Pixel Circuit Research Articles

P‐4: ΔVTH and VSS IR rise Compensable PAM and PWM Combined Oxide TFTs Mini‐LED Pixel Circuit Working at Low Voltage

This paper presents a PAM and PWM combined Mini‐LED pixel circuit based on oxide TFTs. In the PAM part, a constant current (1mA) allows Mini‐LEDs work at peak efficiency and reduce power consumption. A source follower structure compensates for ΔVTH and VSS IR rise, which has only one TFT on current path, so that the pixel circuit works at low voltage. In the PWM part, a diode connected structure compensates for ΔVTH. Simulation results show that the current error rate is less than 0.81% when ΔVTH of PAM part is within ±3 V. When VSS rises less than 3 V, the current error rate is less than 7.22%. When ΔVTH of PWM part is +3 V, emission time variation is reduced by 99.29% compared with the circuit without compensation. The supply‐voltage can be reduced from 19.7 V to 15 V thanks to the proposed Mini‐LED pixel circuit.

P‐23: Micro Light‐Emitting Diode Pixel Circuit Based on IGZO TFTs using a Stepwise Control Signal

We proposed an IGZO TFT‐based micro light‐emitting diode (μLED) pixel circuit using a stepwise control signal. The proposed pixel circuit expressed the gray levels by modulating the emission time with the constant driving current to suppress a wavelength shift of μLEDs. The proposed pixel circuit successfully expressed 8‐bit grayscale using a quaternary digital driving. The simulated μLED current waveforms showed a delay time of approximately 2 μs. Furthermore, the proposed circuit exhibited a luminance error rate below 5% under a threshold voltage variation of ±1 V.

P‐22: Scan Driver Circuit with Multiple Outputs employing Low‐temperature Polysilicon and Oxide TFTs for Mobile Displays

In this paper, we propose a scan driver circuit with two outputs employing low‐temperature polycrystalline silicon and oxide (LTPO) thin‐film transistors (TFTs). The proposed scan driver circuit can generate two types of scan signals that can drive p‐ type and n‐type switching TFTs. In addition, by using the Q[n] node bootstrapping method, the output is stably generated even in the depletion mode of the amorphous indium‐gallium‐zinc‐ oxide (a‐IGZO) TFTs. As it is a scan driver circuit that can be applied to LTPO TFT‐based pixel circuits, variable refresh rate operation is possible. Therefore, the proposed scan driver circuit can be applied to displays requiring high resolution, narrow bezel, and low power consumption.

7‐1: A New a‐IGZO TFT Pixel Circuit for High‐Resolution Mobile AMOLED Display with Highly Stable at Low Gray Levels

This paper proposes a highly stable amorphous indium‐gallium zinc‐oxide (a‐IGZO) thin‐film transistor (TFT) pixel circuit for the active‐matrix organic light‐emitting diode (AMOLED) displays, which can compensate for the non‐uniform threshold voltage (VTH) of the TFT at a low gray level. The stable operation was achieved by minimizing the operation number of the switch connected to the VTH storage capacitor after the VTH compensation stage. The HSPICE simulation verified that the relative current error rate is within only ‐15.7% at 567 pA (13 gray level) against the VTH variations.

7‐3: A New Line‐by‐Line SWEEP Signal Generation Method for PWM Driving MicroLED TFT Pixel Circuit

This paper proposes a novel method to generate a SWEEP signal line‐by‐line for PWM driving microLED pixel circuit. The proposed method generates a SWEEP signal based on the linearly increasing voltage of the capacitor by flowing a constant current to the horizontal line parasitic capacitive load. The constant current is supplied by the sweep generation (SG) circuit in each line. Since the SG circuit utilizes the existing SCAN signals for the pixel circuit, additional SCAN or clock signals are not required. By using the proposed method, an individual SWEEP signal for each line can be generated without a significant increase in operational complexity in the display panel.

P‐26: IGZO TFT‐based Micro Light‐Emitting Diode Pixel Circuit with Progressive Emission Using Pulse Width Modulation

IGZO TFT‐based µLED pixel circuit was proposed. We utilized PWM to improve the wavelength shifts of µLED and applied progressive emission, which easily controls the emission current and emission time for low power consumption. We demonstrated the stable operation in the proposed pixel circuit based on depletion mode IGZO TFTs.

P‐25: An AMOLED Pixel Circuit Compensating for Variation of Sub‐threshold Swing and Threshold Voltage Based on Double Gate a‐IGZO TFTs †

In this paper, an AMOLED pixel circuit based on amorphous indium‐gallium‐zinc‐oxide (a‐IGZO) thin‐film transistors (TFTs) compensating for variations in sub‐threshold swing (SS) and threshold voltage (VTH) of driving TFT is proposed. The proposed pixel circuit is composed of five TFTs and one capacitor. From the simulation results, when the SS and VTH shifts of the driving up to ±10% and ±1 V, the maximum OLED current error can be suppressed to 3.6% and 7.2%, respectively.

P‐63: Simple Pixel Circuit with External Compensation for High PPI Active‐Matrix MicroLED Displays Using PWM Method

This work presents a novel pixel circuit that consists of five transistors and two capacitors for active‐matrix micro‐LED (AMμLED) displays. The proposed pixel circuit uses external compensation to compensate for variations in the low‐temperature polycrystalline‐silicon thin‐film transistor (LTPS TFT) threshold voltage and adopts pulse width modulation (PWM) to modulate the grayscale. According to the simulation results and a layout example, the proposed pixel circuit is expected to be applied in high pixels per inch (PPI) car displays in the future.

P‐10: The Novel OLED Compensation TFT Circuit for Enhanced Outside Visibility

We proposed a novel active‐matrix compensation pixel circuit for the displays using self‐emitting devices (e.g. organic light emitting diodes, micro‐LEDs, perovskite LEDs, et al.). This pixel circuit has two operation modes, the bright mode and the dark mode. This circuit provides both high luminance and excellent gradation quality which are crucial for outside visibility.

P‐9: LTPO TFTs Based PWM Pixel Circuit for AM‐Micro‐LED Display with Falling‐time <10μs

A pulse‐width‐modulation (PWM) pixel circuit for Micro‐LED display based on low‐temperature poly‐silicon and oxide (LTPO) thin‐film transistor (TFT) is proposed. The PWM falling‐time is reduced to 10 μs, which is only 1/10 of conventional PWM pixel schematics, thanks to the high gain dynamic CMOS inverter for converting data voltage to PWM waveforms and an independent diode‐connected compensation structure for generating constant driving current. The maximum error of PWM conversion and driving current of Micro‐LED is less than 1%, despite VTH shift of oxide TFT ±2 V or VTH variation of LTPS TFT ±1 V. Furthermore, high display resolution above 270 PPI is obtained as the pixel layout reduced to 94 μm×94 μm due to the simplified pixel structure.

Hybrid SiC Pixel Detector for Charged-Particle Beam Monitor

We report on the hybrid SiC pixel detector for charged-particle beam monitors in high-energy physics experiments. To demonstrate comparable performances of the state-of-the-art Si pixel detectors, SiC pixel sensors with a die size of 25 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were fabricated based on the p-n diode structure. We combined 12 × 12 diodes of a pixel size of 160 μm and 270 μm pitch with the dedicated readout ASIC by In/Au stud-bump technology. The front-end chip was designed in 0.35 μm CMOS technology, with consideration of a hybrid configuration. Each pixel circuit consists of a charge-sensitive amplifier and band-pass filter, optimized for the SiC sensor. The low noise performance achieved an equivalent noise charge of 55 ± 5e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–</sup> (rms). The spectrum of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup> Am shows an energy resolution of 1.72 keV (FWHM) at 17.8 keV γ-rays at room temperature. The obtained image with <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90</sup> Sr is a proof of a single MIP detection capability with all pixels. Those results prospect a future application of the SiC pixel sensors to high-intensity proton extinction monitors in the COMET muon experiment at Japan Proton Accelerator Research Complex.

15‐2: A Novel Compensation Algorithm for Multi‐Frame‐Drop on AMOLED Display

In this paper, a novel compensation algorithm is proposed for improving multi‐frame drop (MFD) on AMOLED display. The MFD phenomenon occurs as the image switching or sliding when the display with extremely low brightness, resulting in the color shift and distortion at the edge of moving objects. Because the current is too small and the emission time is too short at extremely low brightness, more than one frame is needed to alleviate the impact of the parasitic capacitance between the gate and drain node of driving TFT (DTFT), and the gradual changing of drain voltage (VD) causes the DTFT to move from the linear to saturation region, which also aggravates this phenomenon. This paper analyzes the MFD phenomenon based on the existing 7T1C pixel circuits, records the grayscale changes of multiple frames, and gives the different offsets at dropped frames. By experiment results, this method is not changing the pixel circuit, such as the 8T1C, which could compensate for the dropped frames for reducing MFD issues and is easily implemented in the display driver IC for many different OLED panels.

35‐4: Carbon Nanotube Thin‐Film Transistors for Active‐Matrix Micro‐LED Display – Device Performances, Bias Stress Stability and Compact Modeling

Carbon nanotube thin‐film transistor (CNT‐TFT) is expected to be a promising candidate for backplane technologies for emerging displays including micro‐LED, due to its extraordinary driving capability. In this work, CNT‐TFTs are characterized, which exhibit excellent on‐state current, high carrier mobility, and low subthreshold swing. Different from conventional TFTs, a positive threshold voltage shift under negative bias stress is observed in these devices. The mechanism is explored and explained, and an improved stretched‐exponential model is developed for such phenomenon. To predict the driving capability of CNT‐TFT backplane technology, compact models for CNT‐TFTs and micro‐LEDs are built based on experimental data, and SPICE simulation of a 3T1C pixel circuit is carried out. The results indicate that CNT‐TFTs enables high driving current in active‐matrix micro‐LED display with relative ease, paving the way for practical applications of CNT‐TFTs.

P‐156: Late‐News Poster: Effect of Light Emission Delay on AMOLED Panel

The Active Matrix Organic Light Emitting Diode (AMOLED) display causes the current deviation between pixels due to the dispersion based on device characteristics such as Threshold voltage (Vth) of Thin Film Transistors (TFT). This dispersion is compensated with pixel circuits inside the AMOLED panel, but the compensation time gradually decreases because of the performance improvement of AMOLED display such as high resolution and high‐speed driving, and as a result, it is difficult to solve the problem of the current deviation at low luminance which is using low current. The deviation of charging time for the capacitance of OLED (OLED Cap) by the current deviation based on device characteristic causes the light emission delay deviation, which in turn causes Mura on the panel. In this paper, we analyze the effect of the light emission delay deviation on Mura of AMOLED panel.

7‐4: A Novel a‐IGZO TFT Micro‐LED Circuit with Improved Stability and Area Efficiency

In this paper, a novel amorphous Indium‐Gallium‐Zinc‐Oxide (a‐IGZO) thin‐film transistor (TFT) pixel circuit for pulse width modulation (PWM) driving micro light emitting diode (μLED) display is proposed. During the circuit operation, single signal is used for two different purposes and data line for constant current generation (CCG) unit is eliminated. Furthermore, the impact of connection structure between PWM unit and CCG unit on compensation error is investigated, and showed that the structure used in the proposed circuit is effective in reducing current error rate of CCG unit, especially in low gray level area.

P‐2: Narrow Bezel Gate Driver Generating Positive Pulse for AMOLED Display Using LTPO Technology with Depletion Mode Oxide TFTs

We propose a novel gate driver generating positive pulse for oxide switching TFTs in AMOLED pixel circuits using low‐temperature polycrystalline silicon and oxide (LTPO) thin‐film transistors (TFTs). The proposed gate driver circuit has only five TFTs without capacitor. The proposed circuits can operate perfectly when all oxide TFTs in the circuits have threshold voltage (VTH) of ‐3.5V, depletion mode. The circuit can be easily designed for narrow bezel due to simple structures. The one stage of the circuit was designed with the size of 50 μm × 100 μm. The fabricated circuit works well with the depletion mode oxide TFTs with VTH of ‐3.5V. The circuit operated at the pulse width of 1 μs, corresponding to operating speed of 500 kHz. The proposed gate driver can be applicable for narrow bezel AMOLED display.

P‐11: A Novel AMOLED Pixel Circuit using Double‐Gate LTPO TFTs for Variable Refresh Rate with Low Power Consumption

This paper proposes an Active Matrix Organic Light Emitting Diode (AMOLED) pixel circuit for Variable Refresh Rate (VRR) driving from 1Hz to 240Hz. The circuit is based on a double‐gate Low Temperature Polycrystalline Silicon and Oxide (LTPO) Thin Film Transistor (TFT) backplane. For low frame rate driving we used oxide TFTs as the Switching (SW) TFTs. Moreover, we applied double‐gate Low Temperature Polycrystalline Silicon (LTPS) process to the Driving (DR) TFT for compensation of the Threshold Voltage (Vth) by controlling the bottom gate bias at a high refresh rate. In this paper, operation principle and simulation results will be illustrated to demonstrate the performance of the proposed circuit.

A novel LTPO AMOLED pixel circuit and driving scheme for variable refresh rate

This paper proposes a novel pixel circuit and driving scheme that adopts low-temperature polycrystalline silicon and oxide thin-film transistors (LTPO TFTs) for mobile devices using active-matrix organic light-emitting diode (AMOLED) displays. The proposed pixel circuit and driving scheme provide uniform luminance and render flicker invisible at variable refresh rates (VRRs) from 1 to 120 Hz. Using the proposed pixel circuit with extended compensation time (tCOMP) improves the luminance uniformity at a high-frame rate. The proposed driving scheme applies a voltage to the driving TFTs (D-TFTs) higher than the programmed data voltage during the skip frame. This reduces the flicker caused by the hysteresis of D-TFTs during low-frame rate driving. A 6.0-inch quad high-definition (QHD) LTPO-based AMOLED display was fabricated using the new pixel circuit and driving scheme. Experimental results of the proposed pixel circuit show that the standard deviation of luminance was reduced from 0.056 to 0.008 by extending tCOMP from 2 to 8 µs. The flicker level was −51 dB, so there was no visual artifact during 1 Hz driving. A flicker-free LTPO-based AMOLED display with low power consumption is possible; driving can proceed in 1–120 Hz range.

Ultrasensitive Near-Infrared InAs Colloidal Quantum Dot-ZnON Hybrid Phototransistor Based on a Gradated Band Structure.

Amorphous metal oxide semiconductor phototransistors (MOTPs) integrated with colloidal quantum dots (QDs) (QD-MOTPs) are promising infrared photodetectors owing to their high photoconductive gain, low off-current level, and high compatibility with pixel circuits. However, to date, the poor mobility of conventional MOTPs, such as indium gallium zinc oxide (IGZO), and the toxicity of lead (Pb)-based QDs, such as lead sulfide and lead selenide, has limited the commercial applications of QD-MOTPs. Herein, an ultrasensitive QD-MOTP fabricated by integrating a high-mobility zinc oxynitride (ZnON)-based MOTP and lead-free indium arsenide (InAs) QDs is demonstrated. A new gradated bandgap structure is introduced in the InAs QD layer that absorbs infrared light, which prevents carriers from moving backward and effectively reduces electron-hole recombination. Chemical, optical, and structural analyses confirm the movement of the photoexcited carriers in the graded band structure. The novel QD-MOTP exhibits an outstanding performance with a responsivity of 1.15 × 105 A W-1 and detectivity of 5.32 × 1016 Jones at a light power density of 2 µW cm-2 under illumination at 905nm.

A Pixel Circuit for Compensating Electrical Characteristics Variation and OLED Degradation

In recent years, the active-matrix organic light-emitting diode (AMOLED) displays have been greatly required. A voltage compensation pixel circuit based on an amorphous indium gallium zinc oxide thin-film transistor is presented for AMOLED displays. The circuit is composed of five transistors–two capacitors (5T2C) in combination with an OLED. In the circuit, the threshold voltages of both the transistor and the OLED are extracted simultaneously in the threshold voltage extraction stage, and the mobility-related discharge voltage is generated in the data input stage. The circuit not only can compensate the electrical characteristics variation, i.e., the threshold voltage variation and mobility variation, but also can compensate the OLED degradation. Furthermore, the circuit can prevent the OLED flicker, and can achieve the wide data voltage range. The circuit simulation results show that the OLED current error rates (CERs) are lower than 3.89% when the transistor’s threshold voltage variation is ±0.5V, lower than 3.49% when the mobility variation is ±30%.