Threshold Voltage Compensation Research Articles

Driving scheme for residual image reduction in active-matrix organic light-emitting diodes display

Residual image is a frequent issue in active-matrix organic light-emitting diode (AMOLED) display, due to hysteresis effects of the internal devices. Up to now, some optimized methods have been researched mainly from process perspective. However, the approaches from driving scheme perspective and their electrical mechanism have rarely been demonstrated systematically. In this work, a technical proposal has been proposed to separate the influences of different devices including both thin film transistors (TFTs) and OLEDs furtherly. It was found that the current curve gap was nearly equal to the luminance curve gap and the main factor of residual image was shown to be the hysteresis of TFTs. An equivalent circuit of two thin film transistors and one capacitor (2T1C) was also adopted to substantiate the influences of threshold voltage (Vth) compensation for reducing residual image, and a series of experiments of tuning capacitance were carried out for corresponding compensation improvements. Moreover, other driving scheme optimizations were also studied to reduce residual image furtherly, including increasing the data input time and resetting the driving TFTs. The research opens the possibility of considering reduction of residual image from driving scheme perspective and more systemic analysis from the internal electrical mechanism in AMOLED display.

63‐2: Metal Oxide Thin‐Film Transistors‐Based Pixel Circuit with Progressive Emission Using Pulse Width Modulation for Micro Light‐Emitting Diode Displays

We proposed a metal oxide thin‐film transistor (TFT‐based μLED pixel circuit. Pulse width modulation (PWM) and progressive emission were applied to the circuit. The threshold voltage (VTH) compensation and data addressing were operated using a series capacitor structure connected through switching TFTs. We proved stable operations for the proposed circuit.

Lifetime estimation of thin-film transistors in organic emitting diode display panels with compensation

Oxide semiconductor thin-film transistors (TFTs) are used in the pixel array and gate driver circuits of organic light emitting diode (OLED) display panels. Long-term reliability characteristics of the TFTs are a barometer of the lifetime of OLED display panels. The long-term reliability of the driver TFTs is evaluated in a short time under high voltages and high temperature for an accelerated degradation test. If reliability parameters from the power law or stretched-exponential functions are the same for individual devices and devices in an operating panel, the lifetime of the panel can be accurately estimated. However, since compensation circuits are designed into operating panels, an environmental discrepancy exists between the accelerated test of single devices and the operation of devices in the panel. Herein, we propose a novel compensation stretched-exponential function (CSEF) model which captures the effect of the threshold voltage compensation circuit in the panel. The CSEF model not only bridges the discrepancy between individual devices and panel devices, but also provides a method to accurately and efficiently estimate the long-term lifetime of all display panels that utilize compensation circuits.

38.2: An Accurate and Stable PWM Pixel Circuit Based on LTPO TFTs for AM‐Micro‐LED Display

A pulse-width-modulation (PWM) pixel circuit for Micro-LED displays is proposed based on low-temperature poly-silicon and oxide (LTPO) thin-film transistors (TFTs). The pixel circuit features fast PWM transition, as the PWM falling-time is less than 20 μs thanks to the high-gain dynamic inverter. Furthermore, the proposed pixel circuit can compensate the degradation and non- uniformities of TFT characteristics of LTPO TFTs through auto- zeroing configuration of the dynamic inverter and a diode- connected threshold voltage compensation structure. The error rate of luminous time is less than 3.2% and the error rate of current is less than 1% under VTH shift of oxide TFT ±2 V or LTPS TFT ±1 V.

180 nm FLASH ANALOG TO DIGITAL CONVERTOR

In this paper, Threshold Inverter Quantizer (TIQ) is a novel idea which can effectively replace the reference voltage generator, resistive/capacitive voltage divider network and array of differential comparators in a conventional flash Analog to Digital Converter (ADC) by an internal reference comparator array constructed of Complementary Metal Oxide Semiconductor (CMOS) inverters. The inverter threshold voltage serving as the reference voltage, this architecture claims large improvements in terms of silicon area, power consumption and operating speed. Perturbations due to sensitivity of the inverter threshold voltage to operating temperature/process variations pose impediments in such ADC designs and strongly demand a compensation scheme to such variations. This paper presents a TIQ based flash ADC, with inverter threshold voltage compensation. In this project 5 bit flash adc using TIQ Comparator is designed and simulated using TANNER EDA in 180nm technology. Key Words: flash, adc, tiq(threshold inverter quantization)

Micro LED Display Pixel Circuit Using Amorphous Indium-Gallium-Zinc Oxide Thin-Film Transistor

Micro LED (μ-LED) refers to a micro-light emitting diode with a chip size of 5 to 100 μm. Since micro-LED displays use individual LED chips as a light source of the pixel, they do not need backlight and color filters, which are suitable for implementing flexible displays. In addition, micro-led displays have many advantages over OLED displays, such as excellent color gamut, power consumption, lifetime, and response time. With these advantages, micro-LED displays are attracting attention as next-generation displays, and accordingly, interest in micro-LED driving technology is increasing. While existing OLED displays express grayscale by controlling the amount of current flowing through the light-emitting device, micro-LED can have a problem of color change depending on the grayscale because the light-emitting wavelength shifts when the amount of current changes. Therefore, in order to express a certain color, it is necessary to express the grayscale by adjusting the emission time at the same current density. That is, it is more suitable to use the PWM(pulse width modulation) for each pixel than to use the PAM(pulse amplitude modulation) for the grayscale representation of the OLED display. Therefore, unlike OLED, the micro-LED display requires a new pixel circuit because it must use a PWM driving. In addition, unlike LCDs, micro-led displays are sensitive to changes in electrical characteristics of TFTs depending on the location of each pixel or external stress. Thus, threshold voltage compensation is essential because changes and degradation of the characteristics of the thin film transistor backplane, such as threshold voltage shift, cause non uniform brighteness. Therefore, we propose a PWM compensation pixel circuit based on a-IGZO TFT that can be fabricated on a flexible substrate and has a lower process cost than LTPS and greater mobility than a-Si TFT.The operation of proposed pixel is divided into (1) reset 1, (2) PWM data compensation, (3) reset 2, (4) CCG (constant current generation)compensation, and (5) emission. The circuit is divided into a CCG unit that generates a constant current and a PWM unit that adjusts emission time by a ramping signal. Figure 1 (a) shows the output current simulation result according to PWM data voltage variation. When the PWM data voltages vary from -1 to ~ - 5V, the currents were constant at 70.9 μA, and the smaller the PWM data voltage, the longer the emission time was. In addition, Figure 1 (b) shows the effect of the threshold voltage compensation. The current and emission time are constant for various threshold voltages such as 1, 2, and 3 V. Figure 1

Real-time threshold voltage compensation on dual-gate electrolyte-gated organic field-effect transistors

Electrolyte-Gated Organic Field-Effect Transistors (EGOFETs) offer many opportunities for the development of low-cost and low-power electronics suitable for applications like sensors and point-of-care tests; however, EGOFETs can be affected by the drift of their operative point that causes signals distortion and loss of information during sensing applications. Here, a blend of 2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) and polystyrene (PS) is used as the active material for the fabrication of dual-gate EGOFETs. We exploited the dual-gate architecture to improve EGOFETs stability by implementing digital feedback that uses the back-gate electrode to compensate dynamically for the transistor threshold voltage allowing us to fix its operative point for prolonged tests (>10 h) with different aqueous solutions (Milli-Q water, NaCl 0.1 M and a physiological solution). The presented real-time threshold voltage compensation does not only allow to steady EGOFETs DC output current, but it also preserves EGOFETs sensing capability for the detection of signals with frequencies as low as 1 Hz. • Design and characterization of a dual-gate EGOFET based on diF-TES-ADT:PS: extraction of the top capacitance. • Real-time compensation of the threshold voltage on a dual-gated EGOFET. • Real-time monitoring of action potentials (frequency 1 Hz) applied to the top gate.

P‐32: Investigation on Mechanism of Illumination Mura in AMOLED Display with LTPS‐TFT Backplane after Long Term Localized Illumination

Low‐temperature polycrystalline silicon (LTPS) thin film transistors have been widely used in AMOLED display applications due to the high mobility and the capability of realizing peripheral and active matrix circuits on glass. Though LTPS TFTs exhibited relatively good electrical performance and stability, illumination Mura on AMOLED display with LTPS backplane could be observed by naked eye after several hours’ illumination on localized display area. The Mura boundary separates the areas with and without illumination when the AMOLED display is lighted on without pattern. The electrical instability of LTPS TFT was proved to be the main underlying cause. The DTFT threshold voltage compensation effect and SS variation in 7T1C pixel circuit has certain relationship with the illumination Mura detail.

Improvement of threshold voltage compensation of LTPS AMOLED pixel circuit

Improvement of threshold voltage compensation of LTPS AMOLED pixel circuit

Submicrowatt CMOS Rectifier for a Fully Passive Wake-Up Receiver

This work presents a wide-range, submicrowatt CMOS rectifier, which is designed to efficiently work at input power levels down to −31 dBm. The designed rectifier uses a novel threshold voltage compensation design, namely, the design of a custom-built single-stage rectifier that supplies a nanowatt current reference circuit. The generated bias current is mirrored to the core rectifier in order to precharge its CMOS diodes. The rectifier was optimized for a minimum number of gain stages while trying to minimize the overall input parasitic capacitance of the rectifier. The proposed CMOS rectifier was designed in an in-house 130-nm CMOS technology and includes a 1.6-kV human body level ESD protection at the input terminals. Measurements reveal a high 30% power conversion efficiency (PCE) over a wide input power range from −23 to −13 dBm for a 1- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm{ M}}\Omega $ </tex-math></inline-formula> output load at 868 MHz. PCE of over 10% is achieved for a 1- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm{ M}}\Omega $ </tex-math></inline-formula> load at an input power of −28 dBm. The enhanced startup at submicrowatt input power levels of the CMOS rectifier enables the design of an ultralow-power passive wake-up receiver. The measured sensitivity of −31 dBm for a 1-V output voltage across a capacitive load is the lowest reported in the literature so far.

Active-Matrix Micro-LED Display Driven by Metal Oxide TFTs Using Digital PWM Method

This article presents a new micro-light-emitting diode (LED) pixel circuit driven by the digital pulsewidth modulation (PWM) method, in which the threshold voltage compensation of the driving thin-film transistor (TFT) is included. A scan driver integrated by metal oxide (MO) TFTs is also designed to provide the scan signals for pixel circuits array. A micro-LEDs array with <inline-formula> <tex-math notation="LaTeX">$76\,\, \times $ </tex-math></inline-formula> BBB <inline-formula> <tex-math notation="LaTeX">$\times \,\,78$ </tex-math></inline-formula> and a scan driver with 80 stages are simultaneously fabricated by MO TFTs backplane with back-channel etch (BCE) structure. A respective micro-LED display is consequently realized by a peripheral driving system based on a field-programmable gate array (FPGA), which is used as a core control unit to synchronize the scan signals with the data signals. It is shown that the micro-LED in the pixel circuit can operate at a constant current even at the 5-<inline-formula> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> pulsewidth of the emission (EM) signal. The micro-LED display can achieve 256 linear grayscales with an approximately 60 Hz flash rate under the minimum pulsewidth of 40 <inline-formula> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> of EM signal.

Mechanical strain and bias-stress compensated, 6T-1C pixel circuit for flexible AMOLED displays

This paper proposes a novel topology for a pixel circuit with a voltage-programmed threshold voltage compensation technique for amorphous indium gallium zinc oxide thin-film transistor (a-IGZO TFT) used in flexible AMOLED (Active-Matrix Organic Light-Emitting Diodes) displays. Simulation of the proposed circuit was done in Cadence Spectre using SPICE models for a-IGZO TFT, and the circuit utilizes six a-IGZO TFTs and one storage capacitor. Moreover, a detailed analytical analysis of the circuit is also done to estimate the error introduced during the emission phase. The simulation results showed that this pixel circuit offers excellent immunity to threshold voltage variation of driving TFT caused by bending and prolonged operation under gate bias. It can compensate for a wide range of VTH variations, i.e., 0 V–2.1 V, from the original 0.7 V of the TFT model, and the maximum percentage error in OLED current obtained was below 1.14%. The circuit proposed also alleviates the effect of electrical instability caused by threshold voltage shifts due to the compressive and tensile strain in the flexible substrate of the AMOLED display. The OLED current error was found to be below 0.0018% for a ±0.3% strain in the driving TFT.

A Fully Integrated Ferroelectric Thin‐Film‐Transistor – Influence of Device Scaling on Threshold Voltage Compensation in Displays

AbstractThin‐film transistors (TFTs) based on amorphous indium‐gallium‐zinc‐oxide (a‐IGZO) have attracted vast attention for use in organic light‐emitting diode (AMOLED) displays due to their high electron mobility and large current on–off ratio. Although amorphous oxide semiconductors show considerably less threshold voltage (Vth) variation than poly‐silicon, large‐area processing and degradation effects can impede the characteristic parameters of a‐IGZO TFTs, which manifests in an uneven brightness distribution across the display panel. Such Vth variations are usually reduced by additional compensation circuits consisting of TFTs and capacitors. Herein, a new approach to compensate such variabilities is demonstrated: the integration of a programmable ferroelectric (FE) film in the gate stack of the TFT. This simplifies the complexity of the pixel cell and potentially minimizes the need for compensation circuits, which is crucial for transparent displays. To test this new approach, fully integrated FE‐TFTs (i.e., with vias contacting a structured bottom gate electrode from the top) based on a‐IGZO and FE hafnium‐zirconium oxide (HZO) are developed. A single low‐temperature post‐fabrication treatment at 350 °C for 1 h in air is used to simultaneously crystallize the HZO film in the FE phase and to reduce the number of defects in the a‐IGZO channel. The structural and electrical characterizations provide comprehensive guidance for the design of effective FE‐TFT gate stacks and device geometries. An accurate control of the polarization state and linear switching between multiple intermediate states is shown by using programming pulses of various amplitudes and widths. Furthermore, a direct correlation between the channel length and the applied pulse width for programming is observed.

Design of Pixel Circuit Using a-IGZO TFTs to Enhance Uniformity of AMOLED Displays by Threshold Voltage Compensation

This article presents a novel voltage-programmed pixel circuit using a-IGZO TFTs to effectively compensate threshold voltage (VTH) variations of driving TFT. The compensation is very important to maintain the pixel brightness of active-matrix organic light-emitting diodes (AMOLED) displays. The proposed pixel design uses four-phase clocking schemes: reset period, VTH detection period, data input period and emission period. To maintain uniformity in phases, the duration of all the phases is taken to be same which results in easier design of the multi-phase clock. Moreover, the switching speed of control signals is as high as 25 kHz, ensuring a high frame rate. The circuit has been simulated extensively, and the operation has been verified using SPICE models of a-IGZO TFT and OLED on Cadence Spectre. The circuit also achieves high contrast as OLED does not emit light in any periods except the emission period. The OLED current error in the proposed design for a VTH variation of 170% is below 0.1%. This circuit also provides a high aperture ratio of 51.6%, which ensures high resolution of AMOLED displays.

A Novel Real-Time TFT Threshold Voltage Compensation Method for AM-OLED Using Double Sampling of Source Node Voltage

In this article, we propose a novel real-time threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{th}$ </tex-math></inline-formula> ) compensation method for AM-OLED display. The proposed method predicts <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{th}$ </tex-math></inline-formula> shift of driving thin-film transistors (TFTs) based on the equation derived from the sensing line charging with TFT’s drain current. The prediction equation requires two consecutive source node voltages to consider the mobility variation, and only takes hundreds of microseconds to predict <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{th}$ </tex-math></inline-formula> shift. It is short enough to monitor the real-time <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{th}$ </tex-math></inline-formula> degradation, since the prediction can be carried out during the vertical blank time. Also, very high prediction accuracy was confirmed through Smart SPICE simulation. We simulated severe mobility variation and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{th}$ </tex-math></inline-formula> shift circumstances, and the simulation results showed that the prediction error was less than one just noticeable difference (JND) margin though out all gray levels. We expect that the proposed method can achieve more accurate and faster real-time <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{th}$ </tex-math></inline-formula> degradation compensation for high resolution and large-sized OLED displays than conventional one.

A 3-Transistor Low Power Rectifier for Wideband RF Energy Harvesting with a Threshold Voltage Compensation Technique using 45 nm Technology

With the advent of modern wireless communication technology and increasing requirement of high speed network, network life-time is becoming a major area of concern. The need of network power management is gaining attention with the high data network in place and is making a paradigm shift towards green communication. Hence embedding the RF energy harvesting (EH) capability in a wireless network is becoming inevitable. To make RF EH a reality a high frequency rectifier is indeed indispensable along with other circuits in the system. The RF energy needs to be harvested from the available sources in the ambience. It is also seen that the current generation of RF sources radiates at a very low signal power. So, to successfully convert and store this energy, the rectifier must not only be able to provide a sufficiently higher percentage conversion ratio (PCE) but also be able to cater it at a lower range of signal power. This paper presents the design and analysis of a simplified 3-transistor high frequency rectifier. A threshold voltage compensation technique is also incorporated and it achieves a PCE upto 85% at -2dBm in its single stage implementation. This is observed to be one of the highest in-class efficiency as compared to recently reported designs. From the frequency response it is seen to exhibit wide band performance spanning almost all popular wireless bands. The dynamic power dissipation (DPD) is calculated to be 6.25pW at -2dB, whereas the leakage power (LP) is observed to be zero.

An RF-to-DC Rectifier With High Efficiency Over Wide Input Power Range for RF Energy Harvesting Applications

This paper presents, a wide input range, 4-stage threshold voltage compensated RF-to-DC power converter, designed to efficiently convert RF signals to dc voltages by applying an optimum compensation voltage produced by subthreshold auxiliary transistors. The proposed optimally compensated rectifiers can achieve higher efficiency over a wider input power range compared to other threshold voltage compensation circuits where the level of the compensation is limited by the circuit structure and varies with input power. The designed rectifier is implemented in three possible ways. This proposed compensation technique can be applied to a rectifier chain with a relatively low number of stages. Designed and implemented in a 130 nm CMOS technology, the proposed rectifier exhibits a measured PCE of above 20% over the 8.5-dB input power range while driving a 1- $\text{M}\Omega $ load resistor at 896-MHz. For the same load and utilizing a minimal number of compensated rectifier stages, the proposed circuit exhibits a maximum PCE of 43% at −11 dBm for single-ended Dickson-based CMOS rectifiers. The proposed circuit demonstrates a −20.5 dBm sensitivity for 1 V output across a 1- $\text{M}\Omega $ resistive load.

A Novel OLED Pixel Circuit with Controllable Threshold Voltage Compensation Time

A Novel OLED Pixel Circuit with Controllable Threshold Voltage Compensation Time

A Novel OLED Pixel Circuit with Controllable Threshold Voltage Compensation Time

A Novel OLED Pixel Circuit with Controllable Threshold Voltage Compensation Time

A RF PVT Compensated Negative Resistance Circuit for Q-factor Improvement of Passive Elements in CMOS

A RF process, voltage and temperature (PVT) compensated negative resistance (NR) circuit is proposed to improve the quality factor (Q-factor) of CMOS passive devices while ensuring stability over PVT variations. It is implemented in the form of a grounded NR circuit to support single-ended RF circuit applications. Various circuit techniques such as a diode-connected load, a proportional-to-absolutetemperature (PTAT) current source and an adaptive body-biasing based threshold voltage compensation have been adopted to obtain a stable negative resistance value. Simulation results show that the Qfactor of on-chip spiral inductors loaded by the proposed NR circuit is improved from 6 to 60 at 1 ㎓ under typical corner conditions (tt, 1.2V, 27 ℃). The simulated Q-factor variation in corner conditions is approximately 37, which is approximately 6.5 times more stable than conventional NR circuit without compensation.