Jae Chul Park

High performance amorphous oxide thin film transistors with self-aligned top-gate structure

We have demonstrated self-aligned top-gate amorphous oxide TFTs for large size and high resolution displays. The processes such as source/drain and channel engineering have been developed to realize the self-aligned top gate structure. Ar plasma is exposed on the source/drain region of active layer to minimize the source/drain series resistances. To prevent the conductive channel, N2O plasma is also treated on the channel region of active layer. We obtain a field effect mobility of 5.5 cm2/V·s, a threshold voltage of 1.1 V, and a sub-threshold swing of 0.35 V/decade at sub-micron a-GIZO TFTs with the length of 0.67#x00B5;m. Furthermore, a-IZO TFTs fabricated for gate and data driver circuits on glass substrate exhibit excellent electrical properties such as a field effect mobility of 115 cm2/V·s, a threshold voltage of 0.2 V, a sub-threshold swing of 0.2 V/decade, and low threshold voltage shift less than 1 V under bias temperature stress for 3 hr.

Highly Stable Transparent Amorphous Oxide Semiconductor Thin‐Film Transistors Having Double‐Stacked Active Layers

Recently, amorphous oxide semiconductors (AOSs) have extensively been studied for applications as display devices because AOSs have many advantages over conventional amorphous and polycrystalline silicon that are used for the channel layers of thin-fi lm transistors (TFTs). [ 1 ] As a representative AOS material, amorphous gallium-indium-zinc-oxide (a-GIZO) has intensively been studied as an active layer of TFTs for switching/driving devices in active-matrix liquid crystal display (AMLCD) and active-matrix organic light-emitting diode display (AMOLED) backplanes [ 2–5 ] because of its advantages, such as a good short-range uniformity, a high fi eld-effect mobility ( μ FE ), a large area uniform integration, a low cost and low temperature fabrication process, transparency, etc. Therefore, AOSs can be used in novel application areas, including transparent and/ or fl exible electronic devices. Up until now, many prototype active-matrix displays have been demonstrated, including 12.1 inch wide extended graphics array (WXGA) OLED displays, [ 2 ]

Modeling and characterization of metal-semiconductor-metal-based source-drain contacts in amorphous InGaZnO thin film transistors

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 [JTFE∝exp(VR,Schottky/Vo)] and trap-assisted generation (Jgen∝VR,Schottky) in addition to the thermionic emission current (JS: 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.

High performance oxide thin film transistors with double active layers

We successfully integrated the high performance oxide thin film transistors with double active layers. The active layer is composed of IZO (or ITO) and GIZO layers. The TFT with ITO/GIZO double active layer shows a high mobility of 104 cm2/V.sec, the acceptable threshold voltage of about 0.5 V and the low Vth shift less than 1 V under voltage stress. These are very promising results for applications in driving large area AMOLED and AMLCD display.

Highly Stable Ga2O3-In2O3-ZnO TFT for Active-Matrix Organic Light-Emitting Diode Display Application

We, for the first time, have successfully fabricated amorphous Ga ₂O₃-In₂O₃-ZnO thin film transistor (TFT) with excellent electrical properties and good stability under constant current stress. This transistor shows a field effect mobility of 10 cm²/Vs, an off current below 2 pA and a drain current on-to-off ratio of above 10⁸. The threshold voltage shift was less than 0.2 V for 100 hours at 3 muA and 60 degC. Such stable oxide transistors can be utilized as driving transistor for large area OLED display

High Reliable and Manufacturable Gallium Indium Zinc Oxide Thin-Film Transistors Using the Double Layers as an Active Layer

High reliable bottom gate amorphous gallium indium zinc oxide (a-GIZO) thin-film transistors (TFTs) have been fabricated by using the double active layers. Top and bottom layers were CuGaInZnO (CGIZO) and GIZO, respectively. When the plasma-enhanced processes were introduced during fabrication of the TFTs, the TFT with a-GIZO single active layer did not exhibit electrically reliable performance due to forming the conducting surface layer from the plasma damages. The double-active-layer TFT with a CGIZO layer had the reliable performance (μ FE of 5.1 cm 2 /V s, V th of 3.25 V, subthreshold gate swing value of 0.68 V/decade, I off of 3.8 X 10 -12 A) even under the same processes. This suggested that the Cu atom of CGIZO layer suppressed the carrier concentration during plasma-enhanced processes. The TFTs with double active layer showed excellent stability, which has the threshold voltage shift of <0.6 V at 3 μA drain current under 60°C for 100 h.

Self-Aligned Top-Gate Amorphous Indium Zinc Oxide Thin-Film Transistors Exceeding Low-Temperature Poly-Si Transistor Performance

Thin-film transistor (TFT) is a key component of active-matrix flat-panel displays (AMFPDs). These days, the low-temperature poly silicon (LTPS) TFTs are to match with advanced AMFPDs such as the active matrix organic light-emitting diode (AMOLED) display, because of their high mobility for fast pixel switching. However, the manufacturing process of LTPS TFT is quite complicated, costly, and scale-limited. Amorphous oxide semiconductor (AOS) TFT technology is another candidate, which is as simple as that of conventioanl amorphous (a)-Si TFTs in fabrication but provides much superior device performances to those of a-Si TFTs. Hence, various AOSs have been compared with LTPS for active channel layer of the advanced TFTs, but have always been found to be relatively inferior to LTPS. In the present work, we clear the persistent inferiority, innovating the device performaces of a-IZO TFT by adopting a self-aligned coplanar top-gate structure and modifying the surface of a-IZO material. Herein, we demonstrate a high-performance simple-processed a-IZO TFT with mobility of ∼157 cm(2) V(-1) s(-1), SS of ∼190 mV dec(-1), and good bias/photostabilities, which overall surpass the performances of high-cost LTPS TFTs.

Light Effect on Negative Bias-Induced Instability of HfInZnO Amorphous Oxide Thin-Film Transistor

For the first time, a comprehensive study is done regarding the stability under simultaneous application of light and gate dc bias in amorphous hafnium-indium-zinc-oxide (α-HIZO) thin-film transistors (TFTs). Subthreshold swing (SS) degradation, a negative threshold voltage (Vth) shift, and the occurrence of hump are observed in transfer curves after applying a negative gate bias and light stress. Based on the retention test at room temperature and the hysteresis analysis, it is revealed that all these phenomena result from hole trapping in the gate insulator. Moreover, it is proven that the SS degradation and hump occurrence are mainly attributed to hole trapping in SiO2 at the edge regions along the channel length/width directions and that a negative Vth shift is derived from hole trapping in the gate insulator far from the SiO2/HIZO interface.

Low-frequency noise in amorphous indium-gallium-zinc oxide thin-film transistors from subthreshold to saturation

We investigate the low-frequency noise (LFN) behaviors of amorphous indium-gallium-zinc oxide thin-film transistors in the subthreshold, Ohmic, and saturation regimes. Measured LFNs are proportional to 1/fγ, with γ=0.8–0.9 in all operation regimes. It is found that the LFN behavior follows the carrier number fluctuation model in the subthreshold regime, whereas in the Ohmic and saturation regimes, it agrees well with the bulk mobility fluctuation model. We also observe that the origin of 1/f noise in the Ohmic regime changes from the bulk mobility fluctuation to the carrier number fluctuation as the channel length decreases.

High-performance low-cost back-channel-etch amorphous gallium-indium-zinc oxide thin-film transistors by curing and passivation of the damaged back channel.

High-performance, low-cost amorphous gallium-indium-zinc oxide (a-GIZO) thin-film-transistor (TFT) technology is required for the next generation of active-matrix organic light-emitting diodes. A back-channel-etch structure is the most appropriate device structure for high-performance, low-cost a-GIZO TFT technology. However, channel damage due to source/drain etching and passivation-layer deposition has been a critical issue. To solve this problem, the present work focuses on overall back-channel processes, such as back-channel N2O plasma treatment, SiOx passivation deposition, and final thermal annealing. This work has revealed the dependence of a-GIZO TFT characteristics on the N2O plasma radio-frequency (RF) power and frequency, the SiH4 flow rate in the SiOx deposition process, and the final annealing temperature. On the basis of these results, a high-performance a-GIZO TFT with a field-effect mobility of 35.7 cm(2) V(-1) s(-1), a subthreshold swing of 185 mV dec(-1), a switching ratio exceeding 10(7), and a satisfactory reliability was successfully fabricated. The technology developed in this work can be realized using the existing facilities of active-matrix liquid-crystal display industries.