Byung-Wook Yoo

42.4L: Late‐News Paper: 4 inch QVGA AMOLED Driven by the Threshold Voltage Controlled Amorphous GIZO (Ga2O3‐In2O3‐ZnO) TFT

We successfully fabricated GIZO (Ga2O3-In2O3-ZnO) TFTs with high mobility of 2.6 cm2/Vs and threshold voltage standard deviation of 0.7V which is comparable to that of a-Si TFTs. Because conventional 5 mask process and bottom gate TFT structure of back channel etch type with channel length of 5 μm is used, it is expected to be transferred to mass production line in near future. Also we report the dependency of threshold voltage on the post process after the back surface of GIZO is exposed and suggest the effective method for controlling the threshold voltage of amorphous GIZO TFTs. Finally we demonstrate 4 inch QVGA AMOLED display driven by GIZO TFTs.

Threshold Voltage Control of Amorphous Gallium Indium Zinc Oxide TFTs by Suppressing Back-Channel Current

Effects of plasma treatments on the back-channel of amorphous Ga 2 O 3 -In 2 O 3 -ZnO (GIZO) thin film transistors (TFTs) are compared for N 2 and N 2 O plasma. Acceptor-like states originating from the oxygen adsorbed on the back-channel of the GIZO TFTs suppress the back-channel current by capturing the electrons in the GIZO active layer and thus shift the threshold voltage to the positive direction. It is also shown that the oxygen in a silicon oxide passivation layer reduces the back-channel current. An enhancement-mode GIZO TFT has been successfully fabricated by combining the N 2 O plasma treatment and the silicon oxide passivation layer.

Reliable and Uniform Thin‐Film Transistor Arrays Based on Inkjet‐Printed Polymer Semiconductors for Full Color Reflective Displays

Stable uniform performance inkjet-printed polymer transistor arrays, which allow demonstration of flexible full-color displays, were achieved by new ambient processable conjugated copolymer semiconductor, and OTFT devices incorporating this material showed high mobility values>1.0 cm2 V(-1) s(-1). Bias-stress stability of the devices was improved with a channel-passivation layer, which suppresses the density of trap states at the channel interface.

Stability Improvement of Gallium Indium Zinc Oxide Thin Film Transistors by Post-Thermal Annealing

The effects of post-thermal annealing on the stability of Ga2O3In2O3-ZnO (GIZO) thin film transistors (TFT) were investigated by comparing the GIZO TFTs annealed for 3 hour and for 65 hours under high-field bias stress, light illumination, and long-term storage in air. We found that the poor stability of the GIZO TFTs under these stresses was remarkably improved after 65 hours’ postthermal annealing at 250 O C. The improvement of the stability is ascribed to the reduction of the trap sites in the GIZO layer and curing of weak atomic bonds otherwise susceptible to breaking during the stress. pioonauaoeni Recently amorphous oxide semiconductor thin film transistors (TFT) have attracted much attention for large-area electronics such as active-matrix liquid displays (AMLCDs) and active-matrix organic light emitting diode (AMOLED) displays because the mobility is higher than that of the amorphous silicon (a-Si) TFT facilitating the integration of driving circuits and because the uniformity is expected to be superior to that of the lowtemperature polycrystalline silicon (LTPS) TFT due to structural homogeneity. In addition, the lower process temperature enables to use low-cost soda-lime glass substrate (1, 2). Although the stability of a TFT under electrical and environmental stresses is no less important than the other characteristics in commercialization, there have been few reports on the improvement of the poor stability of the oxide semiconductor TFTs. We investigated extensively to improve the stability of the oxide semiconductor TFTs especially for the amorphous Ga2O3-In2O3-ZnO (GIZO) TFT because of its high performances such as high mobility and steep subthreshold slope. We have found that a post-thermal annealing for a long duration is the most effective method to enhance the stability. In this letter we report the effects of the post-thermal annealing on the stability of the GIZO TFTs under electrical bias stress, light illumination, and long-term storage in air.

Characterization of bias stress induced electrical instability in liquid-crystalline semiconducting polymer thin-film transistors

Bias stress effects in organic thin-film transistors were investigated. A donor-acceptor type liquid-crystalline semiconducting copolymer, poly(didodecylquaterthiophene-alt-didodecylbithiazole), PQTBTz-C12, was used as the active channel material. This substance contains both electron-donating quaterthiophene and electron-accepting 5,5’-bithiazole units. The threshold voltage (VT) shifts induced by direct current (DC) bias stress were studied under different gate-source and drain-source voltages. By fitting ΔVT versus stress time in compliance with a stretched exponential relationship, characteristic charge trapping time constants (τ) and dispersion parameters (β) for the VT shifts were determined for each stress condition. The time constants decrease with increasing gate-drain voltages. It was also observed that the VT shift due to charge trapping can be recovered by releasing the device from bias stress for several hours. The recovery rate from DC OFF bias stress is slightly slower than the recovery fro...