Large‐area FEDs with carbon‐nanotube field emitter

Abstract— Application of carbon nanotubes (CNTs) as field emitters for large‐area FED panels is described. In 1998, we presented the first experimental devices: light‐source tubes for outdoor large‐area displays and a diode‐type flat‐panel display, both with screen‐printed CNT cathodes. The fisrt practical high‐luminance color CNT‐FED panel was built in 1999. It employed the new triode‐structure panel was x‐y addressable. The CNT‐FED structure was further optimized for large‐area display panels by improving the luminous uniformity. This paper also describes the design and performance of a new, experimental, 40‐in.‐diagonal panel, which showed that the CNT‐FED technology is suitable for use in large‐area displays.     

Uniform carbon‐nanotube emitter for field‐emission displays

Abstract— The synthesis of carbon‐nanotube (CNT) field emitters for FEDs by thermal chemical vapor deposition (CVD) and their structural and emission characterization are described. Multi‐walled nanotubes (MWNTs) were grown on patterned metal‐base electrodes by thermal CVD, and the grown CNTs formed a network structured layer covering the surfaces of the metal electrode uniformly, which realized uniform distribution of electron emission. A technique for growing narrow MWNTs was also developed in order to reduce the driving voltage. The diameter of MWNT depends on the growth temperature, and it has changed from 40 nm at the low temperature (675°C) to 10–15 nm at the high temperature (900–1000°C). Moreover, narrower MWNTs were grown by using the metal‐base electrode covered with a thin alumina layer and a metal catalyst layer. Double‐walled nanotubes (DWNTs) were also observed among narrow MWNTs. The emission from the narrow CNTs showed a low turn‐on electric field of 1.5 V/μm at the as‐grown layer.     

Characteristics of surface‐conduction electron emitters made by ion‐exchange technology

Abstract— The principle and preparation method of the ion‐exchange technology used in surface‐conduction electron emitters is introduced in this paper. Surface‐conduction electron emitters through the use of ion‐exchange technology were developed. The relationships between device current and emission current on device voltage has been determined. The electronic emission characteristic of surface‐conduction electron emitters were also determined. The results indicate that the relationship between emission current and device voltage is non‐linear. A device‐voltage range between 20 and 27 V results in the best emission efficiency.     

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.     

Switching of carbon‐nanotube emitters by an integrated MOSFET

Abstract— The integration of carbon‐nanotube (CNT) emitters with a metal‐oxide‐semiconductor field‐effect transistor (MOSFET) can stabilize and control the emission current of CNTs. CNTs were grown by using the resist‐assisted patterning (RAP) process and plasma‐enhanced chemical vapor deposition (PECVD) and were connected to the drain part of an external MOSFET. The electron‐emission current of CNTs was switched by applying a low gate voltage to the MOSFET, and the switching current was very stable because the MOSFET was operated in the saturation region. Based on these results, the emission current of CNTs was stabilized and switched by using a low‐voltage‐driven MOSFET.     

Characteristics and stability of improved re‐crystallized metal‐induced laterally crystallized polycrystalline‐silicon thin‐film transistors for display applications

Abstract— The performance of high‐temperature re‐crystallized (RC) metal‐induced laterally crystallized (MILC) polycrystalline‐silicon (poly‐Si) thin‐film transistors (TFT) have been improved by (1) patterning the active islands before MILC, (2) removing nickel‐containing residues using acid cleaning, (3) using heavily boron‐doped poly‐Si gates to achieve threshold voltage symmetry, and (4) double‐implanting n‐type source/drain junctions. A 30‐MHz driver circuit based on this improved technology was demonstrated. The reliability of optimized RC‐MILC poly‐Si TFTs has not been adversely affected by residual nickel‐containing contaminants in the TFT channel regions.     

Practical CNT‐FED structure for high‐luminance character displays

Abstract— A high‐luminance CNT‐FED character display using a simple line rib structure was constructed. The display panel had 48 × 480 dots and the subpixel pitch was 1 mm. The greatest benefit of a display using CNT technology is high luminance performance with low‐power consumption. The luminance of the green‐color dot wasca. 10,000 cd/m² under 1/1 6 duty‐cycle driving at a 6‐kV anode voltage. The high luminance of the display panel can provide good visibility when installed even in outdoor locations, and the power consumption was ca. 4 W at the character displaying module. This, a CNT‐FED for character displays also has potential multifunctionality, which could be battery driven. It should be useful for public displays even under emergency no‐power conditions. In this work, a practical structure and process technologies for making ribs with reasonable cost were developed. The newly introduced 2‐mm‐tall line ribs as spacers were formed by using innovative production processes; i.e., the rib paste was pushed out of a multi‐slit nozzle, and the rib shape was formed by UV‐light irradiation. The developed panel structure and manufacturing processes also had the advantages of size flexibility and high production yield.     

Low‐temperature poly‐Si TFT data drivers for an SVGA a‐Si TFT‐LCD panel

Low‐temperature poly‐Si TFT data drivers for an SVGA a‐Si TFT‐LCD panel have been developed. The data drivers include shift registers, sample‐and‐hold circuits, and operational amplifiers, and drive LCD panels using a line‐at‐a‐time addressing method. To reduce the power consumption of the shift register, a dot‐clock control circuit has been developed. Using this circuit, the power consumption of the shift register has been reduced to 36% of that of conventional circuits. To cancel the offset voltage generated by the operational amplifier, an offset cancellation circuit for low‐temperature poly‐Si TFTs has been developed. This circuit is also able to avoid any unstable operation of the operational amplifier. Using this circuit, the offset voltage has been reduced to one‐third of the value without using the offset cancellation circuit. These data drivers have been connected to an LCD panel and have realized an SVGA display on a 12.1‐in. a‐Si TFT‐LCD panel.     

The effect of gas‐dynamic factors on selective carbon‐nanotube synthesis by injection CVD method for field‐emission cathodes

Abstract— Reversible selective growth of carbon‐nanotube (CNT) arrays on Si/SiO₂ topologies was investigated for field‐emission‐display applications. The method used was that of high‐temperature pyrolysis of fluid hydrocarbon (p‐xylene [C₈H₁₀]) in a mixture with volatile catalyst (ferrocene [Fe(C₅H₅)₂]) using Ar as the gas carrier. The synthesized CNT arrays were analyzed by SEM, TEM, Raman, and TGA analyses. Reversible CNT growth both on Si and SiO₂ surfaces was found to be sensitive to the gas‐carrier flow rate and the catalyst/hydrocarbon solution injection rate into the synthesis zone. This phenomenon can be explained by inverse domination of active sites on Si and SiO₂ surfaces at different flow rates of gas mixture, causing different types of catalyst precipitation followed by subsequent CNT growth. In principle, the possibility of growing CNTs using the proposed technology will allow the creation of precise geometries of field‐emission cathodes excluding the step of catalyst localization.     

A novel TFT‐OLED integration for OLED‐independent pixel programming in amorphous‐Si AMOLED pixels

Abstract— The direct voltage programming of active‐matrix organic light‐emitting‐diode (AMOLED) pixels with n‐channel amorphous‐Si (a‐Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new “inverted” integration technique which makes the direct programming possible by connecting the driver n‐channel a‐Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn‐on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active‐matrix OLED displays with a‐Si technology and conventional bottom‐anode OLEDs.     

Threshold‐voltage recovery of a‐Si:H digital circuits

Abstract— A‐Si:H thin‐film transistors demonstrate threshold voltage recovery of several volts after room‐temperature rest with no applied voltage. The extent to which this phenomenon can be used to extend the operational lifetime of a‐Si:H digital circuits is examined and is shown to be strictly limited.     

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.     

Fabrication of low‐temperature‐polysilicon thin‐film transistors on flexible substrates using excimer‐laser crystallization

Abstract— Low‐temperature‐polysilicon thin‐film transistors (LTPS TFTs) were fabricated on polymer substrates using sputtered amorphous‐Si (a‐Si) films and excimer‐laser crystallization. The in‐film argon concentration of a‐Si films was minimized as low as 1.6% by using an argon/helium gas mixture as the sputtering gas. By employing XeCl excimer‐laser crystallization, poly‐Si films were successfully fabricated on polymer substrates with an average grain size of 400 nm. With a four‐mask process, a poly‐Si TFT was fabricated with a fully self‐aligned top‐gate structure, and the pMOS TFT device showed a field‐effect mobility of 63.6 cm²/V‐sec, ON/OFF ratio of 10⁵, and threshold voltage of ?1.5 V.     

New applications for LTPS array technology including lab‐on‐chip and MEMS actuators

Abstract— The use of low‐temperature poly‐Si technology for new applications beyond displays is presented. These applications include lab‐on‐chip, MEMS actuators, and sensors. As a key example, the use of high‐voltage poly‐Si TFTs for rapid heating and temperature control, as is required for DNA amplification within lab‐on‐chip, is described in detail. Other examples given include MEMS ink‐jet printer heads and the formation of photosensors and impedance sensors for optical and electronic input, which can be used not only in displays and lab‐on‐chip, but also for new applications such as fingerprint sensing and particle counting.     

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.     

Noise‐margin analysis of a‐Si:H digital circuits

Abstract— The noise margin is one of the fundamental metrics in evaluating the viability and robustness of digital circuits. An analytical model of amorphous‐silicon digital‐circuit noise margin was developed, including the effects of circuit aging. The threshold voltage of a‐Si:H transistors increases over time with electrical stress, degrading the performance and eventually leading to circuit wear‐out. Since static and dynamic inverters are the basic digital‐circuit design elements, they are the basis for this analysis. The analytical model is verified with experimental measurements. The lifetime of dynamic a‐Si:H digital circuits is found to exceed the lifetime for static a‐Si:H circuits by a factor of 2–3. Although the lifetimes are relatively short (～10⁵ sec) and under continuous electrical stress, they are sufficient for low‐duty‐cycle applications.     

Solution‐processed oxide semiconductors for low‐cost and high‐performance thin‐film transistors and fabrication of organic light‐emitting‐diode displays

Abstract— High‐performance solution‐processed oxide‐semiconductor (OS) thin‐film transistors (TFTs) and their application to a TFT backplane for active‐matrix organic light‐emitting‐diode (AMOLED) displays are reported. For this work, bottom‐gated TFTs having spin‐coated amorphous In‐Zn‐O (IZO) active layers formed at 450°C have been fabricated. A mobility (μ) as high as 5.0 cm²/V‐sec, ?0.5 V of threshold voltage (VT), 0.7 V/dec of subthreshold swing (SS), and 6.9 × 10⁸ of on‐off current ratio were obtained by using an etch‐stopper (ES) structure TFT. TFTs exhibited uniform characteristics within 150 × 150‐mm² substrates. Based on these results, a 2.2‐in. AMOLED display driven by spin‐coated IZO TFTs have also been fabricated. In order to investigate operation instability, a negative‐bias‐temperature‐stress (NBTS) test was carried out at 60°C in ambient air. The IZO‐TFT showed ?2.5 V of threshold‐voltage shift (ΔVT) after 10,800 sec of stress time, comparable with the level (ΔVT = ?1.96 V) of conventional vacuum‐deposited a‐Si TFTs. Also, other issues regarding solution‐processed OS technology, including the instability, lowering process temperature, and printable devices are discussed.     

Effects of post‐annealing on a‐Si:H TFT characteristics fabricated on stainless‐steel substrate

Abstract— A 14.1‐in.‐diagonal backplane employing hydrogenated amorphous‐silicon thin‐film transistors (a‐Si:H TFTs) was fabricated on a flexible stainless‐steel substrate. The TFTs exhibited a field‐effect mobility of 0.54 cm²/V‐sec, a threshold voltage of 1.0 V, and an off‐current of 10^?13 A. Most of the electrical characteristics were comparable to those of the TFTs fabricated on glass substrates. To increase the stability of a‐Si:H TFTs fabricated on stainless‐steel substrate, the specimens were thermally annealed at 230°C. The field‐effect mobility was reduced to 71% of the initial value because of the strain of the released hydrogen atoms and residual compressive stress in a‐Si:H TFT under thermal annealing at 230°C.     

Temperature instability of low‐temperature deposited a‐Si:H TFTs fabricated on plastic substrate

Abstract— Low‐temperature deposited a‐Si:H TFTs have been successfully fabricated on colorless polyimide (CPI) substrate for flexible‐display applications. A serious degradation in threshold voltage was observed after applying external thermal stress. The threshold‐voltage shift saturates after applying several thermal stress cycles. In addition, the TFTs show instability under long periods of thermal stress with fixed temperature. This phenomenon was composed of thermally induced traps and substrate‐expansion‐induced mechanical stress. Finally, the a‐Si:H TFT backplane fabricated on a PI substrate at low temperature has been successfully demonstrated for flexible AMLCDs.     

Towards a low‐cost high‐quality carbon‐nanotube field‐emission display

Abstract— We have developed field‐emission‐display technology driven by chemical‐vapor‐deposition‐grown carbon‐nanotube emitters incorporated in a simple, low‐cost device structure. Here, we report on frit‐sealed test displays with a brightness of 3000 cd/m² at 3 kV and a lifetime of 9000 hours with only 45% degradation. We also demonstrate the scalability of the technology with a uniform high‐brightness 6‐in. QVGA that displays video images with a switching voltage of 40 V.