Integrated gate driver circuit technology with IGZO TFT for sensing operation

In this paper, we propose a new integrated gate driver circuit for random sensing operation of external compensated organic light‐emitting diode (OLED) display using oxide thin‐film transistor (TFT). Using this technology, we successfully launched 55‐inch and 65‐inch ultrahigh definition OLED TVs with gate in panel (GIP) circuit. The structure of the existing OLED TVs implemented gate signals through the gate integrated circuits (ICs) attached to the left and right sides of the panel. The structure using the gate IC was inferior to the panel structure using the GIP in terms of process and product design and cost. Thus, we propose a new oxide GIP circuit for OLED TV. Like the previous gate IC model, the proposed GIP circuit successfully implemented the random sensing function during the display operation. This GIP circuit is also designed to overcome the problems caused by the negative Vth characteristics of the oxide device.     

Evaluation of liquid‐crystal‐display motion blur with moving‐picture response time and human perception

Abstract— The moving‐picture response time (MPRT) for measuring liquid‐crystal‐display (LCD) motion blur was studied by several organizations in 2001. To determine the LCD motion blur that humans perceive, subjective evaluation experiments using the method of adjustment was conducted to find a strong correlation between perceived motion blur and extended blurred edge time (EBET) of the MPRT measurements. MPRT thus clearly indicates the degree of which humans perceive motion blur.     

Analysis of response‐time compensation for black‐frame insertion

Abstract— An impulsive driving technique has been widely adopted for the elimination of motion blur in LCDs. Although the problem of slow temporal response time is very well known for LCDs, the inherent motion blur of moving objects in hold‐type displays has a more‐serious impact on display performance. It is well known that even very fast LCDs with zero response time still suffer from the motion‐blur artifact due to hold‐type driving effects. However, a fast temporal response is also critical in order to maximize the blur‐reduction effect even in the case of impulsive driving. In this paper, the special behavior of LC molecules in an impulsive driving environment has been analyzed especially for the case of black‐frame insertion, and we propose an effective means to implement optimized response‐time compensation (RTC) for the black‐frame insertion technique.     

Comparisons of motion‐blur assessment strategies for newly emergent LCD and backlight driving technologies

Abstract— Compared to the conventional cathode‐ray‐tube TV, the conventional liquid‐crystal TV has the shortcoming of motion blur. Motion blur can be characterized by the motion‐picture response‐time metric (MPRT). The MPRT of a display can be measured directly using a commercial MPRT instrument, but it is expensive in comparison with a photodiode that is used in temporal‐response (temporal luminance transition) measurements. An alternative approach is to determine the motion blur indirectly via the temporal point‐spread function (PSF), which does not need an accurate tracking mechanism as required for the direct “spatial” measurement techniques. In this paper, the measured motion blur is compared by using both the spatial‐tracking‐camera approach and the temporal‐response approach at various backlight flashing widths. In comparison to other motion‐blur studies, this work has two unique advantages: (1) both spatial and temporal information was measured simultaneously and (2) several temporal apertures of the display were used to represent different temporal PSFs. This study shows that the temporal method is an attractive alternative for the MPRT instrument to characterize the LCD's temporal performance.     

New pixel circuits for controlling threshold voltage by back‐gate bias voltage using crystalline oxide semiconductor FETs

We devised a threshold voltage compensation pixel circuit using back‐gate bias voltage. Variation in threshold voltages can be reduced to 10% in simulation while improving the saturation characteristics of a driving transistor. The pixel circuit can compensate not only threshold variation but also mobility variation. We fabricated a 5.29‐in Quad‐VGA organic light emitting diode display using this pixel circuit.     

Anode‐voltage‐control circuit for compensation of luminance deterioration

Abstract— An external driving circuit that has realized long lifetime, power‐consumption control, and peak luminance for organic light‐emitting diode (OLED) displays have been developed. This circuit realizes an effective method for constant‐anode‐voltage (CV) driving refered to as clamped inverter (CI) driving. The feature of CV driving is to achieve low‐power consumption compared with constant‐anode‐current (CC) driving and to control the power consumption and peak luminance according to the image because display luminance can be easily changed by controlling the anode voltage. On the other hand, CV driving has the problem that luminance deterioration appears to be serious compared with that of CC driving because the current of the OLED element decreases according to usage time. To cope with this, a lifetime compensation circuit that has increased the anode voltage so that it compensates for the luminance deterioration has been developed. This circuit can compensate not only the decrease in current but also the decrease in luminance at a constant current that CC driving cannot. However, increasing the anode voltage causes an increase in stress on the OLED element. The influence of stress on OLED lifetime was verified. As a result, it was confirmed that this circuit can extend the lifetime by 32% even if the anode voltage is increased, causing stress on the OLED structure.     

AMOLED panel driven by time‐multiplexed clamped inverter circuit to reduce complex control signals

Abstract— A time‐multiplexing technique employing a pulse‐width‐modulation (PWM) charge‐pump scheme for driving active‐matrix organic light‐emitting‐diodes (AMOLEDs) is described. This scheme greatly reduces the number of control lines. The two‐ or four‐phase PWM driving technique not only reduces costs through the simplification of manufacture but also improves the uniformity and lifetime of OLED panels. Experimental results show that the proposed circuit effectively and precisely controls the timing of OLED data‐writing and light emission.     

Development of 55‐in. 8K AMOLED TV based on coplanar oxide thin‐film transistors and inkjet printing process

In this paper, we presented 55‐in. 8K4K AMOLED TV employing coplanar oxide thin‐film transistor (TFT) backplane, top emissive inkjet‐printing organic light‐emitting diode (OLED) device, gate driver on array (GOA), and compensation technologies. It is so far the largest prototype AMOLED TV fabricated by using inkjet printing process with 8K resolution. It shows the stunning display quality, thanks to the high resolution and fast refresh frequency. It proves that the inkjet printing process is not only cost competitive but also can deliver premium display.     

Threshold‐voltage‐compensation methods for AMOLED pixel and analog buffer circuits

Abstract— New pixel‐circuit designs for active‐matrix organic light‐emitting diodes (AMOLEDs) and a new analog buffer circuit for the integrated data‐driver circuit of active‐matrix liquid‐crystal displays (AMLCDs) and AMOLEDs, based on low‐temperature polycrystalline‐silicon thin‐film transistors (LTPS‐TFTs), were proposed and verified by SPICE simulation and measured results. Threshold‐voltage‐compensation pixel circuits consisting of LTPS‐TFTs, an additional control signal line, and a storage capacitor were used to enhance display‐image uniformity. A diode‐connected concept is used to calibrate the threshold‐voltage variation of the driving TFT in an AMOLED pixel circuit. An active load is added and a calibration operation is applied to study the influences on the analog buffer circuit. The proposed circuits are shown to be capable of minimizing the variation from the device characteristics through the simulation and measured results.     

Compact and power‐saving polysilicon data driver with common‐decoder DA converter (CD‐DAC)

Abstract— A common‐decoder architecture for a data‐driver circuit fabricated by using a polysilicon process has been developed. The architecture achieves a compact circuit and low‐power consumption. In application to an integrated polysilicon data driver for small‐sized displays, this architecture reduces the area of the data driver by removing the vertical bus lines that occupy a large area. It also suppresses the power consumption of the data bus by reducing the number of driven lines in the data bus during word‐to‐word transitions from six to two. By using a conventional 4‐μm design rule, we fabricated an active‐matrix OLED (AMOLED) panel with an integrated six‐bit data‐driver circuit with 384 outputs. The driver circuit had a height of 2.6 mm and a pitch between output lines of 84 μm. The maximum power consumption of the driver was only 5 mW, i.e., 3.8 mW for logic‐data transfer and 1.2 mW for reference‐voltage source. Furthermore, we also fabricated an active‐matrix LCD (AMLCD) panel including driver circuits of the same type as the integrated elements. Six‐bit full‐color images were successfully displayed on both panels.     

An external compensation method for AMOLED using the concept of ramp‐stop

In this paper, we propose an external feedback method to compensate the device variation for active‐matrix organic light‐emitting diode. The pixel data current is controlled by ramping the gate voltage and converted to the sensed voltage V_sense in real time. When V_sense is equal to a preset voltage V_data, the switching block outputs the low potential to stop the ramping. Therefore, the gate voltage is locked at the value corresponding to the target data current. This circuit is implemented with three thin‐film transistors in the active area and some functional blocks in driver integrated circuit (IC), namely, sentinel block, current‐voltage converting block, and switching block. Unlike the other usual methods of external compensation requiring double number of connections between driver IC and glass, by using the common ramping signal and a simple circuit made on glass, the proposed method can be implemented with only one pin per column.     

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 new driving method introducing a display period for AMOLEDs

Abstract— We have proposed an Advanced‐Clamped‐Inverter driving method for the fabrication of AMOLEDs, in which one frame period is divided into an addressing period and a display period. This driving method enables the AMOLEDs to produce excellent moving images without motion blur and false pixels, and has peak‐luminance characteristics because of its unique light‐emission scheme of the OLED elements. Good inter‐pixel uniformity was also achieved in a previous clamped‐inverter driving method. We fabricated AMOLEDs and experimentally confirmed their characteristics.     

Circuits using uniform TFTs based on amorphous In‐Ga‐Zn‐O

Abstract— High‐performance and excellent‐uniformity thin‐film transistors (TFTs) having bottom‐gate structures are fabricated using an amorphous indium‐gallium‐zinc‐oxide (IGZO) film and an amorphous‐silicon dioxide film as the channel layer and the gate insulator layer, respectively. All of the 94 TFTs fabricated with an area 1 cm² show almost identical transfer characteristics: the average saturation mobility is 14.6 cm²/(V‐sec) with a small standard deviation of 0.11 cm²/(V‐sec). A five‐stage ring‐oscillator composed of these TFTs operates at 410 kHz at an input voltage of 18 V. Pixel‐driving circuits based on these TFTs are also fabricated with organic light‐emitting diodes (OLED) which are monolithically integrated on the same substrate. It is demonstrated that light emission from the OLED cells can be switched and modulated by a 120‐Hz ac signal input. Amorphous‐IGZO‐based TFTs are prominent candidates for building blocks of large‐area OLED‐display electronics.     

Electrical characterization and modelling of top‐emitting PIN‐OLEDs

Abstract— Positively doped, intrinsic, negatively doped organic light‐emitting diodes (PIN‐OLEDs) have been shown to exhibit high efficiency and a long lifetime compared to conventional small‐molecule OLEDs (SM‐OLEDs). The improved performance of PIN‐OLEDs makes them attractive for use in display applications. Knowledge of the electrical load exhibited by these devices is used to develop an equivalent electrical‐circuit model. Such models are used by circuit designers to assist with the precise design of active‐matrix‐display driver circuits used in such applications. In this paper, the development of a SPICE model for a top‐emitting PIN‐OLED stack is reported.     

A perceived sharpness metric based on the luminance slope and overshoot of the motion‐induced edge‐blur profile

To reduce perceived motion blur on liquid crystal displays, typically various techniques such as overdrive, scanning backlight, black‐data insertion, black‐field insertion, and frame rate up‐conversion are widely employed by the liquid crystal display industry. These techniques aim to steepen the edge transitions by improving the dynamic behavior of the light modulation. However, depending on the implementation, this may result in the perception of irregularly shaped motion‐induced edge‐blur profiles. It is not yet fully understood how these irregularities in the steepened edge‐blur profiles contribute to the perceived sharpness of moving objects. To better understand the consequences of several motion‐blur reduction techniques, a perception experiment is designed to evaluate the perceived sharpness of typical motion‐induced edge‐blur profiles at several contrast levels. Relevant characteristics of these profiles are determined on the basis of the perception results by means of regression analysis. As a result, a sharpness metric with two parameters is established, where one parameter relates to the edge slope and the other to the overshoot/undershoot part of the motion‐induced edge‐blur profile.     

Static active‐matrix OLED display without pixel refresh enabled by amorphous‐silicon non‐volatile memory

Abstract— An active‐matrix organic light‐emitting‐diode (AMOLED) display which does not require pixel refresh is demonstrated. This was achieved by replacing the thin‐film transistor (TFT) that drives the OLED with a non‐volatile memory TFT, in a 2‐transistor pixel circuit. The threshold voltage of the non‐volatile‐memory TFT can be changed by applying programming voltage pulses to the gate electrode. This approach eliminates the need for storage capacitors, increases the pixel fill factor, and potentially reduces power consumption. Each pixel can be individually programmed or erased using a standard active‐matrix addressing scheme. The programmed image is stored in the display even if power is turned off.     

Guiding principles for high‐quality moving picture in LCD TVs

Abstract— The moving‐picture quality of several LCD modules was evaluated by using the quantitative parameter, normalized blurred edge width (N‐BEW), or the N‐BEW value normalized by time (N‐BET), measured and calculated by the developed time‐sequence‐image integration system which has taken LCD‐response characteristics and human‐vision characteristics into consideration. The quality of several LCD TVs is also discussed by using subjective evaluation and the unified quantitative parameter moving‐picture response time (MPRT), which is based on N‐BEW. According to the experimental and calculated results, it is clear that the value of N‐BET can express moving‐picture quality, which depends on the liquid‐crystal response time and the hold‐type character of LCDs. Also, it is confirmed that the value of MPRT can express the moving‐picture quality by comparison with subjective evaluation. The target values of MPRT and N‐BET for the motion‐blur‐less picture are deduced by extrapolating the subjective evaluation results. Then, guidelines to improve the moving picture quality are demonstrated.     

Novel analog feedback current programming architecture compatible with 2‐transistor 1‐capacitor pixel for active matrix organic light‐emitting diode displays

A new feedback current programming architecture is described, which is compatible with active matrix organic light‐emitting diode (AMOLED) displays having the 2T1C pixel structure. The new pixel programming approach is compatible with all TFT technologies and can compensate for non‐uniformities in both threshold voltage and carrier mobility of the pixel OLED drive TFT. Based on circuit simulations, a pixel drive current of less than 10 nA can be programmed in less than 50 µ. This new approach can be implemented within an AMOLED external or integrated display data driver.     

Bezel free design of organic light emitting diode display via a‐InGaZnO gate driver circuit integration within active array

In this article, we described an innovative design technology of active matrix organic light emitting diode (AMOLED) display, to provide a bezel free design. We designed gate driver circuit of amorphous indium‐gallium‐zinc oxide thin‐film transistors (TFTs) not on the bezel area but within the active array. Although we applied challengeable design, no degradation of electrical/optical properties of panel was observed. Because we effectively prevented capacitive coupling and interference between the emission circuit and integrated gate driver circuit in active array, finally, we successfully demonstrated a bezel free designed AMOLED display of 18.3″ HD (1366 × 768) driven by a‐InGaZnO TFTs.