Edward Namkyu Cho

Analysis of Bias Stress Instability in Amorphous InGaZnO Thin-Film Transistors

In this paper, we report an analysis of electrical bias stress instability in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs). Understanding the variations of TFT characteristics under an electrical bias stress is important for commercial goals. In this experiment, the positive gate bias is initially applied to the tested a-IGZO TFTs, and subsequently, the negative gate bias is applied to the TFTs. For comparison with the subsequently negative-gate-bias-applied TFTs, another experiment is performed by directly applying the negative gate bias to the tested TFTs. For the positive gate bias stress, a positive shift in the threshold voltage (Vth) with no apparent change in the subthreshold swing (SSUB) is observed. On the other hand, when the negative gate bias is subsequently applied, the TFTs exhibit higher mobility with no significant change in SSUB, whereas the shift of the Vth is much smaller than that in the positive gate bias stress case. These phenomena are most likely induced by positively charged donor-like subgap density of states and the detrapping of trapped interface charge during the positive gate bias stress. The proposed mechanism was verified by device simulation. Thus, the proposed model can explain the instability for both positive and negative bias stresses in a-IGZO TFTs.

Density-of-States Modeling of Solution-Processed InGaZnO Thin-Film Transistors

The effects of Ga composition on the performance of InGaZnO (IGZO) thin-film transistors (TFTs) prepared by a sol-gel method are investigated, and the density of states (DOS) is characterized by the device modeling. The TFT mode is changed from a depletion type to an enhancement type, and the extracted DOS parameters are reduced with the increase of Ga contents. The extracted DOS distribution has a higher peak value than that of an IGZO TFT prepared by physical vapor deposition.

Effects of channel thickness variation on bias stress instability of InGaZnO thin-film transistors

Here, we report on the effects of channel (or active) layer thickness on the bias stress instability of InGaZnO (IGZO) thin-film transistors (TFTs). The investigation on variations of TFT characteristics under the electrical bias stress is very crucial for commercial applications. In this work, the initial electrical characteristics of the tested TFTs with different channel layer thicknesses (40, 50, and 60 nm) are performed. Various gate bias (VGS) stresses (10, 20, and 30 V) are then applied to the tested TFTs. For all VGS stresses with different channel layer thickness, the experimentally measured threshold voltage shift (DVth )a s a function of stress time is precisely modeled with stretched-exponential function. It is indicated that the DVth is generated by carrier trapping but not defect creation. It is also observed that the DVth shows incremental behavior as the channel layer thickness increases. Thus, it is verified that the increase of total trap states (NT) and free carriers resulted in the increase of DVth as the channel layer thickness increases. 2011 Elsevier Ltd. All rights reserved.

Mobility enhancement in amorphous InGaZnO thin-film transistors by Ar plasma treatment

The effects of Ar plasma treatment on the back-channel of amorphous InGaZnO (a-IGZO) thin-film transistors are investigated. A decrease in metallic ion-oxygen bonding in the Ar plasma-treated a-IGZO channel layer was observed by X-ray photoelectron spectroscopy (XPS) depth profile analysis. An increase in the channel charge carrier concentration is estimated from the increased oxygen vacancy atomic ratio using XPS curve decomposition analysis. The plasma-treated area of the a-IGZO back-channel is varied with a photoresist screening layer with a varied open window length (Lp). From the Lp-dependent channel resistance analysis, a carrier concentration-dependent field-effect mobility enhancement is observed.The effects of Ar plasma treatment on the back-channel of amorphous InGaZnO (a-IGZO) thin-film transistors are investigated. A decrease in metallic ion-oxygen bonding in the Ar plasma-treated a-IGZO channel layer was observed by X-ray photoelectron spectroscopy (XPS) depth profile analysis. An increase in the channel charge carrier concentration is estimated from the increased oxygen vacancy atomic ratio using XPS curve decomposition analysis. The plasma-treated area of the a-IGZO back-channel is varied with a photoresist screening layer with a varied open window length (Lp). From the Lp-dependent channel resistance analysis, a carrier concentration-dependent field-effect mobility enhancement is observed.

Threshold voltage shift prediction for gate bias stress on amorphous InGaZnO thin film transistors

Abstract The demand for amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) has increased due to the high mobility and suitability for low temperature fabrication. A prediction of the threshold voltage shift (Δ V th ) under bias stress is required for the commercial use of a-IGZO TFTs. We have investigated effects of the channel length and alternating pulse bias (positive and negative gate bias stress in sequence) with different positive gate bias values ( V GS+ ) on Δ V th . We found that Δ V th increases as the channel length decreases or V GS+ increases, due to the increase in the charge trapping rate. Finally, the degradation behaviors of a-IGZO TFTs are predicted.

Modeling and optimization of ITO/Al/ITO multilayer films characteristics using neural network and genetic algorithm

Highlights? We investigated ITO/Al/ITO multilayer films characteristics. ? Variations of Al film thickness and annealing temperature. ? NNet models well represent the characteristics of ITO/Al/ITO multilayer films. ? GA is used to find optimum process condition for maximum figure of merit. In this paper, ITO/Al/ITO multilayer films are fabricated with the variations of Al film thickness and annealing temperature. The effects of Al film thickness and annealing temperature on sheet resistance, optical transmittance, and the figure of merit are analyzed in the aid of the artificial neural network (NNet) models. In order to verify the fitness of NNet model, the root mean square error (RMSE) of training and testing data are calculated. The NNet models well represent the measured sheet resistance, optical transmittance, and the figure of merit. After NNet model is established, genetic algorithm (GA) is used to find the optimum process condition for the ITO/Al/ITO multilayer films to obtain maximum figure of merit in the design space.

Characterization of Al 2 O 3 films grown by electron beam evaporator on Si substrates

We report the characterization of aluminum oxide (Al<inf>2</inf>O<inf>3</inf>) films which are grown on Si substrates by electron beam evaporator. This paper focuses on the characteristic variation of Al2O3 films depending on the different annealing techniques, such as post-deposition annealing and post-metallization annealing. The capacitance-voltage (C-V) curves indicate a negative charge and interface trap charge density between the Al<inf>2</inf>O<inf>3</inf> film and Si interface. The current-voltage (I-V) curves show a leakage current. The x-ray diffraction (XRD) patterns show the crystallinity of Al<inf>2</inf>O<inf>3</inf> films. Based on the results, the annealing effect is important condition to increase negative fixed charge in the Al<inf>2</inf>O<inf>3</inf> films.

Channel doping-dependent analytical model for symmetric double gate metal-oxide-semiconductor field-effect transistor. I. Extraction of subthreshold characteristics

An analytical 2D model of subthreshold current (IDSsub), subthreshold swing (Ssub), and threshold voltage (VTH) roll-off with a variation of channel doping concentration (NA) for symmetric double gate (DG) metal-oxide-semiconductor field-effect transistor (MOSFET) is presented. First of all, the channel potential is obtained by solving the 2D Poissons equation with the help of the evanescent method. Based on the obtained channel potential, IDSsub, Ssub, and VTH roll-off expressions are derived in the analytical model. It is shown that the subthreshold characteristics predicted by the analytical model are in good agreement with commercially available 2D numerical simulation results for different channel length (L), channel film thickness (tsi), gate oxide (tox), and NA.

Channel Length-Dependent Charge Detrapping on Threshold Voltage Shift of Amorphous InGaZnO TFTs Under Dynamic Bias Stress

The demand for amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) has been increasing due to their high mobility and transparent properties. In this paper, we report on the channel length (L)-dependent charge detrapping phenomenon of a-IGZO TFTs by observing a threshold voltage Vth shift under dynamic bias stress. Dynamic gate bias stresses are applied to the devices with three types of L: 25, 50, and 100 μm. The positive gate bias with different stress durations is followed by the negative gate bias. Under the sequential negative gate bias stress, the reversible shift of Vth increases due to charge detrapping of the previously trapped charges. As L increases, the negative shift of Vth increases due to the decreased charge detrapping time and high electric field induced by a small subgap density of states.

Effects of Alternating Pulse Bias Stress on Amorphous InGaZnO Thin Film Transistors

Amorphous InGaZnO (a-IGZO) TFTs attracted more demands due to the transparency and high mobility. Since TFTs are always exposed to both positive and negative gate bias, the alternating pulse bias stress tests are required to ensure stable TFT characteristics. In this paper, the effects of alternating pulse bias stress on the threshold voltage shift (ΔVth) of a-IGZO TFTs with respect to the channel length (L) and the stress time interval (Tinterval) are investigated using the concepts of the stretchedexponential function and the density of total trap states (NT).