Hee-Sun Shin

Low-Voltage Driven P-Type Polycrystalline Silicon Thin-Film Transistor Integrated Gate Driver Circuits for Low-Cost Chip-on-Glass Panel

P-type low-temperature polycrystalline silicon (LTPS) thin-film transistor (TFT) integrated driver circuits are proposed for low-cost chip-on-glass (COG) panel. In order to reduce the process cost of panel, gate driver employing level-shifter, shift register and DC–DC converter is integrated by p-type polycrystalline silicon (poly-Si) TFTs. The gate drivers are composed of the level-up and level-down voltage shifters and the robust two-clock shift registers. The DC–DC converters are designed using diode-connected type charge pumps and regulators. The proposed p-type circuits were verified successfully by the simulations and the measurements.

New Current-Scaling Pixel Circuit Compensating Non uniform Electrical Characteristics for Active Matrix Organic Light Emitting Diode

We have proposed and successfully fabricated a new serial current mirror pixel design for an active-matrix organic light-emitting diode which consists of four polycrystalline silicon thin-film transistors (poly-Si TFTs) and one capacitor. The non uniformity of OLED current (IOLED) due to the nature of a poly-Si TFT is considerably suppressed. Our experimental results show that the non uniformity of IOLED due to threshold voltage and mobility variation is around 7% whereas that of IOLED is about 25% in the conventional 2-TFT pixel. When supply voltage decreases from 10 V to 9.5 V, the non uniformity of OLED current is suppressed to 2.2% whereas that of OLED current is 35% in the 2-TFT pixel. The proposed pixel can successfully compensate the variation of the electrical characteristics of the poly-Si TFTs as well as provide uniform OLED current despite the supply voltage drop.

A new current‐mirror pixel circuit employing poly‐Si TFTs for active‐matrix organic light‐emitting‐diode displays

— A novel active-matrix organic light-emitting-diode (AMOLED) display employing a new current-mirror pixel circuit, which requires four-poly-Si TFTs and one-capacitor and no additional signal lines, has been proposed and sucessfully fabricated. The experimental results show that a new current mirror can considerably compensate luminance non-uniformity and scale down a data current more than a conventional current-mirror circuit in order to reduce the pixel charging time and increase the minimum data current. Compared with a conventional two-TFT pixel, the luminance non-uniformity induced by the grain boundaries of poly-Si TFTs can be decreased considerably from 41% to 9.1%.

Active-Matrix Organic Light Emission Diode Pixel Circuit for Suppressing and Compensating for the Threshold Voltage Degradation of Hydrogenated Amorphous Silicon Thin Film Transistors

A new hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) pixel circuit for active-matrix organic light emission diodes (AM-OLEDs), which significantly compensates the OLED current degradation by memorizing the threshold voltage of driving TFT and suppresses the threshold voltage shift of a-Si:H TFTs by negative bias annealing, is proposed and fabricated. During the first half of each frame, the driving TFT of the proposed pixel circuit supplies current to the OLED, which is determined by modified data voltage in the compensation scheme. The proposed pixel circuit was able to compensate the threshold voltage shift of the driving TFT as well as the OLED. During the remaining half of each frame, the proposed pixel circuit induces the recovery of the threshold voltage degradation of a-Si:H TFTs owing to the negative bias annealing. The experimental results show that the proposed pixel circuit was able to successfully compensate for the OLED current degradation and suppress the threshold voltage degradation of the driving TFT.

Novel L-Shaped Dual-Gate Structure of Polycrystalline Silicon Thin-Film Transistors for the Reduction of the Kink Current in Sequential Lateral Solidification or Continuous Wave Laser Method

An L-shaped dual-gate device structure, which reduces kink current in polycrystalline silicon thin-film transistors (poly-Si TFTs), has been proposed and fabricated. In the proposed device, the poly-Si TFTs have a lateral grain growth in channels such as TFTs fabricated by sequential lateral solidification (SLS) or CW laser crystallization. The current flow of dual TFTs is strongly affected by grain boundaries showing lateral grain growth. The L-shaped dual gate structure is employed for asymmetry between dual channels. One of the channels is located in a parallel direction of grain growth and the other is located vertically. It is verified by experiment that the proposed L-shaped dual-gate TFT reduced the kink current of poly-Si TFT and showed improved reliability by fixed current flow in the saturation mode.

P-20: An AMOLED Pixel for the VT Compensation of TFT and a p-Type LTPS Shift Register by Employing 1 Phase Clock Signal

A voltage modulated AMOLED pixel design using low temperature polycrystalline silicon thin film transistor (LTPS-TFT) is proposed. The proposed pixel, which consists of 5 p-type LTPS TFTs and 1-capacitor, successfully suppressed the threshold voltage variation problem in LTPS TFTs and OLEDs. A new p-type LTPS shift register employing only 1 clock signal is also proposed for the integration driving circuits. In order to eliminate the clock skew problem, only 1 phase clock signal is used in the proposed shift register. The proposed circuits were verified by simulation and experimental results.

A New Thin-Film Transistor Pixel Structure Suppressing the Leakage Current Effects on AMOLED

We propose a new pixel structure employing solid-phase crystallized silicon thin-film transistors which suppresses the leakage current effects on active-matrix organic light-emitting diode (AMOLED) displays. The pixel structure has been fabricated on a glass substrate employing the field-enhanced rapid thermal annealing technology. In the proposed pixel, the charge holding capability is considerably enhanced due to the capacitor located between two series-connected switch transistors. Our experimental results shows that the average variation range of the OLED current is suppressed less than 0.5% while the conventional one exceeded 4%.

Highly efficient current scaling AMOLED panel employing a new current mirror pixel circuit fabricated by excimer laser annealed poly-Si TFTs

We propose and fabricate highly efficient current scaling AMOLED panel employing excimer laser annealed poly-Si TFTs, which successfully compensate the non-uniformity of IOLED due to the grain boundaries and residual image caused by a hysteresis phenomenon in poly-Si TFT. The proposed 2.4 inch panel employing a new current mirror pixel circuit successfully reduces a nonuniformity of the luminance from 41% to 9.1%, and eliminates residual image, compared with conventional 2-TFT pixel array. The proposed pixel circuit can also increase the data current by 57% and decrease the pixel charging time by 21%, compared with a traditional current mirror pixel

P‐18: Highly Stable Amorphous Silicon Thin Film Transistor AMOLED Panel Employing Optical Feedback Compensation Pixel Circuit

The optical feedback scheme can compensate the threshold voltage variation of TFT and OLED as well as the degradation of OLED efficiency. The new optical feedback pixel circuit integrating a-Si:H TFT photo sensor which is designed with the reversed diode-connected TFT is proposed and fabricated. We have fabricated the 0.85 inch AMOLED panels (56 pixels × 84 pixels) employing the new optical feedback pixel circuits. The luminance degradation of AMOLED panel employing the proposed pixel after 12 hours chess board patterned stress is less than 12%, while the conventional one has 37% of luminance degradation.

P-12: A New Stable a-Si:H TFT Pixel for AMOLED by Employing the a-Si:H TFT Photo Sensor

The optical feedback pixel circuit has been attractive compensation scheme for improving AMOLED display quality. We reported that the photoelectric characteristics of the a-Si:H TFT as the integrated photo sensor under two bias condition (Vgs=0V, Vgs<0V). And we proposed a new optical feedback pixel circuit integrating the a-Si:H TFT photo sensor in each pixel. In order to improve the compensation ability, the sensor part is separated to the gate of driving TFT during selecting time. We verified the proposed pixel circuit could compensate the variation of OLED luminance not only by TFT degradation, but also by OLED degradation. We will also report the experimental results.