insulator to the bottom-gate insulator and pH sensing utilizing threshold-voltage shifts in bottom-gate transfer characteristics. The close similarity between top-gate effects and pH sensitivity strongly suggests that a common mechanism underlies the phenomena. We discuss the mechanism on the basis of the material properties of a-InGaZnO and the silicon-on-insulator (SOI) model that relates bottom-and top-gate electric fields in fully depleted operations. We believe that the pH-sensitivity enhancement utilizing top-gate effects is one of the potential applications that would make the most of the intrinsic features of a-InGaZnO TFTs.

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This paper describes a method for utilizing an excimer laser to improve the characteristics of InGaZnO 4 (IGZO) thin-film transistors (TFTs). IGZO-TFTs fabricated at room temperature are irradiated with an excimer laser to raise the temperature of the IGZO films for only a very short time, some tens of ns. The ON current of an irradiated IGZO-TFT is more than one order of magnitude higher than that of an unirradiated TFT. This method is promising for achieving high performance IGZO-TFTs on plastic substrates because the thermal damage to substrates will be much less than that which would result from furnace annealing.