Byung Seong Bae

Threshold Voltage and IR Drop Compensation of an AMOLED Pixel Circuit Without a

This letter proposes a voltage drop by current (IR drop) and threshold voltage compensation pixel circuit, in which the power line is eliminated. The proposed circuit provides a larger fill factor than the conventional one because the circuit does not use a VDD line but instead utilizes the scan line as a power line. The proposed pixel circuit was verified using both circuit analysis and circuit simulations. The proposed circuit could increase the aperture ratio and compensate for the IR drop and threshold voltage variations in active matrix organic light-emitting diode devices, where I and R are current and resistance of the supply power lines. Using the proposed pixel circuit, a large fill factor with both IR drop and threshold voltage compensation was achieved.

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We report the effects of monochromatic illumination on the electrical performance of a-SiGe:H thin-film transistor (TFT) and the use of this device as infrared (IR) image-sensing touch displays. In order to reduce the inevitable incidence of visible light into device, the visible cutting layer was designed, which played a critical role in reducing the optical noise from visible light. When the photo response was observed in real liquid crystal display (LCD) operation environment, it showed the display contents dependence, meaning that the supply of external IR light source for the touch sensing would be effective independent of any ambient light condition. Additionally, the optical noise from the display operation was eliminated using the block operation as well as the backlight IR light-emitting diode (LED) light modulation. From these, the clear touch images were successfully obtained in the a-SiGe:H photosensor embedded LCD panel.

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The structure of cholesteric liquid crystal (CLC) undergoes a helical distortion which is left-handed or righthanded. By the right-hand CLC layer, Left-handed polarized light is reflected and vice versa. The color reflected by the selective reflection depends on the chiral pitch and the anisotropy of the refractive index. However, the reflectance of the single CLC layer is theoretically limited to 50% because only one of right- or left-handed circularly polarized light is reflected. In this paper, we demonstrate the enhanced reflectance of a dual-CLC device which can reflect both right- and left-handed circularly polarized light.

Optical Properties of a-SiGe:H Thin Film Transistor for Infrared Image Sensors in Touch Sensing Display

A threshold voltage compensation pixel circuit was developed for active-matrix organic light emitting diodes (AMOLEDs) using amorphous indium-gallium-zinc-oxide thin-film transistors (a-IGZO-TFTs). Oxide TFTs are n-channel TFTs; therefore, we developed a circuit for the n-channel TFT characteristics. The proposed pixel circuit was verified and proved by circuit analysis and circuit simulations. The proposed circuit was able to compensate for the threshold voltage variations of the drive TFT in AMOLEDs. The error rate of the OLED current for a threshold voltage change of 3 V was as low as 1.5%.

Dual structure of cholesteric liquid crystal device for high reflectance

We investigated the temperature dependent recovery of the threshold voltage shift observed in both ZnO and indium gallium zinc oxide (IGZO) thin film transistors (TFTs) after application of gate bias and light illumination. Two types of recovery were observed for both the ZnO and IGZO TFTs; low temperature recovery (below 110 °C) which is attributed to the trapped charge and high temperature recovery (over 110 °C) which is related to the annihilation of trap states generated during stresses. From a comparison study of the recovery rate with the analysis of hydrogen diffusion isochronal annealing, a similar behavior was observed for both TFT recovery and hydrogen diffusion. This result suggests that hydrogen plays an important role in the generation and annihilation of trap states in oxide TFTs under gate bias or light illumination stresses.

Pixel Circuit with Threshold Voltage Compensation using a-IGZO TFT for AMOLED

A hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) source driver for e-paper was designed with a latch and bootstrap selection circuit. The source driver was optimized by simulation and verified with measurements after circuit fabrication. The output waveforms of the conventional and proposed a-Si:H source drivers were compared. The proposed a-Si:H TFT source driver provided a shorter rise time and higher output voltages than the conventional one.

Trap States of the Oxide Thin Film Transistor

This study examined the electrical properties of a-IGZO thin-film transistors (TFTs) with a gate insulator of SiO₂, in which a more TFT-industry-compatible sputtering technique was used for all processing steps. Instead of plasma enhanced chemical vapor deposition, a room-temperature sputtered SiO₂ gate insulator was used, which is more preferable for the simple and low cost process for oxide TFTs. The dielectric strength of the sputtered SiO₂ film with an oxygen ratio of 6.25% was 6.3 MV/cm, which is sufficient for a gate insulator. The a-IGZO TFTs with the sputtered SiO₂ gate insulators showed the optimal device parameters after post-annealing at 400°C for 1 hour in air: saturation field-effect mobility of 3.80 cm²/V·s, V_th of -2.84 V, on/off ratio of 1.43×10⁵, and S.S of 0.88 V/dec.

Integrated a-Si:H Source Driver With Improved Output Voltage for e-Paper

We investigated the photo responses of an amorphous indium gallium zinc oxide (a-IGZO) thin film under light illumination with various wavelengths and intensities. By using the measured photo-conductivities of a-IGZO thin films, we extracted the photo excitation activation energy and dark relaxation activation energy through extended stretched exponential analysis. The stretched exponential analysis was found to describe well both the photoexcitation and the dark-relaxation characteristics. These analyses indicated that recombination takes place more slowly and through activation processes that are more deeply bound with the broader distribution of activation energies (Eac) than those corresponding to the photo-generation process. The longer wavelength of the incident light, the slower the dark-relaxation occurs because of the formation of higher Eac for the ionized oxygen vacancy (Vo2+) states. For the dark-relaxation process, we also observed that the stretching exponent increases and the distribution of energy levels became narrower for longer wavelengths. This suggests that the neutralization of Vo2+ to Vo is slower for longer wavelengths due to the higher energy barrier height (Eac) for the neutralization of Vo2+.

Electrical Characteristics of a-IGZO TFTs With Gate Insulator Prepared by RF Sputtering

The effects of photo-related stress on the electrical performances of a-SiGe:H thin-film transistors (TFTs) were investigated in comparison with a-Si:H TFTs. Compared with a-Si:H TFTs, the a-SiGe:H TFTs show better stability to the light stress because the number of electrons involved in the creation of dangling bonds are smaller in a-SiGe:H TFTs, resulting in less light-induced degradation. However, a larger threshold voltage shift from the positive gate bias was observed due to the higher number of weak bonds in a-SiGe:H TFTs, which leads to a higher gate bias instability than is observed for a-Si:H TFTs. The temperature dependences of the electrical properties in a-SiGe:H TFTs were observed, and they indicated that a-SiGe:H TFTs follow a thermally activated behavior pattern. Based on the thermally activated behavior, a new model predicting the lifetime of a-SiGe:H TFT image sensors was proposed. The instability of the drain current with respect to the stress time under an electrical bias and light was estimated. Based on the calculated lifetime, the a-SiGe:H TFTs are predicted to be reliable for long-term applications in image sensors.

Stabilities of amorphous indium gallium zinc oxide thin films under light illumination with various wavelengths and intensities

Low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) were developed without ion doping for lower cost. Aluminum (Al), a group 3 element, was used for the source and drain regions instead of ion doping. The effect of Al doping was verified through the Arrhenius plot to check the shift of the Fermi level. Al doping was applied for the metal-induced lateral crystallized polycrystalline silicon (Si) to achieve a p-channel LTPS TFT without ion doping. The TFT developed via Al doping exhibited 35.5 cm2/V s field effect mobility, a−3.7 V threshold voltage (Vth), a 4.0×105 on–off ratio, and a 0.7 V/dec subthreshold slope.