Pixel Circuit Research Articles

Novel Top-Anode OLED/a-IGZO TFTs Pixel Circuit for 8K4K AM-OLEDs

This paper proposes amorphous indium–gallium–zinc oxide thin-film transistors (TFTs) four-transistor-two-capacitor (4T2C) pixel circuit in combination with a top-anode organic light-emitting diode (OLED) for the use in 8K4K active-matrix organic light-emitting diode displays (AM-OLEDs). The proposed pixel circuit compensates for driving TFTs threshold voltage ( ${V} _{\textsf {TH}}$ ) shifts and mobility variations, ${V} _{\textsf {DD}}$ current–resistance ( ${I}$ – ${R}$ ) drops, and ${V} _{\textsf {SS}}~{I}$ – ${R}$ -induced rises. Both the positive and negative ${V} _{\textsf {TH}}$ of the driving TFT can be sensed by the proposed circuit. In this paper, we analyze the impact of the driving TFT compensation time for low gray levels. The current error rates are calculated when TFT mobility variations are considered. The proposed pixel circuit allows an increase in the compensation time to reduce the impact of driving TFT ${V} _{\textsf {TH}}$ and mobility variations on the performance of 4T2C pixel circuit. Conventional two-transistor-one-capacitor, five-transistor-two-capacitor, and proposed 4T2C pixel circuits are simulated and compared in this paper. The proposed pixel 4T2C circuit reduces the current error rates to below 5.79% when the ${V} _{\textsf {TH}}$ shifts of ±2 V, mobility variations of ±10%, ${V} _{\textsf {DD}}~{I}$ – ${R}$ drops of 1 V, and ${V} _{\textsf {SS}}~{I}$ – ${R}$ rises of 1 V are all considered. We show that the proposed 4T2C pixel circuit is suitable for 8K4K AM-OLEDs.

A 4410-ppi Resolution Pixel Circuit for High Luminance Uniformity of OLEDoS Microdisplays

This paper proposes a pixel circuit with high resolution and high luminance uniformity for organic light emitting diode-on-silicon (OLEDoS) microdisplays. The proposed pixel circuit employs a simple structure that consists of four n-channel MOSFETs and one capacitor, resulting in high resolution. In addition, this circuit compensates for the threshold voltage ( $V_{th}$ ) variation of the driving transistor caused by the body effect, which increases the $V_{th}$ as the source-to-body voltage of the driving transistor increases, thus reducing the emission current deviation, resulting in a high luminance uniformity. Moreover, the proposed pixel circuit extends the data voltage range using the capacitive coupling of the storage capacitance and the parasitic capacitance at the gate node of the driving transistor to precisely control the emission current. To verify the performance of the proposed pixel circuit, a test pattern with an array of the proposed 4T1C pixel circuits was fabricated on a single-crystalline silicon wafer as a backplane using a 110 nm standard CMOS process with 5.5 V high-voltage devices. The proposed pixel circuit occupies a unit sub-pixel area of $5.76 \mu \mathrm {m} \times 1.92 \mu \text{m}$ , which corresponds to a resolution of 4410 pixels per inch. The measurement results show that the emission current deviation error of the proposed pixel circuit ranges between −1.16% and +1.14%, which is improved from between −45.97% to +45.42% achieved in the conventional current-source type 2T1C pixel circuit, which does not compensate for the $V_{th}$ variation of the driving transistor. Moreover, the measured data voltage range of the proposed pixel circuit is extended to 1.618 V, which is 8.17 times wider than that of the conventional pixel circuit. Therefore, the proposed pixel circuit is very suitable for high resolution and high luminance uniformity of OLEDoS microdisplays.

A 6-TFT Charge-Transfer Self-Compensating Pixel Circuit for Flexible Displays

In this paper, a self-compensating 6 thin-film transistor (TFT) pixel circuit with special layout considerations has been proposed to mitigate the impact of the electrical instability of hydrogenated amorphous silicon TFTs as well as applied mechanical strain. The proposed pixel circuit has been fabricated onto flexible polyethylene naphthalate (PEN) substrate and the measurement results demonstrated less than ±3% variation of its output current after an accelerated 24-h stress test under flat, tensile strain, and compressive strain conditions. In addition, the proposed pixel circuit only required a pair of signals to operate, which reduced the complexity on external IC drivers.

A Compensation Method for Variations in Subthreshold Slope and Threshold Voltage of Thin-Film Transistors for AMOLED Displays

In this paper, we propose a compensation method for variations in the subthreshold slope (SS) and threshold voltage (Vth) of the low-temperature polycrystalline silicon thin-film transistors (LTPS TFTs) in an attempt to improve the image quality of the active matrix organic light-emitting diode (AMOLED) displays for mobile applications. The proposed compensation method provides a uniform voltage to the OLED according to the gray level without being affected by variations in the SS and Vth of the LTPS TFTs. To verify the performance of the proposed compensation method, a test pattern, including two pixel circuits designed for 5.7-inch quadruple high-definition AMOLED display was fabricated and measured. The measurement results showed that the proposed compensation method achieved an emission current error of the pixel circuit of only ±3.1 LSB at the 255th gray level. Therefore, the proposed compensation method is very suitable for AMOLED displays requiring high image quality.

Improvement of TimePix energy resolution correcting threshold variations

The Timepix detector is an important tool in spectroscopic imaging since it allows the determination of the energy of absorbed photons using its time over threshold capability. In this paper, however, we look at the energy resolution using the single photon counting mode combined with threshold scans. The pixel circuit of the TimePix detector involves a discriminator logic, which compares the collected amount of charge in each pixel with a threshold level (THL). The counter is incremented only if the induced signal surpasses this threshold. The THL is typically calibrated w.r.t. energy in the proximity of the noise edge, however, due to gain mismatch of the individual pixels, the threshold levels of pixels deviate if a DAC value far from the calibration point is set. This energy threshold mismatch far from the calibration point was corrected in this paper based on spectrum matching between individual pixels. The derived method is compared to straightforward calibration using multiple monochromatic peaks. To this end a threshold scan was performed using a TimePix Si-1mm detector (utilizing counting priciples) in presence of a Am-241 gamma source. The peak at energy 59.5 keV was fitted for each pixel to determine the actual THL values for all pixels at this energy. Then the full spectrums for all pixels were matched, using one fixed point. The matching process gave us two matrices of correction factors, the first matrix corrects the THL gain and the second its offset. In a second evaluation we measured and fitted the x-ray fluorescence of zirconium and copper during threshold scan in addition to the Am-241 peak. The gain and offset of each pixel were then determined by linear fit of the peak positions. The two calibration methods were evaluated by measuring the full width of half maximum FWHM of two other fluorescence peaks (23.17 keVand 26.2 keV peaks of cadmium). The evaluation shows that linear fit of multiple peaks results in a comparable FWHM (reduction of 21%) over spectrum matching the (reduction of 20%), while, opposite to spectrum matching, necessitating multiple measurement setps and THL scans.

High-Resolution Flexible AMOLED Display Integrating Gate Driver by Metal–Oxide TFTs

This letter proposes a high-resolution [200 (RGB) * 600, 282 PPI) flexible active matrix organic light emitting display (AMOLED) integrating a new gate driver, of which the substrate is a polyimide material. Flexible In–Zn–O thin-film transistors (TFTs) with back channel etched structure reveal a good stability at different curvature radii. The integrated gate driver employing bi-side driving method consists of seven TFTs and two capacitors with the advantages of simple topology, low power, full-swing output, and small area. The conventional 2T1C pixel circuit with top emission structure is applied to the proposed flexible AMOLED panel. And the flexible panel has a good full-color display quality without obvious defects at the bending condition. It is shown that the output signals of the proposed gate driver have no distortion and good noise suppressed characteristics even up to 600 stages and maintain stable under a long-time continuous operation up to 15 days.

A 250 m Direct Time-of-Flight Ranging System Based on a Synthesis of Sub-Ranging Images and a Vertical Avalanche Photo-Diodes (VAPD) CMOS Image Sensor

We have developed a direct time-of-flight (TOF) 250 m ranging Complementary Metal Oxide Semiconductor (CMOS) image sensor (CIS) based on a 688 × 384 pixels array of vertical avalanche photodiodes (VAPD). Each pixel of the CIS comprises VAPD with a standard four transistor pixel circuit equipped with an analogue capacitor to accumulate or count avalanche pulses. High power near infrared (NIR) short (<50 ns) and repetitive (6 kHz) laser pulses are illuminated through a diffusing optics. By globally gating the VAPD, each pulse is counted in the in-pixel counter enabling extraction of sub-photon level signal. Depth map imaging with a 10 cm lateral resolution is realized from 1 m to 250 m range by synthesizing subranges images of photon counts. Advantages and limitation of an in-pixel circuit are described. The developed CIS is expected to supersede insufficient resolution of the conventional light detection and ranging (LiDAR) systems and the short range of indirect CIS TOF.

Capacitive sensor pixel circuit with single transistor for touch‐fingerprint recognition sensor

Touch-based user identity verification is widely used in smartphones, and fingerprint authentication is the most popular method. The authors developed a capacitive type of fingerprint sensor that uses only one amorphous oxide thin film transistor (TFT) that is integrated into a display screen. To achieve a small sensor area, they removed one bus line from the conventional three bus lines and used only one oxide TFT. The proposed sensor shows good sensitivity. The measured voltage difference between a ridge (touch) and a valley (non-touch) was 5.4 V with an input pulse of 15 V.

A Novel Voltage-Programmed Pixel Circuit with Poly-Si TFTs for AMOLED Displays

In this paper, we propose a novel voltage-programmed pixel circuit with polysilicon thin–flim transistors (poly-Si TFTs) for active matrix organic light-emitting diode (AMOLED) displays, which consists of one programming transistor, one driving transistor, four switching transistors and two storage capacitors, respectively. Specifically, the proposed pixel circuit is able to not only efficiently compensate for the threshold variations of TFTs, but also largely suppresses the electrical degradations of the devices caused by the long-term electrical stress. Moreover, the mobility variation of the driving transistor can be compensated as well. The simulation has been performed by HSPICE, and results indicate that the average values of nonuniformities are, respectively, 7.3% as the threshold-voltage varies by [Formula: see text][Formula: see text]V and 2.1%, as the mobility of the driving transistor varies by [Formula: see text]%, both of which are much lower than that of the conventional two-transistor and one-capacitor (2T1C) pixel. Furthermore, since the OLED is reverse-biased during the nonemission phases, the lifetime of OLED will be extended naturally. As a consequence, the proposed pixel circuit can substantially improve the display performance.

Flicker modeling scheme of liquid crystal displays based on current leakage without information about TFT parameters

It is well known that the current leakage at a switching thin film transistor (TFT) of a pixel circuit is one of key factors for the luminance fluctuation of liquid crystal displays (LCDs) perceived as a flicker artifact. This paper proposes a flicker estimation scheme based on the current leakage model of an amorphous silicon (a-Si) TFT. The proposed scheme consists of four blocks such as gray-to-voltage conversion, pixel voltage update, voltage-to-transmittance conversion, and flicker estimation. Gray-to-voltage and voltage-to-transmittance conversion blocks are built from measured data at a source driver and a panel for gray levels. Flicker estimation is established by applying fast Fourier transform (FFT) to simulated transmittance waveforms. The pixel voltage update block is made up by the voltage leakage model which parameters are estimated by matching simulation results to measurement results at a band gray pattern regarding several gray levels. Any given information of TFTs is not required for parameter extraction. The proposed method is applied to a WSXGA+ in-plane switching (IPS) LCD of a 1-dot inversion and flicker levels are estimated within an error range of ±0.6 dB at full gray patterns of gray levels from 32 to 224, compared to measurement results.

Suppressing &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${V_{\mathrm{TH}}}$ &lt;/tex-math&gt; &lt;/inline-formula&gt; Variations in LTPS Thin-Film Transistors With Current Scaling Ratio Modulation for AMOLED Display

This brief suppresses threshold voltage ( ${V}_{\text {TH}}$ ) variations in low-temperature polycrystalline-silicon thin-film transistors (LTPS TFTs) with a new current scaling ratio modulation method for the application of active-matrix organic light-emitting diode displays. With a newly proposed 3T2C LTPS TFT-based pixel circuit, high-current scaling ratios at low gray levels and low ratios at high gray levels are achieved. Thus, the programming time at low gray levels can be reduced, and the driving currents of the OLED are efficiently generated over the entire data range without consuming extra power for the current source. The driving currents are also independent of the threshold voltage of the driving TFT and the power line voltage, guaranteeing the uniformity of the current. Simulation results demonstrate that the relative current error rates are below 3.7% as the threshold voltage varies ±0.5 V and below 5.3% as the threshold voltage varies ±0.5 V and the power line voltage drops by 1 V; the programming time can be as short as only $12~\mu \text{s}$ .

Environmental Effects of the Technology Transition from Liquid–Crystal Display (LCD) to Organic Light-Emitting Diode (OLED) Display from an E-Waste Management Perspective

Organic light-emitting diode (OLED) displays are applied to various electronic devices such as smartphones and televisions in our society, replacing liquid–crystal display (LCD) due to many advantages: self-emitting property, high contrast, slimness, and flexibility. Although OLED consists mostly of organic substances, because it was developed to reduce the consumptions of rare and precious metals, OLED display has a possibility to contain more metal-based components than LCD; for instance, a pixel circuit for OLED needs two thin-film transistors (TFTs), whereas that for LCD does one. Thus, this study was intended to assess and compare possible environmental impacts due to metals in an OLED display and an LCD on a same screen size basis, to examine environmental effects of the technology transition. Hazardous waste potentials at end-of-life were examined based on metal leachability tests, and resource depletion and toxicity potentials were evaluated based on life cycle impact assessment methods. The leachability test results showed that the OLED display has higher hazardous potential than an LCD due to excessive levels of leachability for many metals under California state regulation. The OLED display had 1000–2300 times higher resource depletion potentials than the LCD due primarily to the high concentrations of gold, selenium, silver, palladium, and tin. The OLED display also had 2–600 times higher toxicity potentials due primarily to the high concentrations of arsenic, cadmium, chromium, and antimony. This study can be used to motivate waste recyclers and managers to actively collect waste OLED displays for circular economy and to direct manufactures to develop more environmental-friendly OLED displays for sustainable society. This study evaluates and compares hazardous, resource depletion, and toxic potentials from metals in a Liquid–Crystal Display (LCD) and an Organic Light-Emitting Diode (OLED) Display to examine the effect of the technology transition on environmental impacts.

Electronic Devices Based on Oxide Thin Films Fabricated by Fiber-to-Film Process

Technical development for thin-film fabrication is essential for emerging metal-oxide (MO) electronics. Although impressive progress has been achieved in fabricating MO thin films, the challenges still remain. Here, we report a versatile and general thermal-induced nanomelting technique for fabricating MO thin films from the fiber networks, briefly called fiber-to-film (FTF) process. The high quality of the FTF-processed MO thin films was confirmed by various investigations. The FTF process is generally applicable to numerous technologically relevant MO thin films, including semiconducting thin films (e.g., In2O3, InZnO, and InZrZnO), conducting thin films (e.g., InSnO), and insulating thin films (e.g., AlO x). By optimizing the fabrication process, In2O3/AlO x thin-film transistors (TFTs) were successfully integrated by fully FTF processes. High-performance TFT was achieved with an average mobility of ∼25 cm2/(Vs), an on/off current ratio of ∼107, a threshold voltage of ∼1 V, and a device yield of 100%. As a proof of concept, one-transistor-driven pixel circuit was constructed, which exhibited high controllability over the light-emitting diodes. Logic gates based on fully FTF-processed In2O3/AlO x TFTs were further realized, which exhibited good dynamic logic responses and voltage amplification by a factor of ∼4. The FTF technique presented here offers great potential in large-area and low-cost manufacturing for flexible oxide electronics.

P‐47: An OLEDoS Pixel Circuit with Extended Data Voltage Range for High Resolution Micro‐Displays

A new OLEDoS pixel circuit is proposed for high resolution micro‐displays with improved image quality. The data voltage range is extended by using two coupling capacitors to raise the voltage at the source electrode of the driving transistor. Furthermore, the image quality is improved by a compensation scheme of the threshold voltage variation of the driving transistor. The simulations on the basis of CSMC 0.25 µm CMOS model are used to verify the performance of the proposed pixel circuit. It is shown that the data voltage of the proposed pixel circuit ranges from 1.5 V to 3.05 V, which is 2.5 times of that of the conventional 2T1C pixel circuit. The emission current error is decreased to −2.6% ~ 2.6% with ±5 mV threshold voltage variation of the driving transistor for the proposed circuit.

23‐1: Distinquished Student Paper: An AMOLED Pixel Circuit for 1000 ppi and 5.87‐inch Mobile Displays with AR and VR Applications

In this paper, an active‐matrix organic light emitting diode (AMOLED) pixel circuit is proposed for 1000 ppi and 5.87‐inch mobile displays with augmented and virtual reality applications. The proposed pixel circuit consisting of 3 thin‐film transistors (TFTs) and 2 capacitors employs a simultaneous emission driving method to reduce the number of TFTs. The simulation results of the proposed pixel circuit showed that the emission current errors caused by the threshold voltage variation of driving TFT and crosstalk error were less than ±0.4 LSB and ±1 LSB, respectively, over the entire gray level. Therefore, the proposed pixel circuit is highly suitable for AMOLED displays.

P‐17: Novel Pixel Circuit with Inverter Structure Based on a‐IGZO TFT for Blue‐Phase Liquid Crystal Displays

This work presents a new pixel circuit with amorphous indium‐gallium‐zinc‐oxide thin‐film transistors (a‐IGZO TFTs) for blue‐phase liquid crystal displays (BPLCDs). The proposed circuit adopts two pairs of inverter structures to ameliorate the frequency effect of the BPLCs for generating the accurate gray level voltages. Furthermore, to achieve the maximum transmittance of BPLCs, the wide range of the operation voltage is performed. Simulation results show that the proposed pixel circuit produces an operation voltage range of −29.329 V to 29.748 V when the practical source driver integrated circuits (ICs) only supplies an output voltage swing of 0 V to 15 V, successfully inverting the polarity of the BPLCs without the common signal.

35‐1: Strategy for Developing an Ultra‐High‐Luminance AMOLED Display

To obtain a high‐luminance active‐matrix organic light‐emitting diode (AMOLED) display compatible with the high‐dynamic‐range (HDR) technology, we investigated the effect of basic components, e.g., field‐effect transistor, pixel circuit, OLED structure, and organic material. Combining these technologies, we fabricated a high‐luminance display with a peak luminance exceeding 3,000 cd/m2.

5‐2: Gate Driver Circuits for Internal Compensation Type OLED Display with High Mobility Oxide TFT

The self‐compensating pixel circuit and the gate driver are fabricated with high mobility (~50cm2/Vs) oxide TFTs. In order to improve operating margin against negative shift of Vth in GIP circuit, the circuit utilizes two low‐voltage‐level signals. Also, the design of the EM driver to be controlled by the scan driver reduces the bezel size. Our 5.5‐in. FHD OLED display panel on PI substrate shows good uniformity and long‐term stability with saturation behavior of clamping voltage in GIP circuit under operation at 60°C for over 1,000 hours.

P‐25: OLED Displays with a Specialized Pixel Circuit for Automotive Applications

A 4.2 inch. OLED display which shows high luminance of peak‐white over 800 cd/m2 has been developed for automotive application. A specialized pixel circuit is proposed to drive OLEDs by current range over 1 µA and reset OLEDs within one frame.

Active matrix field sequential color electrically suppressed helix ferroelectric liquid crystal for high resolution displays

AbstractIn this article, we disclose a 3‐inch 250ppi active matrix field sequential color (FSC) display based on electrically suppressed ferroelectric liquid crystal (ESHFLC). ESHFLC's ultra‐fast response time (~10 μs at 6.67 V/μm) enables the display resolution to be tripled via FSC technique. The photo‐alignment technology provides ESHFLC with optimal anchoring energy that contributes to a high contrast ratio over 10 K:1 on single pixel level measurement. A specific 3T1C pixel circuit is designed to generate continuous gray scale from FLC binary switching by utilizing the pulse width modulation concept. The low temperature poly silicon thin‐film transistor array has been used to fabricate the FSC ESHFLC display panel. We achieved an 8‐bit gray level for each color subframe, that is, R, G, and B colors that results in 24‐bit color images. We believe, because of the good optical quality and cost‐effective fabrication, this display may replace in‐plane switching or fringe‐field switching in the near future for the portable device market. Moreover, high resolution FSC ESHFLCs can find applications in the emerging virtual reality displays.