Pixel Circuit Research Articles

4670‐PPI OLEDoS pixel circuit design for wide data voltage range in a 5 V 0.13 μm CMOS process

AbstractOLEDoS displays hold great promise for AR/VR applications, but the ultra‐high pixel density required for microdisplays, exceeding several thousand PPI, presents significant challenges for pixel circuit design. Above all, the maximum OLED current of each OLEDoS sub‐pixel is extremely low, on the order of several nanoamperes or less, resulting in a data voltage range narrower than a few hundred millivolts. Consequently, for example, when expressing 10‐bit grayscale, designing data driver circuits to divide the data voltage range into 1024 levels becomes exceedingly challenging. Furthermore, circuit techniques that compensate for variations in transistor characteristics are nearly inapplicable due to the ultra‐small pixel area. This paper proposes a pixel circuit design method to expand the data voltage range for a 4670‐PPI OLEDoS display while considering current uniformity. The effectiveness of the proposed pixel circuit was verified through measurements on 40 × 120 pixel arrays fabricated in a 5 V 0.13 μm CMOS process.

Cd0.2Zn0.8O nanowire thin film transistor for low kickback high-speed AMLCD circuit applications

This paper reports the Cd0.2Zn0.8O channel cylindrical gate-all-around nanowire thin-film transistor (CY-GAA-NWTFT) for high-speed active-matrix liquid crystal display (AMLCD) pixel circuit with minimal kickback voltage. The AC/DC, analog, and energy efficiency of this device are investigated against the variation in lateral (channel length) as well as vertical dimension (oxide thickness) using technology computer aided design (TCAD) device simulation environment. Further, the behavior of CY-GAA-TFT is crosschecked by implementing it on resistive load digital inverter circuit followed by transient analysis to calculate delay of the inverter circuit. Moreover, static analysis has been also performed to evaluate its voltage transfer curve (VTC) to derive its noise margin and voltage gain. The execution of the proposed CY-GAA-NWT device on AMLCD pixel circuit indicates tremendous improvement in performance in terms of high-speed applications and low kickback voltage.

An analog PWM pixel circuit with shaping function for low grayscale display of Micro-LEDs

This paper presents a new analog pulse width modulation (PWM) micro light-emitting diode (LED) pixel circuit with a shaping function. The rising/falling time of PWM signal generated by the proposed pixel circuit can be greatly reduced to improve the low grayscale display quality of micro-LED. Metal oxide thin film transistors with the top-gate (TG) coplanar structure are used to realize the proposed pixel circuit on the glass substrate. It is shown that the pulse width of micro-LED current can be linearly modulated by the data voltage. Furthermore, the measured rising time and falling time of the PWM signal are 15.6 μs and 25.5 μs, respectively, which are much smaller than that of the PWM signal in the conventional analog PWM pixel circuits.

Integrated 2T1C pixel circuit with a-Si TFT and NMOS for active matrix mini-LED displays

The 2T1C pixel driver circuit for mini-LED direct display has been proposed, which separates the switching transistor and the driver transistor from the same display substrate, replaces the driver transistor with n-metal oxide semiconductor (NMOS), and combines printed circuit board substrate and thin-film transistor (TFT) substrate to improve the driving capability of the circuit. The NMOS was soldered with mini-LEDs simultaneously onto a substrate which connects to the a-Si TFT array. Two driving modes for a 32-level gray-scale display panel were investigated to compare the voltage-current and optical characteristics. The results demonstrated that the drain-driving mode is better suited for high brightness and high-power display application scenarios as it supports higher-driven currents, but the source-driving mode is more appropriate for precision gray-scale applications due to the higher current linearity of the mode.

3D-integrated pixel circuit for a low power and small pitch SOI sensor

Targeting on low power consumption and high spatial resolution, the CPV-4 SOI pixel sensor has been developed. Its pixel circuit requires about 120 transistors to implement the analog-digital mixed signal processing functionality within a compact pixel area of 17 μm × 21 μm. By utilizing 3D vertical integration, sensing diode and analog front-end are realized in the lower-tier chip, while hit information storage and sparse readout are achieved in the upper-tier chip, thereby minimizing its pixel size and power consumption. This work presents the pixel circuit design and the test results on the completed 3D chips. The feature of SOI pixel process and the 3D integration are also highlighted in this article.

Numerical modeling of defects induced dark current in halide perovskite X-ray detectors

Metal halide perovskites have been widely used in x-ray detection due to their outstanding optoelectronic properties. However, the dark current of perovskite x-ray detectors is not appreciably low for integration on thin-film transistors pixel circuits and thus limits their applications in X-ray imaging. Based on numerical models, we investigate the correlation between the dark current and defects of perovskite x-ray detectors. The deep-level defects are the major factor to induce dark current, which has a proportional relation to the defect density. Compared to deep-level defects, the dark current induced by shallow-level defects depends on both of defect energy level and defect density. At last, simulation results present a guidance to engineer defects with suitable values of density and energy level, which yields desirably low dark current. This work provides implications and theoretical guidance for the optimization of defects in halide perovskites, which is believed to assist the further development of x-ray detectors with a low dark current density.

A 6T1C pixel circuit compensating for TFT electrical characteristics variations, voltage drop, and OLED degradation

A voltage-programmed pixel circuit based on low-temperature poly-silicon (LTPS) thin-film transistors (TFTs) is presented for active-matrix organic light-emitting diode (AMOLED) displays. The circuit consists of six p-type transistors and one capacitor (6T1C). During the extraction stage, the circuit extracts the driving TFT’s threshold voltage and the power supply voltage. During the data input stage, the circuit generates a charging voltage, which is associated with the driving TFT’s mobility and the OLED’s threshold voltage. Consequently, the pixel circuit, using only one capacitor, compensates not only for the TFT’s electrical characteristics variations, i.e., the threshold voltage variation and mobility variation, but also for the power supply voltage drop and the OLED degradation. The circuit simulation results reveal that the OLED current error rates (CERs) are lower than 5.09% when the threshold voltage varies by ±0.5V, lower than 3.61% when the mobility varies by ±30%, and lower than 9.16% when the voltage drop varies by −0.5V in the data voltage range.

A New Pixel Circuit for Micro-Light Emitting Diode Displays With Pulse Hybrid Modulation Driving and Compensation

A New Pixel Circuit for Micro-Light Emitting Diode Displays With Pulse Hybrid Modulation Driving and Compensation

A New 13T4C LTPO MicroLED Pixel Circuit Producing Highly Stable Driving Current by Minimizing Effect of Parasitic Capacitors and Stabilizing Capacitor Nodes

A New 13T4C LTPO MicroLED Pixel Circuit Producing Highly Stable Driving Current by Minimizing Effect of Parasitic Capacitors and Stabilizing Capacitor Nodes

High-performance thin-film transistors based on aligned carbon nanotubes for mini- and micro-LED displays

Inorganic mini-LEDs (mLEDs) and micro-LEDs (μLEDs) have ultrahigh luminance and long lifetimes, and are challenging liquid crystal displays (LCDs) and organic light-emitting diode (OLED) displays for next-generation displays. Backplane thin-film transistors (TFTs) for mLED/μLED displays with high mobility and large driving current are under development as currently widely used backplane active materials such as crystalline silicon (CMOS Si), amorphous silicon (a-Si), low-temperature polycrystalline silicon (LTPS), and metal oxides are facing difficulties to meet the requirements. Semiconducting carbon nanotubes (s-CNTs) have long been considered as promising materials for TFTs because of their unique electronic properties. However, TFTs based on s-CNT random networks show inadequate performance due to large amounts of inter-tube junctions. Here, we report high performance TFTs based on wafer-scale, high-density aligned s-CNT array (A-CNT) for mLED/μLED driving backplanes. These A-CNT TFTs with micrometer scale channel length were fabricated using standard semiconductor manufacturing process, and exhibited mobility as high as 160 cm2/Vs and driving current as large as 417 μA/μm at operation voltage of ∼3 V, exceeding commercially available TFTs with similar device sizes. In addition, these A-CNT TFTs have demonstrated good driving capability for commercial mLEDs in a one-transistor-one-diode (1T1D) pixel circuit configuration, and been capable of driving commercially available mLED/μLED displays. Moreover, a prototype display with 2 × 2 mLED array have been demonstrated with full control of each pixel, manifesting the great potential for A-CNT TFT technology for backplane of mLED/μLED displays.

A Mini-LED Pixel Circuit Achieving Precise Control Capability of Emission Time with PWM Driving Method

A Mini-LED Pixel Circuit Achieving Precise Control Capability of Emission Time with PWM Driving Method

An LTPO-based AMOLED Pixel Circuit with V<sub>TH</sub> Compensation and Leakage Current Prevention for Use in Smartwatch Displays

An LTPO-based AMOLED Pixel Circuit with V<sub>TH</sub> Compensation and Leakage Current Prevention for Use in Smartwatch Displays

High Dynamic Pixel Structure Based on an Adaptive Integrating Capacitor.

Infrared image sensing technology has received widespread attention due to its advantages of not being affected by the environment, good target recognition, and high anti-interference ability. However, with the improvement of the integration of the infrared focal plane, the dynamic range of the photoelectric system is difficult to improve, that is, the restrictive trade-off between noise and full well capacity is particularly prominent. Since the capacitance of the inversion MOS capacitor changes with the gate-source voltage adaptively, the inversion MOS capacitor is used as the capacitor in the infrared pixel circuit, which can solve the contradiction between noise in low light and full well capacity in high light. To this end, a highly dynamic pixel structure based on adaptive capacitance is proposed, so that the capacitance of the infrared image sensor can automatically change from 6.5 fF to 37.5 fF as the light intensity increases. And based on 55 nm CMOS process technology, the performance parameters of an infrared image sensor with a 12,288 × 12,288 pixel array are studied. The research results show that a small-size pixel of 5.5 µm × 5.5 µm has a large full well capacity of 1.31 Me- and a variable conversion gain, with a noise of less than 0.43 e- and a dynamic range of more than 130 dB.

Nonvolatile Memory Organic Light-Emitting Transistors.

In the field of active-matrix organic light emitting display (AMOLED), large-size and ultra-high-definition AMOLED applications have escalated the demand for the integration density of driver chips. However, as Moore's Law approaches the limit, the traditional technology of improving integration density that relies on scaling down device dimension is facing a huge challenge. Thus, developing a multifunctional and highly integrated device is a promising route for improving the integration density of pixel circuits. Here, a novel nonvolatile memory ferroelectric organic light-emitting transistor (Fe-OLET) device which integrates the switching capability, light-emitting capability and nonvolatile memory function into a single device is reported. The nonvolatile memory function of Fe-OLET is achieved through the remnant polarization property of ferroelectric polymer, enabling the device to maintain light emission at zero gate bias. The reliable nonvolatile memory operations are also demonstrated. The proof-of-concept device optimized through interfacial modification approach exhibits 20 times improved field-effect mobility and five times increased luminance. The integration of nonvolatile memory, switching and light-emitting capabilities within Fe-OLET provides a promising internal-storage-driving paradigm, thus creating a new pathway for deploying storage capacitor-free circuitry to improve the pixel aperture ratio and the integration density of circuits toward the on-chip advanced display applications.

The Design of a Low-Noise, High-Speed Readout-Integrated Circuit for Infrared Focal Plane Arrays

This paper describes the design of a low-noise, high-speed readout-integrated circuit for use in InGaAs infrared focal plane arrays, and analyzes the working principle and noise index of the pixel circuit in detail. The design fully considers the dynamic range, noise, and power consumption of the pixel circuit in which a capacitance transimpedance amplifier structure is adopted as the input stage circuit, and chip fabrication via an XFAB 0.18 µm CMOS process is successfully realized. The ROIC adopts monolithic integration and implements various functions, such as windowing, subsampling, and different integration and readout modes. The ROIC reached an array scale of 32 × 32, a frame rate of 100 Hz, and a readout rate of 20 Mbps with an analog power consumption of less than 52 mW. The measurement results show that the input reference noise can be reduced to 143 e- via the CDS, and the fully customized scheme has certain advantages in the research of high-performance ROICs.

Biaxially Stretchable Active‐Matrix Micro‐LED Display with Liquid Metal Interconnects

AbstractA stretchable active‐matrix (AM) display is a next‐generation display that breaks away from displaying limited information on flat and curved screens. However, the implementation of stretchable displays requires much more complex fabrication processes and meticulous efforts compared to traditional flat/curved displays. Here, the biaxially stretchable AM micro‐light emitting diode (LED) display on polydimethylsiloxane (PDMS) with liquid metal (LM) interconnects is reported. The pixel islands consisting of oxide TFTs and micro‐LEDs, which are fabricated on the polyimide (PI) substrate, are formed through green laser cutting and transfer to the PDMS substrate. LM interconnects integrate on the pixel island and PDMS by a lift‐off process. The oxide TFTs and micro‐LEDs comprising the 2T1C (2 TFT + 1 capacitor) pixel circuit on the PI island, along with LM interconnects, demonstrate stable electrical characteristics even under a relaxed state as well as when biaxially stretched up to 24%. For 24% stretching of display pixels with the same ratio of PI island and LM interconnects, the LM interconnects operate at 48% stretching. The combination of pixel island array and LM interconnects successfully demonstrates the implementation of a two‐axis stretchable display capable of displaying characters of “A”, “D”, “R”, and “C” through AM driving.

An Active-Matrix Organic Light-Emitting Diode Pixel Circuit Featuring Mobility Compensation for Portable Applications.

A 6T1C pixel circuit based on low-temperature polycrystalline oxide (LTPO) technology for portable active-matrix organic light-emitting diode (AMOLED) display applications is proposed in this paper. For superior high-end portable applications including 4K high resolution and high PPI (pixels per inch), the proposed pixel circuit employs a single storage capacitor and signal sharing switch-control design and provides low-voltage driving and immunity to the IR-drop issue and OLED degradation. Furthermore, the threshold voltage and mobility-compensating capabilities are improved by both compensation mechanisms, which are based on a negative feedback system, and mobility-related compensation parameters. Simulation results reveal that threshold voltage variations of ±0.33 V in the driving thin-film transistors can be well sensed and compensated while the maximum OLED current shift is 4.25%. The maximum variation in OLED currents within all gray levels is only 1.05% with mobility variations of ±30%. As a result, the proposed 6T1C pixel circuit is a good candidate for portable AMOLED display usage.

Reducing Leakage Current Using LTPS-TFT Pixel Circuit in AMOLED Smartwatch Displays

This work proposes a new pixel circuit that is operated at a low frame rate using low-temperature polycrystalline silicon thin-film transistors (LTPS TFTs) for use in active-matrix organic light-emitting diode (AMOLED) smartwatch displays. The proposed circuit uses a new leakage-prevention method (LPM) to balance the off currents at the gate node of the driving TFTs, eliminating the drop of the driving voltage. Uniform OLED currents are thus generated throughout the extended emission period. Also, the OLEDs are all turned off except in the emission stage to suppress image flicker. To confirm the feasibility of the proposed pixel circuit, a 1.41-inch panel, based on the proposed circuit, is fabricated. Experimental results demonstrate that diminishing the off currents maintains the driving capability of the pixel circuit to stabilize the luminance of the fabricated panel. The exhibitions of red, green, blue, and white images at frame rates from 60 Hz to 15 Hz are uniform, verifying that the proposed pixel circuit with LPM has high potential for use in AMOLED smartwatch displays with a low frame rate.

Pressure Mapping With Piezoresistive Sensor on Oxide TFTs Backplane

We report a pixel circuit with a piezoresistive sensor using amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistor (TFT) backplane for large-area pressure mapping. The proposed pixel circuit consists of TFTs, piezoresistive sensor, and a capacitor. The pressure sensor shows a large resistance change between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> ohm. The RC charging time to the capacitance can be modulated by pressure-dependent resistance within pre-charging period for sensing. The pressure sensor array has a resolution of 0.625 mm, rising and falling times of 40 μs and 10 μs, respectively. The output voltage is between 0.15 V (zero pressure) and 1 V (2 kPa) without an external amplifier. We demonstrate the pressure mapping over 32 x 32 active-matrix pressure sensor array.

14‐4: Efficient Multi‐stage Bayesian Optimization for Optimal Display Circuit Input Signal Design

To correct for threshold variations in the driving transistor, pixel circuits rely on a complex sequence of inputs that drive the circuit through programming and emission phase. In this work we propose a multi‐stage optimization strategy to automate the design of input signals for arbitrary pixel circuits. By solving for the operation phases of pixel circuits separately and in reverse order our algorithm efficiently finds the narrow region of input signal designs that achieve the desired functionality in the large search space. We demonstrate the generality of our algorithm on two common pixel circuits, i.e., 3T1C and 7T1C.