Transistor Pixel Research Articles

Opportunities for cost-effective manufacturing of fully printed high performance displays enabled by vertical light-emitting transistor pixels

With the vertical organic light-emitting transistor (VOLET), we introduce a promising solution that could significantly benefit the manufacturing of displays, accelerating the wide adoption of flexible and printed electronics. The VOLET—like conventional, lateral channel, organic thin film transistors—is compatible with a variety of printing techniques as well as flexible substrates and low-temperature processing. In combination these devices will enable a more cost-effective approach to mass-production that can dramatically extend the market potential of active-matrix organic light-emitting diode (AMOLED) displays. In this paper we discuss the prospects that AMOLED presents for the future of the display market, with a focus on the innovative VOLET device architecture. We assess how the integration of this device into active-matrix displays can contribute to the long range sustained competitiveness of AMOLED technology. We review recent progress in mass production techniques for printed electronics, with a particular emphasis on large-scale carbon nanotube material deposition. Finally, we explore the prospects for fully printed active-matrix light-emitting displays, including a review of high-performance printed components whose integration could facilitate the mass production of low-cost, high-performance, VOLET based AMOLEDs.

A compact model for the well-depleted 1-T pixel sensor based on silicon-on-insulator technology

The silicon-on-I_nsulator (SOI) based one transistor (1-T) pixel sensor is a promising candidate for a high-resolution image sensor and in-sensor computing. In this work, a compact model for the photoresponse of a well-depleted SOI 1-T pixel sensor is proposed from the perspective of the physical process. The model explains the photoelectron accumulation, photoelectron redistribution, and modulation of the transistor according to the electron drift/diffusion mechanism, capacitor–voltage relationship, and current solution from BSIM-IMG. The model agrees well with the experimental results from the 1-T pixel sensor fabricated by the 22 nm fully depleted SOI technology, as well as the simulation results from technology computer-aided design. The extraction scheme of the model parameters is also provided to facilitate the use of the model.

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.

7‐4: 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.

ISFET Pixel Array With Selectable Sensitivity and Bulk-Based Offset-Drift Nullification Capability for Reduction of Non-Ideality Effects

This article presents a monolithic, cost-effective, label free, low power, robust <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times2$ </tex-math></inline-formula> fully differential ion-sensitive field-effect transistor (ISFET) pixel array aimed at pH sensing. Each pixel is based on a differential readout architecture which eliminates need for additional non-ideality compensation circuitry. Additionally, the design offers the capability to correct for residual drift and trapped charge offset through substrate-based voltage adjustment during calibration. The design also provides a unique ability for selecting pH sensitivity according to the desired mode of operation which has not been demonstrated previously. We present experimental results obtained from the pixel array fabricated in an unmodified AMS <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.35~\mu \text{m}$ </tex-math></inline-formula> CMOS process using a 3.3 V power supply. Each pixel demonstrates a differential sensitivity of 61 mV/pH when operated in high sensitivity mode and 25 mV/pH when operated in high non-ideality rejection mode over the entire pH range, with the ability to remove the already small drift of 0.3 mV/min and any offset mismatch by adjusting the substrate voltage.

A Compact Amorphous In-Ga-Zn-Oxide Thin Film Transistor Pixel Circuit With Two Capacitors for Active Matrix Micro Light-Emitting Diode Displays

In this paper, we propose a novel amorphous In-Ga-Zn-oxide (a-IGZO) thin film transistor (TFT) pixel circuit for pulse width modulation (PWM) driving active matrix micro light-emitting diode (μLED) displays. The proposed pixel circuit compensates for wavelength shifts according to gray level and threshold voltage (VTH) variation of TFTs with only two capacitors. This compact operation can be achieved by simultaneous compensation for VTH along with data input in the constant current generation (CCG) circuit and the PWM circuit, which consist the pixel. Since the VTH variation in TFTs is compensated, the proposed pixel circuit can supply a uniform and stable current to the μLED throughout all gray levels. The compensation accuracy was verified by HSPICE with an oxide TFT model based on the measurement data. In addition, we analyzed the optimization method to improve the PWM driving by controlling the electrical properties of TFT, and verified this approach by simulation.

Neuromorphic display system for intelligent display

Ultra-high integration, ultra-high resolution, intelligence and human-machine interaction is the inevitable trend of flat panel displays. However, most intelligent display systems are based on traditional Von Neumann architecture, which need to call data frequently because the storage and computation are separated, resulting in serious loss of power consumption and speed. In this paper, the concept of "neuromorphic display" device is proposed for the first time. The display technology is organically combined with the artificial neural network system to build a neuromorphic display network including the sensor module, processing module and display module, which endows the tiny artificial neural network system with the ability of perception, dispose and execution. The neuromorphic display enable to avoid serious loss of power consumption and speed without calling data frequently, and the color of the emitted light directly represents the result of neuromorphic display’s perception and processing of various external environment haptic stimuli. Furthermore, the neuromorphic element is applied in display that one-transistor one-synaptic transistor (1T1S) pixel circuit is demonstrated to replace two-transistor one-capacitance (2T1C) pixel circuit in traditional display to simplify pixel circuit. Therefore, the neuromorphic display showed great potentials in interactive input/control devices, intelligent wallpaper, robots and medical/health monitoring devices, and the 1T1S pixel circuit can simplify circuit design for active matrix display.

Sol-gel indium-zinc-tin-oxide thin film transistor pixel array with superior stabilityunder negative bias illumination stress

In this paper, we fabricate a back channel etched structure thin film transistor (TFT) pixel array with hafnium-aluminum oxide dielectric and indium-zinc-tin-oxide (IZTO) semiconductor using a solution process. The electrical characteristics of IZTO TFT are modified by N&lt;sub&gt;2&lt;/sub&gt;O plasma treatment. In comparison with the subthreshold swing and saturation mobility of the device untreated by plasma , the subthreshold swing decreases from 204 to 137 mV·dec&lt;sup&gt;–1&lt;/sup&gt;, and the saturation mobility increases from 29.12 to 51.52 cm&lt;sup&gt;2&lt;/sup&gt;·V&lt;sup&gt;–1&lt;/sup&gt;·s&lt;sup&gt;–1&lt;/sup&gt;. Improvement in the mobility and the subthreshold swing (SS) demonstrate that interface states may be passivated by reactive O radicals that are generated by N&lt;sub&gt;2&lt;/sub&gt;O plasma, which is confirmed by the result of X-ray photoelectron spectrum analysis. In addition, the stability of negative bias illumination stress (NBIS) shift is only 0.1V for 3600 s with an illumination intensity of 10000 lux. This result indicates that its superior stability meets the requirements for the display driver. Therefore, N&lt;sub&gt;2&lt;/sub&gt;O plasma treatment is verified to be an effective method to improve device performance and light stability for IZTO TFT pixel array.

Electrically driven mid-submicrometre pixelation of InGaN micro-light-emitting diode displays for augmented-reality glasses

InGaN-based blue light-emitting diodes (LEDs), with their high efficiency and brightness, are entering the display industry. However, a significant gap remains between the expectation of highly efficient light sources and their experimental realization into tiny pixels for ultrahigh-density displays for augmented reality. Herein, we report using tailored ion implantation (TIIP) to fabricate highly efficient, electrically-driven pixelated InGaN micro-LEDs (μLEDs) at the mid-submicrometre scale (line/space of 0.5/0.5 μm), corresponding to 8,500 pixels per inch (ppi) (RGB). Creating a laterally confined non-radiative region around each pixel with a controlled amount of mobile vacancies, TIIP pixelation produces relatively invariant luminance, and high pixel distinctiveness, at submicrometre-sized pixels. Moreover, with the incomparable integration capability of TIIP pixelation due to its planar geometry, we demonstrate 2,000 ppi μLED displays with monolithically integrated thin-film transistor pixel circuits, and 5,000 ppi compatible core technologies. We expect that the demonstrated method will pave the way toward high-performance μLED displays for seamless augmented-reality glasses. Submicrometre-sized InGaN-based light-emitting diodes are fabricated by tailored ion implantation. The devices are free from electrical leakage and show a luminance of 7,440 nit at 4.9 A cm−2 even at the line/space scale of 0.5/0.5 μm (= 8,500 ppi).

47‐1: The World Smallest OLED Microdisplay Projection Device Design Methodology

We developed the world smallest OLED microdisplay projection device. This device consists of just 2 parts which a high‐brightness Microdisplay based Organic Light Emitting diodes on a silicon backplane panel (M‐OLED) ( 1,000,000 [cd/m2] ) and an optimal lens (F/1.1) for the self‐emitting projection device.This high‐ brightness M‐OLED is used 2 key technologies.The first is a supplying the photoexitation potential to multiple emitting layers in OLED using by the new pixel transistor (Tr.) that can withstand very high voltage in 7.8 pm pixel. The second is the controlling light divergence angle from each self‐emitting pixel that made using micro optical resonance effect from cathode until anode electrode in OLED and the optimized micro lens (ML) for each pixel.Next, we evaluated the relationship between the light divergence angle from M‐OLED and F number (Fno.) of the projection lens and we found parameters and a specific method for controlling the light efficiency of the self‐emitting projection device.Based on the results of these fundamental studies, we established a design methodology for the self‐emitting projection device with high efficiency, and we realized the world smallest self‐emitting projection device (W 10 /D 11 /H 6 (mm)).

P‐130: A Full Screen Biometric Identification Approach for OLED Displays by Using Near‐Infrared OLED

In this paper, we propose a full screen biometric identification approach for organic light emitting diode (OLED) displays using a near infra‐red(NIR) OLED coupled to a large area thin film transistor (TFT) pixel sensing circuit. Preliminary estimates for photons propagating from the NIR OLED source and reflecting back to the subpixel are presented. Although the signal landing on the subpixel is small, it may be amplified by the large area active pixel sensor circuits integrated in the TFT layer. The proposed approach has the key advantage of compatibility with existing large area fabrication processes. Although there are still some open technical issues such as NIR OLED lifetime, responsivity and sensor characteristics, the proposed approach demonstrates the potential to implement a full screen biometric identification within a large area display.

Modeling and Analysis of Capacitive Relaxation Quenching in a Single Photon Avalanche Diode (SPAD) Applied to a CMOS Image Sensor.

We present an analysis of carrier dynamics of the single-photon detection process, i.e., from Geiger mode pulse generation to its quenching, in a single-photon avalanche diode (SPAD). The device is modeled by a parallel circuit of a SPAD and a capacitance representing both space charge accumulation inside the SPAD and parasitic components. The carrier dynamics inside the SPAD is described by time-dependent bipolar-coupled continuity equations (BCE). Numerical solutions of BCE show that the entire process completes within a few hundreds of picoseconds. More importantly, we find that the total amount of charges stored on the series capacitance gives rise to a voltage swing of the internal bias of SPAD twice of the excess bias voltage with respect to the breakdown voltage. This, in turn, gives a design methodology to control precisely generated charges and enables one to use SPADs as conventional photodiodes (PDs) in a four transistor pixel of a complementary metal-oxide-semiconductor (CMOS) image sensor (CIS) with short exposure time and without carrier overflow. Such operation is demonstrated by experiments with a 6 µm size 400 × 400 pixels SPAD-based CIS designed with this methodology.

Vertically integrated photo junction-field-effect transistor pixels for retinal prosthesis.

Optoelectronic retinal prostheses transduce light into electrical current for neural stimulation. We introduce a novel optoelectronic pixel architecture consisting of a vertically integrated photo junction-field-effect transistor (Photo-JFET) and neural stimulating electrode. Experimental measurements demonstrate that optically addressed Photo-JFET pixels utilize phototransistive gain to produce a broad range of neural stimulation current and can effectively stimulate retinal neurons in vitro. The compact nature of the Photo-JFET pixel can enable high resolution retinal prostheses with the smallest reported optoelectronic pixel size to help restore high visual acuity in patients with degenerative retinal diseases.

Textile Display with AMOLED Using a Stacked-Pixel Structure on a Polyethylene Terephthalate Fabric Substrate.

An active-mode organic light-emitting diode (AMOLED) display on a fabric substrate is expected to be a prominent textile display for e-textile applications. However, the large surface roughness of the fabric substrate limits the aperture ratio—the area ratio of the organic light-emitting diode (OLED) to the total pixel area. In this study, the aperture ratio of the AMOLED panel fabricated on the polyethylene terephthalate fabric substrate was enhanced by applying a stacked-pixel structure, in which the OLED was deposited above the organic thin-film transistor (OTFT) pixel circuit layer. The stacked pixels were achieved using the following three key technologies. First, the planarization process of the fabric substrate was performed by sequentially depositing a polyurethane and photo-acryl layer, improving the surface roughness from 10 μm to 0.3 μm. Second, a protection layer consisting of three polymer layers, a water-soluble poly-vinyl alcohol, dichromated-polyvinylalcohol (PVA), and photo acryl, formed by a spin-coating processes was inserted between the OTFT circuit and the OLED layer. Third, a high mobility of 0.98 cm2/V∙s was achieved at the panel scale by using hybrid carbon nano-tube (CNT)/Au (5 nm) electrodes for the S/D contacts and the photo-acryl (PA) for the gate dielectric, enabling the supply of a sufficiently large current (40 μA @ VGS = −10 V) to the OLED. The aperture ratio of the AMOLED panel using the stacked-pixel structure was improved to 48%, which was about two times larger than the 19% of the side-by-side pixel, placing the OLED just beside the OTFTs on the same plane.

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 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.

Total-Ionization-Dose Radiation-Induced Noise Modeling and Analysis of a 2k &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\times$ &lt;/tex-math&gt; &lt;/inline-formula&gt; 2k 4T CMOS Active Pixel Sensor for Space Applications

This paper presents modeling techniques of total-ionization-dose (TID) radiation-induced noise characteristics including dark current (DC), fixed pattern noise (FPN), and equivalent noise charge (ENC) of a CMOS active pixel sensor (APS) for space applications. The noise models from theoretical derivation and experimental data analysis are discussed in detail according to the circuit structure of the CMOS APS based on 4-T transistor pixel topology followed by the column-level correlation double sampling and the column-level amplifier. From the simulation of the theoretical models, the amplitude of noise significantly increases with the increase of total dose, and a threshold dose of ~60 krad (Si) exists. When the total dose is below such threshold, DC, FPN, and ENC increase slowly. When the total dose exceeds the threshold value, these noise parameters increase substantially according to the exponential growth. To verify the proposed models, a $\gamma $ -ray radiation experiment using 60Co source is performed on a commercial 2k $\times \,\, 2\text{k}$ CMOS APS chip in 180-nm CMOS technology from Vendor A. The noise performance of two CMOS APS devices at the dose rates of 50 and 10.6 rad(Si)/s has been measured. It shows that experimental results are in accordance with the rules of theoretical deduction. The dose threshold of the abovementioned three noise parameters is around 60 krad (Si). The TID-induced noise models can be applied to both the design of radiation-hardened CMOS APS and the construction of filtering algorithms in the post image processing.

A-InGaZnO Active-Matrix Organic LED Pixel Periodically Detecting Thin-Film Transistor Threshold Voltage Once for Multiple Frames

This letter proposes a new voltage-programmed amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) pixel circuit for active-matrix organic light-emitting diode (AMOLED) displays. The proposed circuit detects the $\text{V}_{\mathrm {TH}}$ of the driving TFT only in a periodically executed compensation frame, allowing the other frames to perform only data input and emission for high-speed applications. An HSPICE model is also established based on the measured electrical characteristics of a fabricated a-IGZO TFT. The simulation results reveal that the proposed circuit can compensate for the degradation of the driving TFT and the OLED without external circuits. Moreover, the current error rates are <6.32%, so the proposed circuit is effective for use in AMOLED displays.

Laser-induced forward transfer of multi-layered structures for OTFT applications

We report on the one-step laser printing of multi-layered organic-based field effect transistors, using thin films of bis(2-phenylethynyl) end-substituted terthiophene (diPhAc-3T) as semiconductor, parylene-C (Py-C) as dielectric and silver (Ag) as gate electrode, respectively. The laser-induced forward transfer (LIFT) process was used to transfer pixels from donor to receiver substrates. The latter included pre-designed source and drain gold electrodes to form complete organic thin films transistors (OTFTs). Such laser-induced forward transfer used a single 50ps duration pulse delivered by a Nd:YAG laser operating at 355nm to print transistor pixel arrays under ambient temperature. The pixels (350μm sized-squares, and 700±40nm in thickness), fabricated in the top gate configuration, were investigated for their current–voltage characteristics immediately after printing. Electrical characterization demonstrated that the laser printed transistor is fully functional with hole mobilities of 4×10−4cm2/Vs, a threshold voltage Vt near −10V, Ion/Ioff ratio near to 104–105 and the sub-threshold slope (S) of 14–18V/decade. The efficient cohesion between the three different layers composing the pixels offers an exceptionally high strength to laser printing, while maintaining the electrical properties.

Contactless Functionality Inspection of Flat-Panel-Display Pixels and Thin-Film Transistors by Capacitive Coupling

A fast and thorough detection of structural and functional defects of flat-panel displays, large-area image detectors, and printed electronics requires a contactless and flexible inspection technique. Moreover, to accelerate the development of new products and to increase yields, efficient device characterization including the analysis of single component functionality and testing under operating conditions is essential. In this contribution, a contactless inspection method solely based on capacitive coupling is used to analyze pixel and thin-film transistor (TFT) functionality of active-matrix liquid crystal and electrophoretic display backplanes. Employing a capacitively coupled sensor, the measurement of the evolution of the pixel electrode voltage during TFT operation (switching) yields display flicker and TFT parameters, such as TFT on- and off-currents, TFT threshold, and intrinsic capacitance. To confirm the measurement results, the pixel voltage was also measured with an active voltage probe brought into contact with the pixel electrodes under test.