Pixel Circuit Research Articles

Double-gate metal–oxide TFT pixel circuit for improved luminance uniformity of mobile OLED display

The small subthreshold swing (SS) of metal–oxide (MOx) thin-film transistors (TFTs) reduces the data voltage (VDAT) range of the organic light-emitting diode (OLED) display pixel circuit. This leads to a large OLED current error when a small change occurs in the gate-to-source voltage (VGS) of the driving TFT in the pixel. Therefore, we propose a new pixel circuit adopting a double-gate TFT structure for the driving TFT, which is mainly driven by the bottom gate with a thicker gate insulator to provide a wide VDAT range. The proposed pixel circuit further expands the VDAT range by employing threshold voltage (VTH) modulation effect depending on the gray level. It also allows for flexible adjustment of the VTH extraction time to reduce OLED current error at low gray levels. SPICE simulation and measured results verify that the proposed pixel circuit reduces the OLED current error rate to less than 10% even with a VTH variation of ±0.5 V or SS variation of ±5% for the current level from 0.1 nA to 30 nA.

Adaptive frequency driving scan driver based on a‐InGaZnO TFTs for extremely low‐power displays

AbstractIn this paper, we propose a novel scan driver combined with a logic circuit using amorphous indium‐gallium‐zinc‐oxide (a‐InGaZnO) thin‐film transistors (TFTs) in order to enhance the electrical stability and reduce power consumption in display panels. The latest mobile displays with high resolution and refresh rates consume more battery power, which inevitably limits portability and usability. The proposed scan driver can mask the signal by combining the output stage with a logic circuit, thereby blocking the output pulse for static images. This allows the display panel to operate in a partial driving mode depending on the display content. Due to the masking of the scan driver's output pulses, the connected pixel circuits are consistently maintained in the same state as the previous frame, leading to a lower refresh rate and reduced power consumption. Furthermore, by constructing additional control signals, the proposed scan driver can operate stably in depletion mode under a ∆VTH = −3 V.

Heterogeneous Integration of Complementary Field-Effect Transistors for High-Performance Micro LED Displays.

This study investigates a micro light-emitting diode (µLED) pixel circuit using the heterogeneous integration of complementary field-effect transistors (CFETs). The CFETs are fabricated using a semiconductor layer composed of tellurium (Te) and indium-gallium-zinc oxide (IGZO) layers. Te and IGZO layers in the heterostructure IGZO/Te film exhibit hexagonal and amorphous phases, respectively, indicating that each layer maintains independent material characteristics. The fabricated IGZO/Te CFETs exhibit ambipolar behavior with a turn-on voltage of 2.0V and field-effect mobility of 0.74 and 1.42 cm2 V-1 s-1 for p-type and n-type channels, respectively. Inverters comprising IGZO/Te CFET and IGZO TFT exhibit inverting behavior. A µLED pixel circuit is designed using IGZO/Te CFETs based on pulse width modulation (PWM). The proposed circuit uses an inverter structure with IGZO/Te and IGZO to control the emission time, suppressing the wavelength shift of µLEDs depending on the µLED current levels. The operation of the proposed pixel circuit is investigated through simulation and measurement of the fabricated circuit. The fabricated µLED pixel circuit successfully exhibits PWM operation, controlling the emission time and luminance. Consequently, IGZO/Te CFETs show promise as devices for high-quality µLED displays.

Research on high dynamic pixel structure based on adaptive integral capacitor

Infrared image sensing technology has attracted wide attention due to its advantages such as being unaffected by the environment, good target recognition, and strong anti-interference ability. However, with the improvement of the integration of infrared focal planes, the constraints between the dynamic range, noise, and full-shade of the optoelectronic system are particularly prominent. Therefore, in order to solve the contradiction between noise in weak light and full-shade capacity in strong light, in a 5 T infrared pixel circuit, the relationship between the capacitance value and voltage of the inverted MOS capacitor in a specific voltage range is used to make the integral capacitance of the infrared image sensor automatically change from 6.5 fF to 37.5 fF. A high dynamic pixel structure based on adaptive integral capacitance is proposed. Based on 55 nm CMOS process technology, 12288 × 12288its performance parameters are studied in a pixel-scale infrared sensor. The research results show that 5.5m × 5.5mthe small-size pixel has a large full-shade capacity of 1.31 Me- and a variable conversion gain, a noise of less than 0.43 e, and a dynamic range of more than 130 dB.

Low-voltage operatable top-gated organic transistors based on the solution-processed binary polymers dielectric for simplifying the manufacturing flow of arrayed AMOLED

The complex process flow is an important factor that hinders the development of active-matrix organic light-emitting diode (AMOLED) displays. Organic thin-film transistors (OTFTs) are one of the promising candidates as the pixel circuits for AMOLEDs. Both the architecture and the fabrication of OTFTs are crucial elements to determine the process flow and cost of the AMOLEDs. In this Letter, we develop a strategy to significantly simplify the process flow and reduce the cost of AMOLEDs by constructing top-gated OTFTs with a solution-processed vertically phase separated binary polymer dielectric as the pixel circuits. The design on the OTFTs considers both the process flow and the device performance in terms of mobility and operating voltages. The mechanism to improve device performances is discussed. Finally, a 3 × 4 arrayed AMOLED is demonstrated, in which a high mobility over 0.3 cm2/Vs is obtained on the switching and driving OTFTs, and luminance over 300 cd/m2 is achieved on the OLEDs at the supplied low operating voltages of 10 V. This strategy provides a competitive technological route for the manufacturing of AMOLEDs.

Design and fabrication of polarization independent LCoS phase modulators with polymer waveplate and analog driving

Abstract Phase-only liquid crystal on silicon (LCoS) spatial light modulators are used in a variety of applications. Despite their wide use, current phase-only LCoS devices are hindered by their single-polarization modulation. We propose a polarization independent LCoS device which uses a polymer quarter-wave plate for polarization conversion. We designed a custom pixel circuit which enables high driving voltage and high pixel grayscale by utilizing an analog driving frame buffer pixel circuit. Our pixel circuit is optimized for small pixel size and the silicon backplanes can be fabricated at a traditional foundry. We demonstrate polarization independent phase modulation through optical beam steering tests for various input polarizations. Our fabricated device lays the foundation for commercial LCoS devices highly suitable for wavelength selective switches, holographic display, wavefront correction, and optical beam shaping.

A new gate driver employing linearly sweep signal generated module for high‐resolution micro‐LED display with line‐by‐line pulse width modulation driving method

AbstractA new gate driver with metal oxide thin‐film transistors (MO‐TFTs) has been presented for micro‐light‐emitting diode (Micro‐LED) display. The proposed gate driver employing linearly decreasing sweep signal generated module could realize the line‐by‐line pulse width modulation (PWM) driving method, which is valuable for application in high‐resolution Micro‐LED display compared with the conventional driving method employing global sweep signal. A simple Micro‐LED pixel circuit including an inverter module driven by the analog PWM method is employed to demonstrate the feasibility of the proposed gate driver. A 10 × 10 green Micro‐LED display array has been fabricated with MO‐TFTs backplane technology. It is shown that a 10‐μs width pulse and linearly scanning sweep signal can be successfully output. Consequently, the pixel circuit array can be driven properly under analog line‐by‐line PWM driving method.

Pixel Circuit Design for X-ray Detection Utilizing a-IGZO Thin-Film Transistors

Abstract In recent advancements, X-ray detectors have made significant progress. Active pixel sensors exhibit superior Signal-to-Noise Ratio (SNR) compared to passive counterparts. The conventional pixel circuit for X-ray detectors has three transistors and one capacitor. We focused on simplifying this pixel to enhance resolution. Our research introduces and verifies a novel pixel circuit developed for high-resolution X-ray detectors. The proposed pixel circuit is composed by two n-type thin-film transistors and one capacitor. It requires two scan pulses and three operational stages. This structure can effectively reduce the component area of the pixel circuit. We designed RPI LEVEL = 35 amorphous TFT model and simulated for verify our proposed pixel circuit. In results, proposed pixel circuit successfully operated and shows 2.5 μV when the equivalent resistance of detector (RDetector) is 10 MΩ, and 8.38 V when RDetector is 1 MΩ. To minimize the harmful effects of X-ray exposure, reducing the dosage is essential. Sensitivity and low noise are crucial factors, and the proposed circuit offers a compact design, increased sensitivity, and higher output voltage.

An improved pixel circuit with low noise event rate and enhanced bright‐light sensitivity for dynamic vision sensor

AbstractDynamic vision sensor (DVS) imaging quality is significantly affected by pixel noise and temporal contrast (TC), which is inversely proportional to sensitivity. To reduce the noise event rate and improve sensitivity in bright‐light conditions in the DVS pixel circuit, this paper proposes improvements to the conventional DVS pixel circuit. The proposed DVS pixel circuit adopts stacked medium‐threshold transistors instead of a single high‐threshold transistor in the photoreceptor and introduces a threshold switching circuit. Compared with the conventional DVS pixel circuit, this design increases event threshold normalized by root mean square (RMS) noise voltage, reducing the dim‐light noise bandwidth. Additionally, it achieves higher sensitivity in bright‐light conditions compared with dim‐light conditions. The proposed DVS pixel circuit is implemented in a 110‐nm complementary metal‐oxide semiconductor (CMOS) process. Post‐simulation results show that, for photocurrents between 5 fA and 100 pA, the proposed DVS pixel circuit achieves a 35 Hz peak event rate at 15% TC, which is reduced to 3.1% of the conventional structure. For photocurrents exceeding 30 pA, the proposed structure can switch TC from 15% to 5%, maintaining a noise event rate below 0.1 Hz.

Unlocking Neuromorphic Vision: Advancements in IGZO-Based Optoelectronic Memristors with Visible Range Sensitivity.

Optoelectronic memristors based on amorphous oxide semiconductors (AOSs) are promising devices for the development of spiking neural network (SNN) hardware in neuromorphic vision sensors. In such devices, the conductance state can be controlled by both optical and electrical stimuli, while the typical persistent photoconductivity (PPC) of AOS materials can be used to emulate synaptic functions. However, due to the large band gap of these materials, sensitivity to visible light (red/green/blue) is difficult to accomplish, which hinders applications requiring color discrimination. In this work, we report a 4 μm2 hydrogen-doped (H-doped) indium-gallium-zinc oxide (IGZO) optoelectronic memristor that emulates all of the important rules of SNNs such as short- to long-term memory transition (STM-LTM), paired-pulse facilitation (PPF), spike-time-dependent plasticity (STDP), and learning and forgetting capabilities. By the incorporation of hydrogen gas in the sputtering deposition of IGZO, visible sensitivity was achieved for green and blue wavelengths. Additionally, extremely high light/dark ratios of 179, 93, and 12 are demonstrated for wavelengths of 365, 405, and 505 nm, respectively, due to hydrogen-induced subgap states and device miniaturization. Therefore, the proposed device shows remarkable potential for integration with the pixel circuits of IGZO-based displays with extreme resolution for a true intelligent self-processing display.

Reduction of short-time image sticking in organic light-emitting diode display through transient analysis of low-temperature polycrystalline silicon thin-film transistor

Accurate compensation operation of low-temperature polycrystalline-silicon (LTPS) thin-film transistor (TFT) in pixel circuits is crucial to achieve steady and uniform luminance in organic light-emitting diode (OLED) display panels. However, the device characteristics fluctuate over time due to various traps in the LTPS thin film transistor and at the interface with the gate insulator, resulting in abnormal phenomena such as short-time image sticking and luminance fluctuation, which degrade display quality during image change. Considering these phenomena, transient analysis was conducted through device simulation to optimize the pixel compensation circuit. In particular, we analyzed the behavior of traps within LTPS TFT in correlation with compensation circuit operation, and based on this, proposed a methodology for designing a reset voltage scheme for the driver TFT to reduce the image sticking phenomenon.

A 64 × 128 3D-Stacked SPAD Image Sensor for Low-Light Imaging.

Low-light imaging capabilities are in urgent demand in many fields, such as security surveillance, night-time autonomous driving, wilderness rescue, and environmental monitoring. The excellent performance of SPAD devices gives them significant potential for applications in low-light imaging. This article presents a 64 (rows) × 128 (columns) SPAD image sensor designed for low-light imaging. The chip utilizes a three-dimensional stacking architecture and microlens technology, combined with compact gated pixel circuits designed with thick-gate MOS transistors, which further enhance the SPAD's photosensitivity. The configurable digital control circuit allows for the adjustment of exposure time, enabling the sensor to adapt to different lighting conditions. The chip exhibits very low dark noise levels, with an average DCR of 41.5 cps at 2.4 V excess bias voltage. Additionally, it employs a denoising algorithm specifically developed for the SPAD image sensor, achieving two-dimensional grayscale imaging under 6 × 10-4 lux illumination conditions, demonstrating excellent low-light imaging capabilities. The chip designed in this paper fully leverages the performance advantages of SPAD image sensors and holds promise for applications in various fields requiring low-light imaging capabilities.

Ultrathin Niobium‐Doped Indium Oxide Active Layer Enables High‐Performance Phototransistors for Driving Quantum‐Dot Light‐Emitting Diodes

AbstractActive materials play a crucial role in the performance of phototransistors. However, the discovery of a novel and versatile active material is a big challenge. For the first time, phototransistors with ultrathin niobium‐doped indium oxide (InNbO) active layer are fabricated. The InNbO phototransistors without additional light‐absorbing layers exhibit the performance with a high average mobility of 22.86 cm2 V−1s−1, a turn‐on voltage of −0.75 V, a low sub threshold swing of 0.18 V/decade, and a high on/off current ratio of 5.74 × 108. Detailed studies show that Nb is the key to suppress the free carrier generation due to the strong bonding strength of Nb─O. In addition, the InNbO phototransistors exhibit a very broad spectral responsivity with a photocurrent of 4.72 × 10−4 A, a photosensitivity of 1.69 × 108, and a high detectivity of 3.33 × 1013 Jones under violet (405 nm) light illumination, which is significantly higher than that of the IGZO phototransistors. Furthermore, an active‐matrix quantum‐dot light‐emitting diode pixel circuit based on InNbO phototransistors is demonstrated. The findings not only indicate that InNbO is a new active material for phototransistors, but also suggest that InNbO‐based phototransistors have a great potential for the next‐generation interactive display technology.

79‐3: Novel Mini‐LED Pixel Circuit with PWM Driving Method for Decreasing Power Consumption

This article proposes a mini‐LED pixel circuit that employs the PWM driving method to decrease power consumption. To achieve uniform current, the circuit compensates for threshold voltage variations of TFTs and VDD I‐R drops. Simulation results show that power consumption is ameliorated over 10.53% more than the 6T1C circuit.

63‐3: An Enhanced Micro‐LED Pixel Circuit: Achieving Low Error Rates through Stable Current Generation with LTPO Technology

This paper proposed a micro‐LED pixel circuit utilizing low‐temperature polycrystalline oxide (LTPO) technology. Unlike conventional designs, our approach leverages coupling effects to nullify threshold voltages in both segments without signal line augmentation. A more stable emission current is achieved through altered emission methods, yielding an error rate below 4%.

38‐2: Multimodal Transistor‐Based 7T2C LTPS Pixel Circuit for Simultaneous PAM and PWM Control in μLED Display

We propose a 7T2C micro‐LED pixel circuit which uses a multimodal transistor (MMT) as driver for compact pulseamplitude and pulse‐width modulation (PAM + PWM) functionality. The LTPS circuit exploits the MMT's ability to maintain a constant drain current, controlled by its source gate, while the channel gate potential varies without interfering with the current magnitude. The MMT also affords a compact footprint and tolerance to large variations in drain‐source voltage caused by LED operating conditions or power supply I × R drops.

Ultra-Efficient Low-Power Retinal Nano Electronic Circuit for Edge Enhancement and Detection Using 7 nm FinFET Technology

Ultra-Efficient Low-Power Retinal Nano Electronic Circuit for Edge Enhancement and Detection Using 7 nm FinFET Technology

P‐68: Oxide Semiconductor Thin‐Film Transistor‐Based Micro‐LED Pixel Circuit with External Current Setting System

We propose an oxide semiconductor thin‐film transistor‐based micro light‐emitting diode pixel circuit with external current setting system. The proposed circuit achieved stable operation using pulse width modulation with maximum error rates 1.2 % and 3.2 % under threshold voltage variations and mobility variations, respectively.

P‐149: The Optical Taste of MLED Display: From the Mini‐/Micro‐LED Optical Property to the Design of Pixel Circuit

Mini‐LED and Micro‐LED have ultrahigh luminance and excellent color saturation to be used in MLED display based on COG (Chip on Class). In this paper, two pitch‐0.6 mm MLED displays with pulse hybrid modulation (PHM) and pulse amplitude modulation (PAM) driving Mode separately is proposed. And we discuss the influences of the intrinsic optical characteristics of Mini‐LED and Micro‐LED on the design of MLED display pixel circuit. In particular, surface brightness distributions of different gray scale are tested to theoretically analyze the pocking phenomenon in different ways.

38‐1: Invited Paper: Micro Light‐Emitting Diode Pixel Circuit and Driving Method Considering Wavelength Shift

We proposed micro light‐emitting diode (μLED) pixel circuits and driving methods. Because the wavelength shift of μLEDs occurs as the current density is varied, we designed pixel circuits based on pulse width modulation (PWM), hybrid PWM and pulse amplitude modulation (PAM), and quaternary digital driving.