Investigation of the hysteresis phenomenon of an a‐Si:H TFT at an elevated temperature for AMOLED displays

Abstract— The temperature dependence of the hysteresis of an a‐Si:H TFT has been investigated. An a‐Si:H TFT pixel driving scheme has been proposed and investigated. This scheme can eliminate changes in the organic light‐emitting diode (OLED) current caused by hysteresis of an a‐Si:H TFT. The VTH of the a‐Si:H TFT was changed according to the gate‐voltage sweep direction because of the hysteresis of the a‐Si:H TFT. The variation of VTH for a a‐Si:H TFT decreased from 0.41 to 0.17 V at an elevated temperature of 60°C because the sub‐threshold slope (s‐slope) of the a‐Si:H TFT, in the reverse voltage sweep direction, increased more than in the forward voltage sweep direction due to a greater increase in the initial electron trapped charges than the hole charges. Although the OLED current variation caused by hysteresis decreased (～14%) as the temperature increased, the error in the OLED current needed to be improved in order to drive the pixel circuit of AMOLED displays. The proposed pixel circuit can apply the reset voltage (?10 V) before the data voltage for the present frame that was written to fix the sweep direction of the data voltage. The variation in the OLED current caused by hysteresis of the a‐Si:H TFT was eliminated by the fixed voltage sweep direction in the proposed pixel circuit regardless of operating temperature.     

Low‐temperature poly‐Si TFT data drivers for an SVGA a‐Si TFT‐LCD panel

Low‐temperature poly‐Si TFT data drivers for an SVGA a‐Si TFT‐LCD panel have been developed. The data drivers include shift registers, sample‐and‐hold circuits, and operational amplifiers, and drive LCD panels using a line‐at‐a‐time addressing method. To reduce the power consumption of the shift register, a dot‐clock control circuit has been developed. Using this circuit, the power consumption of the shift register has been reduced to 36% of that of conventional circuits. To cancel the offset voltage generated by the operational amplifier, an offset cancellation circuit for low‐temperature poly‐Si TFTs has been developed. This circuit is also able to avoid any unstable operation of the operational amplifier. Using this circuit, the offset voltage has been reduced to one‐third of the value without using the offset cancellation circuit. These data drivers have been connected to an LCD panel and have realized an SVGA display on a 12.1‐in. a‐Si TFT‐LCD panel.     

Novel a‐Si:H TFT pixel circuit for electrically stable top‐anode light‐emitting AMOLEDs

Abstract— A novel pixel circuit for electrically stable AMOLEDs with an a‐Si:H TFT backplane and top‐anode organic light‐emitting diode is reported. The proposed pixel circuit is composed of five a‐Si:H TFTs, and it does not require any complicated drive ICs. The OLED current compensation for drive TFT threshold voltage variation has been verified using SPICE simulations.     

Design of dual‐outputs‐single‐stage a‐Si:H TFT gate driver for high resolution TFT‐LCD application

A new gate driver has been designed and fabricated by amorphous silicon technology. With utilizing the concept of sharing the noise free block in a single stage for gate driver, dual‐outputs signals could be generated in sequence. By increasing the number of output circuit block in proposed gate driver, number of outputs per stage could also be adding that improves the efficiency for area reduction. Besides, using single driving thin‐film‐transistor (TFT) for charging and discharging, the area of circuit is also decreased by diminishing the size of pulling down TFT. Moreover, the proposed gate driver has been successfully demonstrated in a 5.5‐inch Full HD (1080xRGBx1920) TFT‐liquid‐crystal display panel and passed reliability tests of the supporting foundry.     

A new a‐Si:H TFT pixel circuit employing data‐reflected negative‐bias annealing for a stable and uniform AMOLED

Abstract— A new a‐Si:H pixel circuit to reduce the VTH degradation of driving a‐Si:H thin‐film transistors (TFTs) by data‐reflected negative‐bias annealing (DRNBA) is presented. The new pixel circuit compensates VTH variation induced by non‐uniform degradation of each a‐Si:H pixel due to various electrical stress. The proposed pixel circuit was verified by SPICE simulations. Although the VTH of the driving a‐Si:H TFT varies from 2.5 to 3.0 and 3.5 V, the organic light‐emitting diode (OLED) current changes by only 1.5 and 2.8% in the emission period, respectively. During the negative‐bias annealing period, the negative VGS is applied to the driving TFT by using its own data signal. It is expected that the VTH shift of the driving TFT can be effectively reduced and the VTH shift can be compensated for in our new pixel circuit, which can contribute to a stable and uniform image from an a‐Si:H TFT active‐matrix OLED.     

Effects of post‐annealing on a‐Si:H TFT characteristics fabricated on stainless‐steel substrate

Abstract— A 14.1‐in.‐diagonal backplane employing hydrogenated amorphous‐silicon thin‐film transistors (a‐Si:H TFTs) was fabricated on a flexible stainless‐steel substrate. The TFTs exhibited a field‐effect mobility of 0.54 cm²/V‐sec, a threshold voltage of 1.0 V, and an off‐current of 10^?13 A. Most of the electrical characteristics were comparable to those of the TFTs fabricated on glass substrates. To increase the stability of a‐Si:H TFTs fabricated on stainless‐steel substrate, the specimens were thermally annealed at 230°C. The field‐effect mobility was reduced to 71% of the initial value because of the strain of the released hydrogen atoms and residual compressive stress in a‐Si:H TFT under thermal annealing at 230°C.     

Touch‐screen panel integrated into 12.1‐in. a‐Si:H TFT‐LCD

Abstract— A touch‐screen‐panel (TSP) embedded 12.1‐in. LCD employing a standard existing a‐Si:H TFT‐LCD process has been successfully developed. Compared with conventional external touch‐screen panels, which use additional components to detect touch events, the new internal TSP exhibits a clearer image and improved touch feeling, as well as increased sensing speed using discrete sensing lines to enable higher‐speed sensing functions including handwriting. The new internal digital switching TSP can be fabricated with low cost because it does not require any additional process steps compared to a standard a‐Si:H TFT‐LCD.     

Dual‐plate OLED display (DOD) based on amorphous‐silicon (a‐Si) TFT backplane

Abstract— An improved AMOLED with an a‐Si TFT backplane based on a unique structure is reported. The new structure is refered to as a dual‐plate OLED display (DOD). While a top‐emission OLED array is directly fabricated on a TFT backplane, the DOD consists of an upper OLED substrate and a lower TFT substrate, which are independently fabricated. Because the OLED substrate, which is fabricated through the process flow of bottom emission, is attached to the TFT substrate, the light is emitted in the opposite direction to the TFT backplane. The DOD enables the design of large‐sized TFTs and a complicated pixel circuit. It can also not only achieve higher uniformity in luminance in large‐sized displays due to the low electrical resistance of the common electrode, but also wider viewing angles.     

Low‐power‐consumption TFT‐LCD with dynamic memory embedded in the pixels

A low‐power‐consumption thin‐film‐transistor liquid‐crystal display (TFT‐LCD) with dynamic memory cells embedded in each pixel using low‐temperature poly‐Si technology has been developed. By holding data in the memory, the operating rate of the data driver can be dramatically reduced to 4 Hz. Eight levels of gray scale with low power consumption can be achieved by using the area‐ratio gray‐scale method. This TFT‐LCD can be used for displaying fine still images, with low power consumption.     

Development of a 14.1‐in. UXGA TFT‐LCD using LTPS technology

A 14.1‐in. UXGA low‐temperature poly‐Si TFT‐LCD has been developed using p‐MOS technology. Both the peripheral driving circuits and the pixel switches are implemented using only p‐channel TFTs. The device performance for the driving circuits and the panel design issues, such as crosstalk and flicker, were investigated. The image quality required for the notebook‐PC display has been achieved by optimizing the panel design and by improving the device performance. In addition, the redundant gate driving structure has been developed to minimize the degradation of the panel yield.     

Integrated a‐Si TFT row driver circuits for high‐resolution applications

Abstract— A 12.1‐in. tablet liquid‐crystal‐display (LCD) panel with integrated amorphous‐silicon row driver circuits has been developed using a standard TFT process and Advanced Fringe‐Field Switching (AFFS) technology. An XGA‐resolution 768‐stage shift‐register circuit with two‐phase clocks has been designed and fabricated. The circuit parameters were optimized in order to obtain a highly reliable a‐Si row‐driver‐circuit structure. Thermal Humidity Operation (THO) test results at 50°C and 80% humidity during 500 hours of operation shows that the fabricated panel is reliable during long‐term operation and any abnormal display phenomenon was not observed at 0°C.     

A 470 × 235‐ppi poly‐Si TFT‐LCD for high‐resolution 2‐D and 3‐D autostereoscopic displays

Abstract— We have developed a 470 × 235‐ppi poly‐Si TFT‐LCD with a novel pixel arrangement, called HDDP (horizontally double‐density pixels), for high‐resolution 2‐D and 3‐D autostereoscopic displays. 3‐D image quality is especially high in a lenticular‐lens‐equipped 3‐D mode because both the horizontal and vertical resolutions are high, and because these resolutions are equal. 3‐D and 2‐D images can be displayed simultaneously in the same picture. In addition, 3‐D images can be displayed anywhere and 2‐D characters can be made to appear at different depths with perfect legibility. No switching of 2‐D/3‐D modes is necessary, and the design's thin and uncomplicated structure makes it especially suitable for mobile terminals.     

Vt Compensated voltage‐data a‐Si TFT AMOLED pixel circuits

Abstract— Active‐matrix organic light‐emitting‐diode (AMOLED) displays are now entering the marketplace. The use of a thin‐film‐transistor (TFT) active matrix allows OLED displays to be larger in size, higher in resolutions and lower in power dissipation than is possible using a conventional passive matrix. A number of TFT active‐matrix pixel circuits have been developed for luminance control, while correcting for initial and electrically stressed TFT parameter variations. Previous circuits and driving methods are reviewed. A new driving method is presented in which the threshold‐voltage (V_t) compensation performance, along with various circuit improvements for amorphous‐silicon (a‐Si) TFT pixel circuits using voltage data, are discussed. This new driving method along with various circuit improvements is demonstrated in a state‐of‐the‐art 20‐in. a‐Si TFT AMOLED HDTV.     

A novel TFT‐OLED integration for OLED‐independent pixel programming in amorphous‐Si AMOLED pixels

Abstract— The direct voltage programming of active‐matrix organic light‐emitting‐diode (AMOLED) pixels with n‐channel amorphous‐Si (a‐Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new “inverted” integration technique which makes the direct programming possible by connecting the driver n‐channel a‐Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn‐on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active‐matrix OLED displays with a‐Si technology and conventional bottom‐anode OLEDs.     

Letter: Mechanisms for on/off currents in dual‐gate a‐Si:H thin‐film transistors using indium‐tin‐oxide top‐gate electrodes

Abstract— Two types of dual‐gate a‐Si:H TFTs were made with transparent indium‐tin‐oxide (ITO) top‐gate electrodes of different lengths to investigate the static characteristics of these devices. By changing the length of the ITO top gate, we found that the variations in the on‐currents of these dual‐gate TFTs with dual‐gate driving are due to the high resistance of the parasitic intrinsic a‐Si:H regions between the back electron channel and the source/drain contact. In the off‐state of the dual‐gate‐driven TFTs, the Poole‐Frenkel effect is also enhanced due to back‐channel hole accumulation in the vicinity of the source/drain contact. Furthermore, we observed for the first time that under illumination the dual‐gate‐driven a‐Si:H TFTs exhibit extremely low photo‐leakage currents, much lower than that of single‐gate‐driven TFTs in a certain range (reverse subthreshold region) of negative gate voltages. The high on/off current ratio under backside illumination makes dual‐gate TFTs suitable devices for use as switching elements in liquid‐crystal displays (LCDs) or for other applications.     

A 23‐in. full‐panel‐resolution autostereoscopic LCD with a novel directional backlight system

Abstract— An autostereoscopic display that shows stereoscopic images with full‐panel resolution has been developed,¹ but it has a problem in terms of unit size. To resolve this problem, a new directional backlight system was developed, and it was applied to a prototype autostereoscopic LCD. The backlight system has two light sources — one for the right eye and the another for the left eye — and an elliptically shaped mirror that controls the direction of light from the light sources. The LCD uses a field‐sequential method which re‐writes an image for one eye and one for the other eye at a frame rate of 120 Hz, and the light sources alternately blink in synchronization with each frame so that the LCD shows full‐panel‐resolution stereoscopic images without flicker. In this paper, the new backlight system is described. The backlight system is effective for large screen such as 23 in. on the diagonal. By using this backlight system, the prototype LCD achieved practible unit size, brightness over the entire screen, and cross‐talk.     

Crystallization of amorphous‐Si films by pulsed YAG2ω green laser for polycrystalline Si TFT fabrication

Abstract— A pulsed YAG2ω green laser with a line‐shaped beam was applied in the crystallization process to fabricate polycrystalline‐Si TFTs, and this procedure was compared with excimer‐laser annealing (ELA). YAG2ω lasers are superior to excimer lasers in that they have fewer items to maintain and a longer working time ratio. The crystallization mechanism for YAG2ω laser annealing (YLA) based on the lateral growth of grain is different from that of ELA. Its fluence margin was obtained from the n‐channel mobility measured from fabricated TFTs and was found to be twice as large as that for ELA. We also found that overlapping irradiation does not change the mobility of TFTs. This is because the distribution of grain size is almost the same as in non‐overlapping regions. This suggests the possibility of overlapping irradiation in YLA.     

LED‐backlight feedback control system with integrated amorphous‐silicon color sensor on an LCD panel

Abstract— Thin‐film‐transistor liquid‐crystal displays (TFT‐LCDs) have the largest market share of all digital flat‐panel displays. An LCD backlighting system employing a three‐color red‐green‐blue light‐emitting diode (RGB‐LED) array is very attractive, considering its wide color gamut, tunable white point, high dimming ratio, long lifetime, and environmental compatibility. But the high‐intensity LED has problems with thermal stability and degradation of brightness over time. Color and white luminance levels are not stable over a wide range of temperature due to inherent long‐term aging characteristics. In order to minimize color point and brightness differences over time, optical feedback control is the key technology for any LED‐backlight system. In this paper, the feasibility of an optical color‐sensing feedback system for an LED backlight by integrating the amorphous‐silicon (a‐Si) color sensor onto the LCD panel will be presented. To minimize the photoconductivity degradation of a‐Si, a new laser exposure treatment has been applied. The integrated color‐sensor optical‐feedback‐controlled LED‐backlight system minimized the color variation to less than 0.008 Δu'v' (CIE1976) compared to 0.025 for an open‐loop system over the temperature range of 42–76°C.     

Active‐matrix organic light‐emitting displays employing two thin‐film‐transistor a‐Si:H pixels on flexible stainless‐steel foil

Abstract— An active‐matrix organic light‐emitting diode (AMOLED) display driven by hydrogenated amorphous‐silicon thin‐film transistors (a‐Si:H TFTs) on flexible, stainless‐steel foil was demonstrated. The 2‐TFT voltage‐programmed pixel circuits were fabricated using a standard a‐Si:H process at maximum temperature of 280°C in a bottom‐gate staggered source‐drain geometry. The 70‐ppi monochrome display consists of (48 × 4) × 48 subpixels of 92 ×369 μm each, with an aperture ratio of 48%. The a‐Si:H TFT pixel circuits drive top‐emitting green electrophosphorescent OLEDs to a peak luminance of 2000 cd/m².     

Temperature instability of low‐temperature deposited a‐Si:H TFTs fabricated on plastic substrate

Abstract— Low‐temperature deposited a‐Si:H TFTs have been successfully fabricated on colorless polyimide (CPI) substrate for flexible‐display applications. A serious degradation in threshold voltage was observed after applying external thermal stress. The threshold‐voltage shift saturates after applying several thermal stress cycles. In addition, the TFTs show instability under long periods of thermal stress with fixed temperature. This phenomenon was composed of thermally induced traps and substrate‐expansion‐induced mechanical stress. Finally, the a‐Si:H TFT backplane fabricated on a PI substrate at low temperature has been successfully demonstrated for flexible AMLCDs.