Multioutputs single‐stage gate driver on array with wide temperature operable thin‐film‐transistor liquid‐crystal display for high resolution application

A hydrogenated amorphous silicon (a‐Si:H) thin‐film transistor (TFT) gate driver with multioutputs (eight outputs per stage) for high reliability, 10.7‐inch automotive display has been proposed. The driver circuit is composed of one SR controller, eight driving TFTs (one stage to eight outputs) with bridging TFTs. The SR controller, which starts up the driving TFTs, could also prevent the noise of gate line for nonworking period. The bridging TFT, using width decreasing which connects between the SR controller and the driving TFT, could produce the floating state which is beneficial to couple the gate voltage, improves the driving ability of output, and reaches consistent rising time in high temperature and low temperature environment. Moreover, 8‐phase clocks with 75% overlapping and dual‐side driving scheme are also used in the circuit design to ensure enough charging time and reduce the loading of each gate line. According to lifetime test results, the proposed gate driver of 720 stages pass the extreme temperature range test (90°C and ?40°C) for simulation, and operates stably over 800 hours at 90°C for measurement. Besides, this design is successfully demonstrated in a 10.7‐inch full HD (1080 × RGB×1920) TFT‐liquid‐crystal display (LCD) panel.     

Amorphous‐silicon gate‐driver circuits of shared‐node dual pull‐down structure with overlapped output signals

Abstract— A novel gate‐driver circuit using amorphous‐silicon (a‐Si) TFTs has been developed. The circuit has a shared‐node dual pull‐down AC (SDAC) structure with a common‐node controller for two neighboring stages, resulting in a reduced number of TFTs. The overlapped clock signals widen the temperature range for stable operation due to the extended charging time of the inner nodes of the circuit. The accelerated lifetime was found to be over 1000 hours at 60°C with good bias‐temperature‐stress (BTS) characteristics. Accordingly, the a‐Si gate‐driver circuit was successfully integrated into a 14.1‐in. XGA (1024 × RGB × 768) TFT‐LCD panel having a single bank form.     

High‐reliability gate driver circuit to prevent ripple voltage

In this paper, a high‐reliability gate driver circuit is proposed to prevent multiple outputs. The proposed circuit ensures reliability of the pull‐up thin‐film transistor (TFT) by periodically discharging the Q node voltage to the low‐level voltage (VGL) in the off stage. In addition, the output node is composed of two pull‐down TFTs that are driven alternately to ensure stability against bias stress. Thus, because the reliabilities of the pull‐up and pull‐down TFTs can be guaranteed simultaneously, the stability of the entire circuit is improved. Based on the simulation results, the rising and falling times of the output pulse are stable within 1.77 and 1.28 μs, respectively, even when the threshold voltage of the entire TFT is shifted by +10.0 V. In addition, the ripple voltage of the proposed circuit is almost eliminated and is within 0.79% of the total swing voltage. Moreover, through current is prevented in the proposed circuit because the turn‐on durations of the pull‐up and pull‐down units are completely nonoverlapping, which suggests that unnecessary power consumption can be eliminated. Therefore, based on 2,160 stages, the total power consumption of the proposed circuit is reduced by 34.7 mW from 276.3 to 241.6 mW.     

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.     

Vertically integrated,double stack oxide TFT layers for high‐resolution AMOLED backplane

We developed a novel vertically integrated, double stack oxide thin‐film transistor (TFT) backplane for high‐resolution organic light‐emitting diode (OLED) displays. The first TFT layer is bulk‐accumulation mode, and the second TFT layer is a single gate with back‐channel etched structure. The extracted mobilities and threshold voltages are higher than 10 cm²/Vs and 0 ~ 1 V, respectively. Both TFTs are found to be extremely stable under the bias and temperature stress. The gate driver with width of 530 μm and a pitch of 18.6 μm was developed, exhibiting well shifted signal up to the last stage of 900 stages without output degradation, which could be used for 1360 ppi TFT backplane.     

TFT‐LCD driver IC with embedded non‐volatile memory for portable applications

Abstract— To improve the display quality and yield of the TFT‐LCD driver IC, non‐volatile multiple‐time‐programmable (MTP) memory, which consists of an EEPROM cell and our proposed sense amplifier and power control circuit (SP), was integrated into a TFT‐LCD driver IC. The proposed SP has a 30% smaller layout area and a 18% faster response time compared to that of the conventional SP. The proposed SP also has lower power consumption because it does not use a static current. The TFT‐LCD quality was also improved by tuning the characteristics of the driver IC and the panel with the VREF, OSC, and VCOM blocks, using non‐volatile MTP memory. When the display quality improved, the yield also improved, along with a reduction in the failure ratio of the display module, which consists of the driver IC and the panel. As a result, the TFT‐LCD driver IC with the non‐volatile MTP memory demonstrated improved display quality and a higher yield compared to conventional driver ICs without such a memory.     

A small‐area and low‐power data driver IC using two‐stage DAC with a capacitor array for active matrix flat‐panel displays

A small‐area and low‐power data driver integrated circuit (IC) using a two‐stage digital‐to‐analog converter (DAC) with a capacitor array is proposed for active matrix flat‐panel displays. The proposed data driver IC employs a capacitor array in the two‐stage DAC so as to reduce the DAC area and eliminate the need for a resistor string, which has high‐power consumption. To verify the proposed two‐stage DAC, a 20‐channel data driver IC with the proposed 10‐bit two‐stage DAC was fabricated using a 0.18‐μm complementary metal–oxide–semiconductor process with 1.8 and 6 V complementary metal–oxide–semiconductor devices. The proposed 10‐bit two‐stage DAC occupies only 43.8% of the area of a conventional 10‐bit two‐stage DAC. The measurement results show that the differential nonlinearity and integral nonlinearity are +0.58/?0.52 least significant bit and +0.62/?0.59 least significant bit, respectively. The measured interchannel deviation of the voltage outputs is 8.8 mV, and the measured power consumption of the 20‐channel data driver IC is reduced to 7.1 mW, which is less than half of the power consumed by the conventional one.     

A novel pixel memory using integrated voltage‐loss‐compensation (VLC) circuit for ultra‐low‐power TFT‐LCDs

Abstract— A novel pixel memory using an integrated voltage‐loss‐compensation (VLC) circuit has been proposed for ultra‐low‐power TFT‐LCDs, which can increase the number of gray‐scale levels for a single subpixel using an analog voltage gray‐scale technique. The new pixel with a VLC circuit is integrated under a small reflective electrode in a high‐transmissive aperture‐ratio (39%) 3.17‐in. HVGA transflective panel by using a standard low‐temperature‐polysilicon process based on 1.5‐μm rules. No additional process steps are required. The VLC circuit in each pixel enables simultaneous refresh with a very small change in voltage, resulting in a two‐orders‐of‐magnitude reduction in circuit power for a 64‐color image display. The advanced transflective TFT‐LCD using the newly proposed pixel can display high‐quality multi‐color images anytime and anywhere, due to its low power consumption and good outdoor readability.     

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.     

Robust integrated shift register circuit over clock noises for in‐cell touch applications

This paper proposes an integrated shift register circuit for an in‐cell touch panel that is robust over clock noises. It is composed of 10 thin film transistors and 1 capacitor, and the time division driving method is adopted to prevent the negative effect of display signals on the touch sensing. Two pre‐charging nodes are employed for reducing the uniformity degradation of gate pulses over time. In particular, the proposed circuit connects a drain of the first pre‐charging node's pull‐up thin film transistor (TFT) to the positive supply voltage instead of clock signals. This facilitates to lower coupling noises as well as to clock power consumption. The simulation program with an integrated circuit emphasis is conducted for the proposed circuit with low temperature poly‐silicon TFTs. The positive threshold voltage that shifts up to 12 V at the first pre‐charging pull‐up TFT can be compensated for without the uniformity degradation of gate pulses. For a 60‐Hz full‐HD display with a 120‐Hz reporting rate of touches, the clock power consumption of the proposed gate driver circuit is estimated as 7.13 mW with 160 stages of shift registers. In addition, the noise level at the first pre‐charging node is lowered to ?28.95 dB compared with 2.37 dB of the previous circuit.     

ITO lift‐off technique for TFT mask‐reduction process

Abstract— In recent years, reducing the number of TFT‐manufacturing steps has become an unavoidable technology development stream for all TFT‐LCD makers for the purpose of cost reduction. In this paper, an advanced photomask‐process‐reduction technique, a three‐mask TFT process, by chemical lift‐off which is inherent of the conventional four‐mask TFT fabrication process, is proposed. The major feature of this three‐mask technique is the combining of the passivation‐layer and pixel‐electrode formation within one photolithography process. A new halftone mask (HTM) design has been applied to the photolithography process. With this new HTM design, a small SiN_x island bridge was formed, located at the TFT source contact‐edge border. And it provided an ITO pixel electrical conducting path and avoided the undercut issue where ITO breaks from the gate insulator (GI). In order to enhance the chemical lift‐off efficiency, different process and structure designs were also implemented and introduced. Furthermore, a new laser lift‐off technology was adopted to improve the ability of ITO lift‐off. By using this new laser lift‐off technology, unnecessary ITO film could be easily lift‐off before photoresist stripping. Finally, the first HTM lift‐off panel was successfully demonstrated by using our new three‐mask TFT design scheme.     

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.     

Amorphous‐oxide TFT backplane for large‐sized AMOLED TVs

Abstract— Amorphous‐oxide thin‐film‐transistor (TFT) arrays have been developed as TFT backplanes for large‐sized active‐matrix organic light‐emitting‐diode (AMOLED) displays. An amorphous‐IGZO (indium gallium zinc oxide) bottom‐gate TFT with an etch‐stop layer (ESL) delivered excel lent electrical performance with a field‐effect mobility of 21 cm²/V‐sec, an on/off ratio of >10⁸, and a subthreshold slope (SS) of 0.29 V/dec. Also, a new pixel circuit for AMOLED displays based on amorphous‐oxide semiconductor TFTs is proposed. The circuit consists of four switching TFTs and one driving TFT. The circuit simulation results showed that the new pixel circuit has better performance than conventional threshold‐voltage (VTH) compensation pixel circuits, especially in the negative state. A full‐color 19‐in. AMOLED display with the new pixel circuit was fabricated, and the pixel circuit operation was verified in a 19‐in. AMOLED display. The AMOLED display with a‐IGZO TFT array is promising for large‐sized TV because a‐IGZO TFTs can provide a large‐sized backplane with excellent uniformity and device reliability.     

IGZO‐TFT technology for large‐screen 8K display

We succeeded in G8 factory for mass production of Indium–Gallium–Zink–Oxide thin‐film transistor (IGZO‐TFT) for the first time in the world. The initial TFT process was an etching stop‐type TFT, but now, we are mass producing channel etching‐type TFTs. And, its application range is smartphones, tablets, PCs, monitors, TV, and so on. In particular, because of recent demands for high‐resolution and narrow frame, our IGZO display has been advanced in technology development with gate driver in panel. In this paper, we report development combining low resistance technology and the latest IGZO‐TFT (IGZO5) for large‐screen 8K display.     

A novel gate driver circuit for depletion‐mode a‐IGZO TFTs

In this paper, a novel gate driver circuit, which can achieve high reliability for depletion mode in a‐InGaZnO thin‐film transistors (TFTs), was proposed. To prevent the leakage current paths for Q node effectively, the new driving method was proposed by adopting the negative gate‐to‐source voltage (V_GS) value for pull‐down units. The results showed all the V_OUT voltage waveforms were maintained at VGH voltage despite depletion‐mode operation. The proposed circuit could also obtain stable V_OUT voltage when the threshold voltage for all TFTs was changed from ?6.5 to +11.5 V. Therefore, the circuit can achieve high reliability regardless of threshold voltage value for a‐IGZO TFTs. In addition, the output characteristics and total power consumption were shown for the alternating current (AC)–driven and direct current (DC)–driven methods based on 120‐Hz full‐HD graphics (1920 × 1080) display panel. The results showed that the AC‐driven method could achieve improved V_OUT characteristics compared with DC‐driven method since the leakage current path for Q node can be completely eliminated. Although power consumption of the AC‐driven method can be slightly increased compared with the DC‐driven method for enhancement mode, consumption can be lower when the operation has depletion‐mode characteristics by preventing a leakage current path for pull‐down units. Consequently, the proposed gate driver circuit can overcome the problems caused by the characteristics of a‐IGZO TFTs.     

Driver‐circuits‐integrated LCDs based on novel amorphous In‐Ga‐Zn‐oxide TFT

Abstract— A liquid‐crystal panel integrated with a gate driver and a source driver by using amorphous In—Ga—Zn‐oxide TFTs was designed, prototyped, and evaluated. By using the process of bottom‐gate bottom‐contact (BGBC) TFTs, amorphous In—Ga—Zn‐oxide TFTs with superior characteristics were provided. Further, for the first time in the world, a 4‐in. QVGA liquid‐crystal panel integrated with a gate driver and a source driver was developed by using BGBC TFTs formed from an oxide semiconductor. By evaluating the liquid‐crystal panel, its functionality was successfully demonstrate. Based on the findings, it is believed that the novel BGBC amorphous In—Ga—Zn‐oxide TFT will be a promising candidate for future large‐screen backplanes having high definition.     

Latest developments for “system‐on‐glass” displays using low‐temperature poly‐Si TFTs

Abstract— A 2.3‐in.‐diagonal QVGA‐formatted “System‐On‐Glass” display has been developed by using low‐temperature poly‐Si TFT‐LCD technology. This display fully integrates 6‐bit RGB digital interface drivers as well as all the power supply circuitry to drive the LCD, which requires neither external driver ICs nor power‐supply ICs. This paper discusses the newly developed TFT circuit technologies used in this LCD. The development trend of the “System‐On‐Glass” display is also reviewed.     

Current programming pixel circuit and data‐driver design for active‐matrix organic light‐emitting diodes

Abstract— We propose a new pixel design for active‐matrix organic light‐emitting diodes (AMOLEDs) employing five polycrystalline thin‐film transistors (poly‐Si TFTs) and one capacitor, which decreases the data current considerably in order to reduce the charging time compared with that of conventional current‐mirror structures. Also, the new pixel design compensates the threshold‐voltage degradation of OLEDs caused by continuous operation and the non‐uniformity of poly‐Si TFTs due to excimer‐laser annealing. The proposed pixel circuit was verified by SPICE simulation, based on measured TFT and OLED characteristics. We also propose current‐data‐driver circuitry that reduces the number of shift‐register signals for addressing the current data driver by one‐half.     

Digital‐to‐analog converter with gamma correction on glass substrate for TFT‐panel applications

Abstract— Low‐temperature polysilicon (LTPS) technology has a tendency towards integrating all circuits on glass substrate. However, the poly‐Si TFTs suffered poor uniformity with large variations in the device characteristics due to a narrow laser process window for producing large‐grained poly‐Si TFTs. The device variation is a serious problem for circuit realization on the LCD panel, so how to design reliable on‐panel circuits is a challenge for system‐on‐panel (SOP) applications. In this work, a 6‐bit R‐string digital‐to‐analog converter (DAC) with gamma correction on glass substrate for TFT‐panel applications is proposed. The proposed circuit, which is composed of a folded R‐string circuit, a segmented digital decoder, and reordering of the decoding circuit, has been designed and fabricated in a 3‐μm LTPS technology. The area of the new proposed DAC circuit is effectively reduced to about one‐sixth compared to that of the conventional circuit for the same LTPS process.     

Design of a low‐power‐consumption a‐IGZO TFT‐based Vcom driver circuit with long‐term reliability

Abstract— An amorphous‐InGaZnO (a‐IGZO) thin‐film transistor (TFT)‐based Vcom driver circuit that has long‐term reliability and can be integrated with the pixel array on a panel has been designed. Owing to the Vcom inversion, the power consumed by the proposed driving scheme is 40% less than that consumed by the conventional line‐inversion method. The high mobility (>10 cm²/V‐sec) of the a‐IGZO TFTs allows the integration of devices with small channel widths (<750 μm) and thus keeps the overall device size small, which is important for displays with narrow bezels. The lifetime of the Vcom driver is improved by AC driving (by clocking the n‐th and (n + 1)‐th frame with 20 and 0 V, respectively) of the buffer TFTs.