Showing papers on "Thin-film transistor published in 2004"

Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors

Abstract: Transparent electronic devices formed on flexible substrates are expected to meet emerging technological demands where silicon-based electronics cannot provide a solution. Examples of active flexible applications include paper displays and wearable computers1. So far, mainly flexible devices based on hydrogenated amorphous silicon (a-Si:H)2,3,4,5 and organic semiconductors2,6,7,8,9,10 have been investigated. However, the performance of these devices has been insufficient for use as transistors in practical computers and current-driven organic light-emitting diode displays. Fabricating high-performance devices is challenging, owing to a trade-off between processing temperature and device performance. Here, we propose to solve this problem by using a novel semiconducting material—namely, a transparent amorphous oxide semiconductor from the In-Ga-Zn-O system (a-IGZO)—for the active channel in transparent thin-film transistors (TTFTs). The a-IGZO is deposited on polyethylene terephthalate at room temperature and exhibits Hall effect mobilities exceeding 10 cm2 V-1 s-1, which is an order of magnitude larger than for hydrogenated amorphous silicon. TTFTs fabricated on polyethylene terephthalate sheets exhibit saturation mobilities of 6–9 cm2 V-1 s-1, and device characteristics are stable during repetitive bending of the TTFT sheet.

Wide-bandgap high-mobility ZnO thin-film transistors produced at room temperature

Abstract: We report high-performance ZnO thin-film transistor (ZnO-TFT) fabricated by rf magnetron sputtering at room temperature with a bottom gate configuration. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 19V, a saturation mobility of 27cm2∕Vs, a gate voltage swing of 1.39V∕decade and an on/off ratio of 3×105. The ZnO-TFT presents an average optical transmission (including the glass substrate) of 80% in the visible part of the spectrum. The combination of transparency, high mobility, and room-temperature processing makes the ZnO-TFT a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics.

Introduction to Organic Thin Film Transistors and Design of n-Channel Organic Semiconductors

Abstract: The development of new organic semiconductors with improved performance in organic thin film transistors (OTFTs) is a major challenge for materials chemists. There is a particular need to develop air-stable n-channel (electron-conducting) organic semiconductors with performance comparable to that of p-channel (hole-conducting) materials, for organic electronics to realize the benefits of complementary circuit design, i.e., the ability to switch transistors with either positive or negative gate voltages. There have been significant advancements in the past five years. In terms of standard OTFT metrics such as the field effect mobility (μFET) and on-to-off current ratio (ION/IOFF), n-channel OTFTs have achieved performance comparable both to that of n-channel amorphous silicon TFTs and to that of the best reported p-channel (hole-conducting) OTFTs; however, issues of device stability linger. This review provides a detailed introduction to OTFTs, summarizes recent progress in the development of new n-channel...

Semiconductor device, manufacturing method, and electronic device

Abstract: In a thin film transistor ( 1 ), a gate insulating layer ( 4 ) is formed on a gate electrode ( 3 ) formed on an insulating substrate ( 2 ). Formed on the gate insulating layer ( 4 ) is a semiconductor layer ( 5 ). Formed on the semiconductor layer ( 5 ) are a source electrode ( 6 ) and a drain electrode ( 7 ). A protective layer ( 8 ) covers them, so that the semiconductor layer ( 5 ) is blocked from an atmosphere. The semiconductor layer ( 5 ) (active layer) is made of, e.g., a semiconductor containing polycrystalline ZnO to which, e.g., a group V element is added. The protective layer ( 8 ) thus formed causes decrease of a surface level of the semiconductor layer ( 5 ). This eliminates a depletion layer spreading therewithin. Accordingly, the ZnO becomes an n-type semiconductor indicating an intrinsic resistance, with the result that too many free electrons are generated. However, the added element works on the ZnO as an accepter impurity, so that the free electrons are reduced. This decreases a gate voltage required for removal of the free electrons, so that the threshold voltage of the thin film transistor ( 1 ) becomes on the order of 0 V. This allows practical use of a semiconductor device which has an active layer made of zinc oxide and which includes an protective layer for blocking the active layer from an atmosphere.

Indium oxide-based thin film transistors and circuits

Abstract: In electronic displays or imaging units, the control of pixels is achieved by an array of transistors. These transistors are in a thin film form and arranged in a two-dimensional configuration to form switching circuits, driving circuits or even read-out circuits. In this invention, thin film transistors and circuits with indium oxide-based channel layers are provided. These thin film transistors and circuits may be fabricated at low temperatures on various substrates and with high charge carrier mobilities. In addition to conventional rigid substrates, the present thin film transistors and circuits are particularly suited for the fabrication on flexible and transparent substrates for electronic display and imaging applications. Methods for the fabrication of the thin film transistors with indium oxide-based channels are provided.

High-performance semiconducting polythiophenes for organic thin-film transistors.

Abstract: Conjugated polymers have been widely studied as potential semiconductor materials for organic thin-film transistors (TFTs). However, they have provided functionally poor transistor properties when the TFTs are fabricated in air. We have developed a class of liquid crystalline regioregular polythiophenes, PQTs, that possess sufficient air stability to enable achievement of excellent TFT properties under ambient conditions. These polythiophenes exhibit unique self-assembly ability and form highly structured thin films when deposited from solution under appropriate conditions. TFTs fabricated in air with PQT channel layers have provided high field-effect mobility to 0.14 cm2 V-1 s-1 and high current modulation to over 107, together with other desirable transistor properties. These high-performance polythiophenes will therefore help bring the long-standing concept of low-cost organic/polymer transistor circuits closer to commercial reality.

Low-voltage organic transistors with an amorphous molecular gate dielectric

Abstract: Organic thin film transistors (TFTs) are of interest for a variety of large-area electronic applications, such as displays, sensors and electronic barcodes. One of the key problems with existing organic TFTs is their large operating voltage, which often exceeds 20 V. This is due to poor capacitive coupling through relatively thick gate dielectric layers: these dielectrics are usually either inorganic oxides or nitrides, or insulating polymers, and are often thicker than 100 nm to minimize gate leakage currents. Here we demonstrate a manufacturing process for TFTs with a 2.5-nm-thick molecular self-assembled monolayer (SAM) gate dielectric and a high-mobility organic semiconductor (pentacene). These TFTs operate with supply voltages of less than 2 V, yet have gate currents that are lower than those of advanced silicon field-effect transistors with SiO2 dielectrics. These results should therefore increase the prospects of using organic TFTs in low-power applications (such as portable devices). Moreover, molecular SAMs may even be of interest for advanced silicon transistors where the continued reduction in dielectric thickness leads to ever greater gate leakage and power dissipation.

Semiconductor device and method for forming the same

Abstract: In fabricating a thin film transistor, an active layer comprising a silicon semiconductor is formed on a substrate having an insulating surface Hydrogen is introduced into The active layer A thin film comprising SiO x N y is formed to cover the active layer and then a gate insulating film comprising a silicon oxide film formed on the thin film comprising SiO x N y Also, a thin film comprising SiO x N y is formed under the active layer The active layer includes a metal element at a concentration of 1×10 15 to 1×10 19 cm −3 and hydrogen at a concentration of 2×10 19 to 5×10 21 cm −3

Semiconductor device, its manufacturing method, and electronic device

Abstract: A thin film transistor (1) wherein a gate electrode (3) is formed on an insulative substrate (2), a gate insulating layer (4) is formed on the gate electrode (3), a semiconductor layer (5) is formed on the gate insulating layer (4), a source electrode (6) and a drain electrode (7) are formed on the semiconductor layer (5), and a protective layer (8) covering them are formed. The semiconductor layer (5) is isolated from the atmosphere. The semiconductor layer (5) (active layer) is formed of a ZnO polycrystalline semiconductor doped with, for example, a group V element. Since the surface state of the ZnO semiconductor is reduced thanks to the protective layer (8) and inward expansion of the depletion layer is prevented, the ZnO semiconductor is of an n-type showing its intrinsic resistance value and contains excessive free electrons. The added element acts as acceptor impurities in the ZnO semiconductor, decreasing the excessive electrons. Thus the gate voltage to eliminate the excessive free electrons lowers, thereby making the threshold voltage around 0 V. A semiconductor device using a zinc oxide for an active layer and having a protective layer for isolating the active layer from the atmosphere can be actually used.

Threshold voltage shift in organic field effect transistors by dipole monolayers on the gate insulator

Abstract: We demonstrate controllable shift of the threshold voltage and the turn-on voltage in pentacene thin film transistors and rubrene single crystal field effect transistors (FET) by the use of nine organosilanes with different functional groups. Prior to depositing the organic semiconductors, the organosilanes were applied to the SiO2 gate insulator from solution and form a self-assembled monolayer (SAM). The observed shifts of the transfer characteristics range from −2to50V and can be related to the surface potential of the layer next to the transistor channel. Concomitantly the mobile charge carrier concentration at zero gate bias reaches up to 4×1012∕cm2. In the single crystal FETs the measured transfer characteristics are also shifted, while essentially maintaining the high quality of the subthreshold swing. The shift of the transfer characteristics is governed by the built-in electric field of the SAM and can be explained using a simple energy level diagram. In the thin film devices, the subthreshold re...

Solution Casting and Transfer Printing Single-Walled Carbon Nanotube Films

Abstract: This paper presents methods for solution casting and transfer printing collections of individual single-walled carbon nanotubes (SWNTs) onto a wide range of substrates, including plastic sheets. The deposition involves introduction of a solvent that removes surfactant from a suspension of SWNTs as it is applied to a substrate. The subsequent controlled flocculation (cF) produces films of SWNTs with densities that can be varied between a few tubes per square micron to thick multilayers in a single deposition step and with orientation determined by the direction of solution flow. High-resolution rubber stamps inked in this manner can be used to print patterns of tubes with geometries defined by the relief structure on the surface of the stamp. Thin film transistors fabricated with these techniques demonstrate their potential use in flexible “macroelectronic” systems.

Threshold Voltage Shift in Organic Field Effect Transistors by Dipole-Monolayers on the Gate Insulator

Abstract: We demonstrate controllable shift of the threshold voltage and the turn-on voltage in pentacene thin film transistors and rubrene single crystal field effect transistors (FET) by the use of nine organosilanes with different functional groups. Prior to depositing the organic semiconductors, the organosilanes were applied to the SiO2 gate insulator from solution and form a self assembled monolayer (SAM). The observed shift of the transfer characteristics range from -2 to 50 V and can be related to the surface potential of the layer next to the transistor channel. Concomitantly the mobile charge carrier concentration at zero gate bias reaches up to 4*10^12/cm^2. In the single crystal FETs the measured transfer characteristics are also shifted, while essentially maintaining the high quality of the subthreshold swing. The shift of the transfer characteristics is governed by the built-in electric field of the SAM and can be explained using a simple energy level diagram. In the thin film devices, the subthreshold region is broadened, indicating that the SAM creates additional trap states, whose density is estimated to be of order 1*10^12/cm^2.

Organic Thin Film Transistors Based on N-Alkyl Perylene Diimides: Charge Transport Kinetics as a Function of Gate Voltage and Temperature

Abstract: We report structural and electrical transport properties of a family of π-stacking soluble organic semiconductors, N,N‘-dialkyl-3,4,9,10-perylene tetracarboxylic diimides (alkyl − pentyl [1], octyl [2], and dodecyl [3]). The structures of evaporated polycrystalline films of 1−3 were studied using X-ray diffraction and atomic force microscopy. Films of 1−3 pack similarly with the direction of π−π overlap in the substrate plane. Organic thin film transistors (OTFTs) based on 1−3 deposited on SiO2 gate dielectric showed linear regime electron mobilities of 0.1, 0.6, and 0.2 cm2/(V s), respectively, corrected for contact resistance. OTFTs of 2 had saturation electron mobilities as high as 1.7 cm2/(V s) with on-to-off current ratios of 107. Variable temperature measurements were used to examine the charge transport kinetics in the range 80−300 K and revealed (1) thermally activated electron mobilities with activation energies dependent on gate voltage and (2) the presence of well-defined isokinetic points, i.e...

Thin Film Deposition, Patterning, and Printing in Organic Thin Film Transistors

Abstract: Organic thin film transistors (OTFTs) will play an important role in future plastic electronic devices. The device performance is greatly affected by the molecular structure and morphology of the organic semiconductors. Various methods for organic semiconductor deposition are reviewed. Recent progress in printing and patterning of OTFTs are also surveyed.

A printable form of silicon for high performance thin film transistors on plastic substrates

Abstract: Free-standing micro- and nanoscale objects of single crystal silicon can be fabricated from silicon-on-insulator wafers by lithographic patterning of resist, etching of the exposed top silicon, and removing the underlying SiO2 to lift-off the remaining silicon. A large collection of such objects constitutes a type of material that can be deposited and patterned, by dry transfer printing or solution casting, onto plastic substrates to yield mechanically flexible thin film transistors that have excellent electrical properties. Effective mobilities of devices built with this material, which we refer to as microstructured silicon (μs-Si), are demonstrated to be as high as 180cm2∕Vs on plastic substrates. This form of “top down” microtechnology might represent an attractive route to high performance flexible electronic systems.

Structural Characterization of a Pentacene Monolayer on an Amorphous SiO2 Substrate with Grazing Incidence X-ray Diffraction

Abstract: Grazing incidence X-ray diffraction reveals that a pentacene monolayer, grown on an amorphous SiO2 substrate that is commonly used as a dielectric layer in organic thin film transistors (OTFTs), is crystalline. A preliminary energy-minimized model of the monolayer, based on the GIXD data, reveals that the pentacene molecules adopt a herringbone arrangement with their long axes tilted slightly from the substrate normal. Although this arrangement resembles the general packing features of the (001) layer in single crystals of bulk pentacene, the monolayer lattice parameters and crystal structure differ from those of the bulk. Because carrier transport in pentacene OTFTs is presumed to occur in the semiconductor layers near the dielectric interface, the discovery of a crystalline monolayer structure on amorphous SiO2 has important implications for transport in OTFTs.

p-Channel, n-Channel Thin Film Transistors and p−n Diodes Based on Single Wall Carbon Nanotube Networks

Abstract: This paper demonstrates the use of arrays of networks of single wall carbon nanotubes (SWNTs) and electrical breakdown procedures for building thin film transistors (TFTs) that have good, reproducible performance and high current output. Channel length scaling analysis of these TFTs indicates that the resistance at the source/drain contacts is a small fraction of the device resistance, in the linear regime. When measured with the channel exposed to air or coated by poly(methyl methacrylate) (PMMA), these transistors operate in the unipolar p mode. By spin-coating the polymer polyethylenimine (PEI) on the channel region, these transistors can be switched to operate in the unipolar n mode. Patterning the exposure of a single channel to PMMA and PEI yields p -n diodes. These results indicate that SWNT-TFTs can provide the building blocks of complex complementary circuits for a range of applications in macroelectronics, sensors, and other systems.

Enhancement of Field-Effect Mobility by Surface-Mediated Molecular Ordering in Regioregular Polythiophene Thin Film Transistor

Abstract: With the aim of enhancing the field-effect mobility by promoting surface-mediated two-dimensional molecular ordering in self-aligned regioregular poly(3-hexylthiophene) (P3HT) we have controlled the intermolecular interaction at the interface between P3HT and the insulator substrate by using self-assembled monolayers (SAMs) functionalized with various groups (–NH2, –OH, and –CH3). We have found that, depending on the properties of the substrate surface, the P3HT nanocrystals adopt two different orientations—parallel and perpendicular to the insulator substrate—which have field-effect mobilities that differ by more than a factor of 4, and that are as high as 0.28 cm2 V–1 s–1. This surprising increase in field-effect mobility arises in particular for the perpendicular orientation of the nanocrystals with respect to the insulator substrate. Further, the perpendicular orientation of P3HT nanocrystals can be explained by the following factors: the unshared electron pairs of the SAM end groups, the π–H interactions between the thienyl-backbone bearing π-systems and the H (hydrogen) atoms of the SAM end groups, and interdigitation between the alkyl chains of P3HT and the alkyl chains of the SAMs.

Photochemical Stability of Pentacene and a Substituted Pentacene in Solution and in Thin Films

Abstract: The organic semiconductor pentacene (1) has shown the highest field effect mobilities in thin films of any organic semiconductor, yet suffers from instability toward oxidation. 6,13-Bis(triisopropylsilylethynyl)pentacene (2) has been reported as an interesting functionalized pentacene which is soluble in common organic solvents and exhibits high carrier mobility (>0.1 cm2/Vs) in thin film transistor devices. In our investigations of 2, we were surprised by its remarkable stability in solution. Using UV−vis spectroscopy we observe that under ambient light conditions, 2 is approximately 50× more stable toward degradation in air-saturated tetrahydrofuran solution as compared to unsubstituted pentacene. Previous investigators have implicated oxygen in the mechanism of photodegradation of pentacene. In this study, quantum chemical calculations have been performed which demonstrate that alkynyl functionalization at the 6 and 13 positions reduces the rate of photooxidation in two ways. First, alkynyl substitutio...

Electro-optical device and electronic apparatus

Abstract: An electro-optical device of the present invention includes pixel electrodes, TFTs electrically connected to the pixel electrodes, and scanning lines and data lines connected to the TFTs on a TFT array substrate. Each scanning line has a broad width portion as a gate electrode in a portion facing a channel area of the TFT and the narrow width portion. As a result, an electro-optical device capable of easily adjusting an arrangement of the gate electrodes and the scanning lines and at the same time precisely driving the TFTs is provided.

Influence of the dielectric roughness on the performance of pentacene transistors

Abstract: The properties of the dielectric strongly influence the performance of organic thin-film transistors. In this letter, we show experimental results that quantify the influence of the roughness of the dielectric on the mobility of pentacene transistors and discuss the cause of it. We consider the movement of charge carriers out of the “roughness valleys” or across those valleys at the dielectric–semiconductor interface as the limiting step for the roughness-dependent mobility in the transistor channel.

ZnO-channel thin-film transistors: Channel mobility

Abstract: ZnO-channel thin-film transistor (TFT) test structures are fabricated using a bottom-gate structure on thermally oxidized Si; ZnO is deposited via RF sputtering from an oxide target, with an unheated substrate. Electrical characteristics are evaluated, with particular attention given to the extraction and interpretation of transistor channel mobility. ZnO-channel TFT mobility exhibits severe deviation from that assumed by ideal TFT models; mobility extraction methodology must accordingly be recast so as to provide useful insight into device operation. Two mobility metrics, μavg and μinc, are developed and proposed as relevant tools in the characterization of nonideal TFTs. These mobility metrics are employed to characterize the ZnO-channel TFTs reported herein; values for μinc as high as 25 cm2/V s are measured, comprising a substantial increase in ZnO-channel TFT mobility as compared to previously reported performance for such devices.

Tin oxide transparent thin-film transistors

Abstract: A SnO2 transparent thin-film transistor (TTFT) is demonstrated. The SnO2 channel layer is deposited by RF magnetron sputtering and then rapid thermal annealed in O2 at 600°C. The TTFT is highly transparent, and enhancement-mode behaviour is achieved by employing a very thin channel layer (10–20 nm). Maximum field-effect mobilities of 0.8 cm2 V−1 s−1 and 2.0 cm2 V−1 s−1 are obtained for enhancement- and depletion-mode devices, respectively. The transparent nature and the large drain current on-to-off ratio of 105 associated with the enhancement-mode behaviour of these devices may prove useful for novel gas-sensor applications.

Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic

Abstract: This paper presents design considerations along with measurement results pertinent to hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) drive circuits for active matrix organic light emitting diode (AMOLED) displays. We describe both pixel architectures and TFT circuit topologies that are amenable for vertically integrated, high aperture ratio pixels. Here, the OLED layer is integrated directly above the TFT circuit layer, to provide an active pixel area that is at least 90% of the total pixel area with an aperture ratio that remains virtually independent of scaling. Both voltage-programmed and current-programmed drive circuits are considered. The latter provides compensation for shifts in device characteristics due to metastable shifts in the threshold voltage of the TFT. Various drive circuits on glass and plastic were fabricated and tested. Integration of on-panel gate drivers is also discussed where we present the architecture of an a-Si:H based gate de-multiplexer that is threshold voltage shift invariant. In addition, a programmable current mirror with good linearity and stability is presented. Programmable current sources are an essential requirement in the design of source driver output stages.

Gated four-probe measurements on pentacene thin-film transistors: Contact resistance as a function of gate voltage and temperature

Abstract: We describe gated four-probe measurements designed to measure contact resistance in pentacene-based organic thin-film transistors (OTFTs) The devices consisted of metal source and drain electrodes contacting a 300-A-thick pentacene film thermally deposited on Al2O3 or SiO2 dielectrics with a p-doped Si substrate serving as the gate electrode Voltage-sensing leads extending into the source-drain channel were used to monitor potentials in the pentacene film while passing current during drain voltage (VD) or gate voltage (VG) sweeps We investigated the potential profiles as a function of contact metallurgy (Pt, Au, Ag, and Ca), substrate chemistry, VG, and temperature The contact-corrected linear hole mobilities were as high as 175cm2∕Vs and the film sheet resistance and specific contact resistance were as low as 600kΩ∕◻ and 13kΩ-cm, respectively, at high gate voltages In the temperature range of 50–200K, the pentacene OTFTs displayed an activated behavior with activation energies of 15–30meV Importa

All jet-printed polymer thin-film transistor active-matrix backplanes

Abstract: Thin-film transistor (TFT) backplanes fabricated by using jet printing as the only patterning method are reported. Additive and subtractive printing processes are combined to make 128×128 pixel active matrix arrays with 340μm pixel size. The semiconductor used, a regioregular polythiophene, poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene]; (PQT-12) is deposited by inkjet printing and exhibits average TFT mobility of 0.06cm2∕Vs, on/off ratios of 106, and minimal bias stress. The printed TFTs have high yield with a narrow performance distribution. The pixel design benefits from the registration accuracy of jet printing and it is shown that the electrical performance is suitable for addressing capacitive media displays.

Ambipolar Organic Field-Effect Transistors Based on a Solution-Processed Methanofullerene

Abstract: Organic field-effect transistors (OFETs, see Figure), based on the solution-processible methanofullerene [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM), have been fabricated in a bottom-contact device configuration using gold electrodes. The OFET functions either as a p- or n-channel device, depending upon the bias conditions. This is the first example of ambipolar charge transport in FETs based on pristine PCBM.

Organic thin-film transistors with nanocomposite dielectric gate insulator

Abstract: High-performance organic thin-film transistors (OTFTs) with a nanoparticle composite dielectric layer have been demonstrated. The dielectric layer consists of cross-linked poly-4-vinylphenol (PVP) and high-dielectric titanium dioxide (TiO2) nanoparticles. Because of the nanosize of TiO2, it disperses well in the organic solvent, which makes it possible to use solution-processable methods to prepare the dielectric layer. OTFTs with pentacene as the semiconducting layers have been demonstrated; it was found that the OTFTs with the nanocomposite dielectric layer have higher field-induced current than that of conventional devices because the dielectric constant of the gate insulator is increased. This finding opens an interesting direction for the preparation of high-performance OTFTs without complicated sputtering of high-κ dielectric materials.

Organic TFT array on a paper substrate

Abstract: We demonstrate a high-performance organic thin-film transistor array fabricated on a flexible paper substrate. As a water and chemical barrier layer, 6-/spl mu/m-thick parylene has been coated on the paper substrate by using the vacuum deposition. The parylene layer protects the paper substrate from becoming damp during the wet chemical process. Using poly (3-hexylthiophene) as an active layer, a high-performance organic transistor with field effect mobility up to 0.086 cm/sup 2//V/spl middot/s and an on/off ratio of 10/sup 4/ can be achieved. Organic transistors built on a cheap paper substrate open a channel for future applications in flexible and disposable electronics with extremely low-cost.

Bias-induced threshold voltages shifts in thin-film organic transistors

Abstract: An investigation into the stability of metal-insulator-semiconductor (MIS) transistors based on α-sexithiophene is reported. In particular, the kinetics of the threshold voltage shift upon application of a gate bias has been determined. The kinetics follow stretched-hyperbola-type behavior, in agreement with the formalism developed to explain metastability in amorphous-silicon thin-film transistors. Using this model, quantification of device stability is possible. Temperature-dependent measurements show that there are two processes involved in the threshold voltage shift, one occurring at T≈220 K and the other at T≈300 K. The latter process is found to be sample dependent. This suggests a relation between device stability and processing parameters.