Modeling of threshold‐voltage‐shift dependency on drain bias in amorphous‐silicon thin‐film transistors in active‐matrix organic light‐emitting‐diode displays

Abstract— A theoretical model to interpret appearances of the threshold voltage shift in hydrogenated amorphous‐silicon (a‐Si:H) thin‐film transistors (TFTs) is developed to better understand the instability of a‐Si:H TFTs for the driving transistors in active‐matrix organic light‐emitting‐diode (AMOLED) displays. This model assumes that the defect creation at channel in a‐Si:H is proportional to the carrier concentration, leading to the defect density varying along the channel depending on the bias conditions. The model interprets a threshold‐voltage‐shift dependency on the drain‐stress bias. The model predicts the threshold voltage shift stressed under a given gate bias applying the drain saturation voltage is 66% of that with zero drain bias, and it even goes down to 50–60% of that when stressed by applying twice the drain saturation voltage.     

Letter: A new compensation pixel circuit with metal oxide thin‐film transistors for active‐matrix organic light‐emitting diode displays

This paper presents a novel compensation pixel circuit for active‐matrix organic light‐emitting diode displays, in which the coupling effect mask technology is developed to compensate the threshold voltage of driving thin‐film transistor whether it is positive or negative. Twenty discrete compensation pixel circuits have been fabricated by In‐Zn‐O thin‐film transistors process. It is measured that the non‐uniformity of the proposed pixel circuit is significantly reduced with an average value of 8.6%. Furthermore, the organic light‐emitting diode emission current remains constant during 6 h continuous operation, which also confirms the validity of the proposed pixel circuit.     

A LTPS-TFT pixel circuit for active matrix organic light emitting diode based on improved current mirror

In this paper, a voltage-driving and current compensation method for active matrix organic light emitting diode (AMOLED) displays is proposed. An improved current mirror is introduced into the pixel circuit to overcome the channel length modulation effect of TFTs. The SPICE simulation results show that the proposed pixel circuit not only effectively compensates for non-idealities related with deviations of μ and V_T in TFTs, the OLED degradation, but also offers a less setting time and guarantees a good liner relationship between V_DATA and I_OLED.     

A scalable manufacturing process for flexible active‐matrix e‐paper displays

Abstract— A scalable manufacturing process for fabricating active‐matrix backplanes on low‐cost flexible substrates, a key enabler for electronic‐paper displays, is presented. This process is based on solution processing, ink‐jet printing, and laser patterning. A multilayer architecture is employed to enable high aperture ratio and array performance. These backplanes were combined with E Ink electrophoretic media to create high‐performance displays that have high contrast, are bistable, and can be flexed repeatedly to a radius of curvature of 5 mm.     

In‐pixel temperature sensor for high‐luminance active matrix micro‐light‐emitting diode display using low‐temperature polycrystalline silicon and oxide thin‐film‐transistors

We propose an in‐pixel temperature sensor using low‐temperature polycrystalline silicon and oxide (LTPO) thin‐film transistor (TFTs) for high‐luminance active matrix (AM) micro‐light‐emitting diode (LED) displays. By taking advantage of the different off‐current characteristics of p‐type LTPS TFTs and n‐type a‐IGZO TFTs under temperature change, we designed and fabricated a temperature sensor consists of only LTPO TFTs without additional sensing component or material. The fabricated sensor exhibits excellent temperature sensitivity of up to 71.8 mV/°C. In addition, a 64 × 64 temperature sensor array with 3T sensing pixel and integrated gate driver has also been fabricated, which demonstrates potential approach for maxing out the performance of high‐luminance AM micro‐LED display with real‐time in‐pixel temperature monitoring.     

Vt Compensated voltage‐data a‐Si TFT AMOLED pixel circuits

Abstract— Active‐matrix organic light‐emitting‐diode (AMOLED) displays are now entering the marketplace. The use of a thin‐film‐transistor (TFT) active matrix allows OLED displays to be larger in size, higher in resolutions and lower in power dissipation than is possible using a conventional passive matrix. A number of TFT active‐matrix pixel circuits have been developed for luminance control, while correcting for initial and electrically stressed TFT parameter variations. Previous circuits and driving methods are reviewed. A new driving method is presented in which the threshold‐voltage (V_t) compensation performance, along with various circuit improvements for amorphous‐silicon (a‐Si) TFT pixel circuits using voltage data, are discussed. This new driving method along with various circuit improvements is demonstrated in a state‐of‐the‐art 20‐in. a‐Si TFT AMOLED HDTV.     

Full‐color active‐matrix OLED based on a‐Si TFT technology

Abstract— We have successfully demonstrated a 4‐in. full‐color active‐matrix OLED display based on amorphous‐Si (a‐Si) TFT technology. With improvements in the TFT manufacturing process and structure, a‐Si TFTs provide abundant capability to drive OLEDs. This demonstration clearly shows the possibility of using a‐Si TFTs as driving backplanes in the manufacture of full‐color AMOLEDs.     

Surface‐light‐emitting transistors based on vertical‐type metal‐base organic transistors

Abstract— Light‐emitting transistors having a metal‐base organic transistor (MBOT) structure demonstrate both the function of an organic thin‐film transistor (OTFT) and organic light‐emitting diode (OLED). The MBOT is a vertical‐type organic transistor having a simple structure composed of organic/metal/organic layers demonstrating high‐current and low‐voltage operation. The light‐emitting MBOT was fabricated simply by inserting additional layers of hole‐transporting and emissive materials used in the OLED into the col lector layer. The device showed perfect surface emission similar to an OLED. A luminance modulation of 370 cd/m² was observed at a collector voltage of 20 V and a base voltage of 3 V. This device can be applied to an OLED display device to increase the numerical aperture or reduce the required current of the TFT backplane.     

Ink‐jet‐printable phosphorescent organic light‐emitting‐diode devices

Abstract— A novel method for the fabrication of ink‐jet‐printed organic light‐emitting‐diode devices is discussed. Unlike previously reported solution‐processed OLED devices, the emissive layer of OLED devices reported here does not contain polymeric materials. The emission of the ink‐jet‐printed P²OLED (IJ‐P²OLED) device is demonstrated for the first time. It shows good color and uniform emission although it uses small‐molecule solution. Ink‐jet‐printed green P²OLED devices possess a high luminous efficiency of 22 cd/A at 2000 cd/m² and is based on phosphorescent emission. The latest solution‐processed phosphorescent OLED performance by spin‐coating is disclosed. The red P²OLED exhibits a projected LT50 of >53,000 hours with a luminous efficiency of 9 cd/A at 500 cd/m². The green P²OLED shows a projected LT50 of >52,000 hours with a luminous efficiency of 35 cd/A at 1000 cd/m². Also discussed is a newly developed sky‐blue P²OLED with a projected LT50 of >3000 hour and a luminous efficiency of 18 cd/A at 500 cd/m².     

Static active‐matrix OLED display without pixel refresh enabled by amorphous‐silicon non‐volatile memory

Abstract— An active‐matrix organic light‐emitting‐diode (AMOLED) display which does not require pixel refresh is demonstrated. This was achieved by replacing the thin‐film transistor (TFT) that drives the OLED with a non‐volatile memory TFT, in a 2‐transistor pixel circuit. The threshold voltage of the non‐volatile‐memory TFT can be changed by applying programming voltage pulses to the gate electrode. This approach eliminates the need for storage capacitors, increases the pixel fill factor, and potentially reduces power consumption. Each pixel can be individually programmed or erased using a standard active‐matrix addressing scheme. The programmed image is stored in the display even if power is turned off.     

Fabrication of poly‐Si complementary metal oxide semiconductor inverter by all sputtering deposition process

Direct current sputtering was used for deposition of Si film for precursor film of excimer laser annealing, n⁺‐Si/p⁺‐Si film for source/drain contact, and SiO₂ film for gate insulator of polycrystalline silicon thin‐film transistor. Using these methods, poly‐Si thin‐film complementary metal oxide semiconductor inverter was fabricated by all sputtering process for the first time. The field‐effect mobility was, respectively, 6.5 and 12.5 cm²/Vs for n‐TFTs and p‐TFTs. This inverter exhibits a full rail‐to‐rail swing and abrupt voltage transfer characteristics over the entire voltage range, and the output voltage gain was ~117 at V_dd = 20 V.     

Inorganic light‐emitting diode displays using micro‐transfer printing

Large quantities of microscopic red, green, and blue light‐emitting diodes (LEDs) made of crystalline inorganic semiconductor materials micro‐transfer printed in large quantities onto rigid or flexible substrates form monochrome and color displays having a wide range of sizes and interesting properties. Transfer‐printed micro‐LED displays promise excellent environmental robustness, brightness, spatial resolution, and efficiency. Passive‐matrix and active‐matrix inorganic LED displays were constructed, operated, and their attributes measured. Tests demonstrate that inorganic micro‐LED displays have outstanding color, viewing angle, and transparency. Yield improvement techniques include redundancy, physical repair, and electronic correction. Micro‐transfer printing enables revolutionary manufacturing strategies in which microscale LEDs are first assembled into miniaturized micro‐system “light engines,” and then micro‐transfer printed and interconnected directly to metallized large‐format panels. This paper reviews micro‐transfer printing technology for micro‐LED displays.     

Active‐matrix organic light‐emitting diode displays with indium gallium zinc oxide thin‐film transistors and normal,inverted, and transparent organic light‐emitting diodes

Abstract— Active‐matrix organic light‐emitting diode (AMOLED) displays have gained wide attention and are expected to dominate the flat‐panel‐display industry in the near future. However, organic light‐emitting devices have stringent demands on the driving transistors due to their current‐driving characteristics. In recent years, the oxide‐semiconductor‐based thin‐film transistors (oxide TFTs) have also been widely investigated due to their various benefits. In this paper, the development and performance of oxide TFTs will be discussed. Specifically, effects of back‐channel interface conditions on these devices will be investigated. The performance and bias stress stability of the oxide TFTs were improved by inserting a SiO^x protection layer and an N²O plasma treatment on the back‐channel interface. On the other hand, considering the n‐type nature of oxide TFTs, 2.4‐in. AMOLED displays with oxide TFTs and both normal and inverted OLEDs were developed and their reliability was studied. Results of the checkerboard stimuli tests show that the inverted OLEDs indeed have some advantages due to their suitable driving schemes. In addition, a novel 2.4‐in. transparent AMOLED display with a high transparency of 45% and high resolution of 166 ppi was also demonstrated using all the transparent or semi‐transparent materials, based on oxide‐TFT technologies.     

Flexible active‐matrix OLED displays: Challenges and progress

Abstract— Organic light‐emitting‐device (OLED) devices are very promising candidates for flexible‐display applications because of their organic thin‐film configuration and excellent optical and video performance. Recent progress of flexible‐OLED technologies for high‐performance full‐color active‐matrix OLED (AMOLED) displays will be presented and future challenges will be discussed. Specific focus is placed on technology components, including high‐efficiency phosphorescent OLED technology, substrates and backplanes for flexible displays, transparent compound cathode technology, conformal packaging, and the flexibility testing of these devices. Finally, the latest prototype in collaboration with LG. Phillips LCD, a flexible 4‐in. QVGA full‐color AMOLED built on amorphous‐silicon backplane, will be described.     

Suspension‐deposited carbon‐nanotube networks for flexible active‐matrix displays

Abstract— The unique properties of carbon nanotubes (CNTs) promise innovative solutions for a variety of display applications. The CNTs can be deposited from suspension. These simple and low‐cost techniques will replace time‐consuming and costly vacuum processes and can be applied to large‐area glass and flexible substrates. Single‐walled carbon nanotubes (SWNTs) have been used as conducting and transparent layers, replacing the brittle ITO, and as the semiconducting layer in thin‐film transistors (TFTs). There is no need for alignment because a CNT network is used instead of single CNTs. Both processes can be applied to glass and to flexible plastic substrates. The transparent and conductive nanotube layers can be produced with a sheet resistance of 400 Ω/□ at 80% transmittance. Such layers have been used to produce directly addressed liquid‐crystal displays and organic light‐emitting diodes (OLED). The CNT‐TFTs reach on/off ratios of more than 10⁵ and effective charge‐carrier mobilities of 1 cm²/V‐sec and above.     

Flexible electronic‐paper active‐matrix displays

Abstract— A100‐μm‐thick 320 × 240‐pixel active‐matrix display integrated into a functional‐device prototype is presented. The active matrix is composed of alternating layers of organic materials and gold. A six‐mask photolithographic process is used. An electrophoretic electronic imaging film is laminated on top of the active matrix. The display is bendable to a radius of 7.5 mm for more than 30,000 repetitions.     

A controller design for micro‐cup active‐matrix electrophoretic displays

Abstract— The active‐matrix electrophoretic display (AMEPD) has been commonly used for the applications of smart handheld reading devices such as e‐books and e‐newspapers. This paper presents a controller IC design for the AMEPD backend system which reduces the total hardware cost compared to that of the conventional design. By contrast, this study also provides a driving method for image displays. The prototyped controller is connected to a 6‐in. AMEPD panel, and good display quality has demonstrated the effectiveness of the proposed controller design.     

Fast voltage‐driving scheme for AMOLED displays based on internal feedback

Abstract— A new driving scheme for active‐matrix organic light‐emitting diodes (AMOLED) displays based on voltage programming is proposed. While conventional voltage drivers have a trade‐off between speed and accuracy, the new scheme is inherently fast and accurate. Based on the new driving scheme, a fast pixel circuit is designed using amorphous‐silicon (a‐Si) thin‐film transistors (TFTs). As the simulation results indicate, this pixel circuit can compensate the threshold‐voltage shift (VT shift) of the driver transistors. This pixel can be programmed in just 10 μsec, and it can compensate the threshold‐voltage shifts over 5 V with an error rate of less than 5% for a 1 ‐μA pixel current.     

Active‐matrix organic light‐emitting displays employing two thin‐film‐transistor a‐Si:H pixels on flexible stainless‐steel foil

Abstract— An active‐matrix organic light‐emitting diode (AMOLED) display driven by hydrogenated amorphous‐silicon thin‐film transistors (a‐Si:H TFTs) on flexible, stainless‐steel foil was demonstrated. The 2‐TFT voltage‐programmed pixel circuits were fabricated using a standard a‐Si:H process at maximum temperature of 280°C in a bottom‐gate staggered source‐drain geometry. The 70‐ppi monochrome display consists of (48 × 4) × 48 subpixels of 92 ×369 μm each, with an aperture ratio of 48%. The a‐Si:H TFT pixel circuits drive top‐emitting green electrophosphorescent OLEDs to a peak luminance of 2000 cd/m².     

A course on thin‐film transistor circuit design for modern displays

A new subject‐specific course on thin‐film transistor (TFT) circuit design is introduced, covering related knowledge of display technologies, TFT device physics, processing, characterization, modeling and circuit design. A design project is required for students to deepen the understanding even more and get hands‐on design experience. This course can be an intense 1‐week course to offer a full training of design engineers in an organized way to meet the ever‐increasing needs in display industry for TFT circuit design specialists. It can also be organized in one semester for electrical engineering Master's and Ph.D. students.