Thin-film transistor

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Several types of TFT constructions. Thin-film transistor variants EN.svg
Several types of TFT constructions.

A thin-film transistor (TFT) is a special kind of field-effect transistor made by depositing thin films of an active semiconductor layer as well as the dielectric layer and metallic contacts over a supporting (but non-conducting) substrate. A common substrate is glass, because the primary application of TFTs is in liquid-crystal displays (LCDs). This differs from the conventional transistor, where the semiconductor material typically is the substrate, such as a silicon wafer.

The field-effect transistor (FET) is an electronic device which uses an electric field to control the flow of current. This is achieved by the application of a voltage to the gate terminal, which in turn alters the conductivity between the drain and source terminals.

A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, LEDs, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.

A semiconductor material has an electrical conductivity value falling between that of a metal, like copper, gold, etc. and an insulator, such as glass. Their resistance decreases as their temperature increases, which is behaviour opposite to that of a metal. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, and gallium arsenide. After silicon, gallium arsenide is the second most common semiconductor used in laser diodes, solar cells, microwave frequency integrated circuits, and others. Silicon is a critical element for fabricating most electronic circuits.

Contents

Manufacture

TFTs can be made using a wide variety of semiconductor materials. A common material is silicon. The characteristics of a silicon-based TFT depend on the silicon's crystalline state; that is, the semiconductor layer can be either amorphous silicon, [1] microcrystalline silicon, [1] or it can be annealed into polysilicon.

Silicon Chemical element with atomic number 14

Silicon is a chemical element with symbol Si and atomic number 14. It is a hard and brittle crystalline solid with a blue-grey metallic lustre; and it is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic table: carbon is above it; and germanium, tin, and lead are below it. It is relatively unreactive. Because of its high chemical affinity for oxygen, it was not until 1823 that Jöns Jakob Berzelius was first able to prepare it and characterize it in pure form. Its melting and boiling points of 1414 °C and 3265 °C respectively are the second-highest among all the metalloids and nonmetals, being only surpassed by boron. Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure element in the Earth's crust. It is most widely distributed in dusts, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates. More than 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust after oxygen.

Crystal solid material whose constituent atoms, molecules, or ions are arranged in an ordered pattern extending in all three spatial dimensions

A crystal or crystalline solid is a solid material whose constituents are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.

Amorphous silicon

Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs.

Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenide, [2] [3] or metal oxides such as zinc oxide [4] or hafnium oxide. An application for hafnium oxide is as a high-κ dielectric. [5] TFTs have also been made using organic materials, referred to as organic field-effect transistors or OTFTs.

Cadmium selenide chemical compound

Cadmium selenide is an inorganic compound with the formula CdSe. It is a black to red-black solid that is classified as a II-VI semiconductor of the n-type. Much of the current research on cadmium selenide is focused on its nanoparticles.

The term high-κ dielectric refers to a material with a high dielectric constant κ. High-κ dielectrics are used in semiconductor manufacturing processes where they are usually used to replace a silicon dioxide gate dielectric or another dielectric layer of a device. The implementation of high-κ gate dielectrics is one of several strategies developed to allow further miniaturization of microelectronic components, colloquially referred to as extending Moore's Law.

Organic field-effect transistor

An organic field-effect transistor (OFET) is a field-effect transistor using an organic semiconductor in its channel. OFETs can be prepared either by vacuum evaporation of small molecules, by solution-casting of polymers or small molecules, or by mechanical transfer of a peeled single-crystalline organic layer onto a substrate. These devices have been developed to realize low-cost, large-area electronic products and biodegradable electronics. OFETs have been fabricated with various device geometries. The most commonly used device geometry is bottom gate with top drain and source electrodes, because this geometry is similar to the thin-film silicon transistor (TFT) using thermally grown SiO2 as gate dielectric. Organic polymers, such as poly(methyl-methacrylate) (PMMA), can also be used as dielectric.

By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent. Such transparent TFTs (TTFTs) can be used for construction of video display panels. Because conventional substrates cannot withstand high annealing temperatures, the deposition process must be completed under relatively low temperatures. Chemical vapor deposition and physical vapor deposition (usually sputtering) are applied. The first solution-processed TTFTs, based on zinc oxide, were reported in 2003 by researchers at Oregon State University. [4] The Portuguese laboratory CENIMAT at the Universidade Nova de Lisboa has produced the world's first completely transparent TFT at room temperature. [6] CENIMAT also developed the first paper transistor, [7] which may lead to applications such as magazines and journal pages with moving images.

Electrode electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte or a vacuum)

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. The word was coined by William Whewell at the request of the scientist Michael Faraday from two Greek words: elektron, meaning amber, and hodos, a way.

Indium tin oxide chemical compound

Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can either be described as a ceramic or alloy. Indium tin oxide is typically encountered as an oxygen-saturated composition with a formulation of 74% In, 18% O2, and 8% Sn by weight. Oxygen-saturated compositions are so typical, that unsaturated compositions are termed oxygen-deficient ITO. It is transparent and colorless in thin layers, while in bulk form it is yellowish to grey. In the infrared region of the spectrum it acts as a metal-like mirror.

Chemical vapor deposition chemical process used in the semiconductor industry to produce thin films

Chemical vapor deposition (CVD) is a deposition method used to produce high quality, high-performance, solid materials, typically under vacuum. The process is often used in the semiconductor industry to produce thin films.

Applications

The best known application of thin-film transistors is in TFT LCDs, an implementation of LCD technology. Transistors are embedded within the panel itself, reducing crosstalk between pixels and improving image stability.

Liquid-crystal display display that uses the light modulating properties of liquid crystals

A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals. Liquid crystals do not emit light directly, instead using a backlight or reflector to produce images in color or monochrome. LCDs are available to display arbitrary images or fixed images with low information content, which can be displayed or hidden, such as preset words, digits, and seven-segment displays, as in a digital clock. They use the same basic technology, except that arbitrary images are made up of a large number of small pixels, while other displays have larger elements. LCDs can either be normally on (positive) or off (negative), depending on the polarizer arrangement. For example, a character positive LCD with a backlight will have black lettering on a background that is the color of the backlight, and a character negative LCD will have a black background with the letters being of the same color as the backlight. Optical filters are added to white on blue LCDs to give them their characteristic appearance.

In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit or channel to another.

Pixel a physical point in a raster image

In digital imaging, a pixel, pel, dots, or picture element is a physical point in a raster image, or the smallest addressable element in an all points addressable display device; so it is the smallest controllable element of a picture represented on the screen.

As of 2008, many color LCD TVs and monitors use this technology. TFT panels are frequently used in digital radiography applications in general radiography. A TFT is used in both direct and indirect capture[ jargon ] as a base for the image receptor in medical radiography.

LCD television television sets that use liquid-crystal displays to produce images

Liquid-crystal-display televisions are television sets that use liquid-crystal displays to produce images. They are, by far, the most widely produced and sold television display type. LCD TVs are thin and light, but have some disadvantages compared to other display types such as high power consumption, poorer contrast ratio, and inferior color gamut.

Digital data, in information theory and information systems, is the discrete, discontinuous representation of information or works. Numbers and letters are commonly used representations.

Radiography imaging technique

Radiography is an imaging technique using X-rays, gamma rays, or similar radiation to view the internal form of an object. To create the image, a beam of X-rays or other form of electromagnetic radiation is produced by an X-ray generator and is projected toward the object. A certain amount of the X-rays or other radiation is absorbed by the object, dependent on the object's density and structural composition. The X-rays that pass through the object are captured behind the object by a detector. The generation of flat two dimensional images by this technique is called projectional radiography. In computed tomography an X-ray source and its associated detectors rotate around the subject which itself moves through the conical X-ray beam produced. Any given point within the subject is crossed from many directions by many different beams at different times. Information regarding attenuation of these beams is collated and subjected to computation to generate two dimensional images in three planes which can be further processed to produce a three dimensional image.

AMOLED (active-matrix organic light-emitting diode) screens also contain a TFT layer.

The most beneficial aspect[ neutrality is disputed ] of TFT technology is its use of a separate transistor for each pixel on the display. Because each transistor is small, the amount of charge needed to control it is also small. This allows for very fast re-drawing of the display.

Structure of a TFT-display matrix

This picture does not include the actual light-source (usually cold-cathode fluorescent lamps or white LEDs), just the TFT-display matrix.

Related Research Articles

Organic electronics

Organic electronics is a field of materials science concerning the design, synthesis, characterization, and application of organic small molecules or polymers that show desirable electronic properties such as conductivity. Unlike conventional inorganic conductors and semiconductors, organic electronic materials are constructed from organic (carbon-based) small molecules or polymers using synthetic strategies developed in the context of organic and polymer chemistry. One of the promised benefits of organic electronics is their potential low cost compared to traditional inorganic electronics. Attractive properties of polymeric conductors include their electrical conductivity that can be varied by the concentrations of dopants. Relative to metals, they have mechanical flexibility. Some have high thermal stability.

Semiconductor device fabrication process used to create the integrated circuits that are present in everyday electrical and electronic devices

Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications.

MOSFET transistor used for amplifying or switching electronic signals

The metal-oxide-semiconductor field-effect transistor is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. A metal-insulator-semiconductor field-effect transistor or MISFET is a term almost synonymous with MOSFET. Another synonym is IGFET for insulated-gate field-effect transistor.

OLED light-emitting diode in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current.

An organic light-emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. This organic layer is situated between two electrodes; typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as smartphones, handheld game consoles and PDAs. A major area of research is the development of white OLED devices for use in solid-state lighting applications.

In semiconductor manufacturing, a low-κ is a material with a small relative dielectric constant relative to silicon dioxide. Although the proper symbol for the relative dielectric constant is the Greek letter κ (kappa), in conversation such materials are referred to as being "low-k" (low-kay) rather than "low-κ" (low-kappa). Low-κ dielectric material implementation is one of several strategies used to allow continued scaling of microelectronic devices, colloquially referred to as extending Moore's law. In digital circuits, insulating dielectrics separate the conducting parts from one another. As components have scaled and transistors have gotten closer together, the insulating dielectrics have thinned to the point where charge build up and crosstalk adversely affect the performance of the device. Replacing the silicon dioxide with a low-κ dielectric of the same thickness reduces parasitic capacitance, enabling faster switching speeds and lower heat dissipation.

In semiconductor fabrication, a resist is a thin layer used to transfer a circuit pattern to the semiconductor substrate which it is deposited upon. A resist can be patterned via lithography to form a (sub)micrometer-scale, temporary mask that protects selected areas of the underlying substrate during subsequent processing steps. The material used to prepare said thin layer is typically a viscous solution. Resists are generally proprietary mixtures of a polymer or its precursor and other small molecules that have been specially formulated for a given lithography technology. Resists used during photolithography are called photoresists.

Microfabrication processes of fabrication of miniature structures

Microfabrication is the process of fabricating miniature structures of micrometre scales and smaller. Historically, the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" or "semiconductor device fabrication". In the last two decades microelectromechanical systems (MEMS), microsystems, micromachines and their subfields, microfluidics/lab-on-a-chip, optical MEMS, RF MEMS, PowerMEMS, BioMEMS and their extension into nanoscale have re-used, adapted or extended microfabrication methods. Flat-panel displays and solar cells are also using similar techniques.

Atomic layer deposition

Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas phase chemical process. ALD is considered a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals, typically called precursors. These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner. Through the repeated exposure to separate precursors, a thin film is slowly deposited. ALD is a key process in the fabrication of semiconductor devices, and part of the set of tools available for the synthesis of nanomaterials.

An ohmic contact is a non-rectifying electrical junction: a junction between two conductors that has a linear current–voltage (I-V) curve as with Ohm's law. Low resistance ohmic contacts are used to allow charge to flow easily in both directions between the two conductors, without blocking due to rectification or excess power dissipation due to voltage thresholds.

Hafnium dioxide chemical compound

Hafnium(IV) oxide is the inorganic compound with the formula HfO2. Also known as hafnia, this colourless solid is one of the most common and stable compounds of hafnium. It is an electrical insulator with a band gap of 5.3~5.7 eV. Hafnium dioxide is an intermediate in some processes that give hafnium metal.

Octadecyltrichlorosilane (ODTS, or n-octadecyltrichlorosilane) is an organometallic chemical. It is used in semiconductor industry to form self-assembled monolayer thin films on silicon dioxide substrates. Its structural chemical formula is CH3(CH2)17SiCl3. It is flammable, reacts with water, and is also sensitive to air. It is corrosive and can severely damage mucous membranes. Its EINECS number is 203-930-7.

Printed electronics

Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography, and inkjet. By electronic industry standards, these are low cost processes. Electrically functional electronic or optical inks are deposited on the substrate, creating active or passive devices, such as thin film transistors; capacitors; coils; resistors. Printed electronics is expected to facilitate widespread, very low-cost, low-performance electronics for applications such as flexible displays, smart labels, decorative and animated posters, and active clothing that do not require high performance.

Hafnium silicate is the hafnium(IV) salt of silicic acid with the chemical formula of HfSiO4.

Indium gallium zinc oxide (IGZO) is a semiconducting material, consisting of indium (In), gallium (Ga), zinc (Zn) and oxygen (O). IGZO thin-film transistor (TFT) is used in the TFT backplane of flat-panel displays (FPDs). IGZO-TFT was developed by Hideo Hosono's group at Tokyo Institute of Technology and Japan Science and Technology Agency (JST) in 2003 and in 2004. IGZO-TFT has 20–50 times the electron mobility of amorphous silicon, which has often been used in liquid-crystal displays (LCDs) and e-papers. As a result, IGZO-TFT can improve the speed, resolution and size of flat-panel displays. It is thus considered to be one of the most promising thin-film transistors for use in organic light-emitting diode (OLED) displays.

An oxide thin-film transistor (TFT) is a particular kind of field-effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate. The principal difference between amorphous silicon TFT and Oxide TFT is that the material of the electron channel is oxide or amorphous silicon. A common substrate is glass, since the primary application of TFTs is in liquid crystal displays and organic light emitting displays (OLEDs). This differs from the conventional transistor where the semiconductor material typically is the substrate, such as a silicon wafer. TFT electrical performance is dramatically degraded if a zinc–tin–oxide TFT is covered with a dielectric layer and does not undergo both types of annealing. In addition to silicon dioxide, successful passivation of zinc–tin–oxide TFTs is accomplished using thermally evaporated calcium fluoride, germanium oxide, strontium fluoride, or antimony oxide as passivation.

Low-temperature polycrystalline silicon (LTPS) is polycrystalline silicon that has been synthesized at relatively low temperatures compared to in traditional methods. LTPS is important for display industries, since the use of large glass panels prohibits exposure to deformative high temperatures. More specifically, the use of polycrystalline silicon in thin-film transistors (LTPS-TFT) has high potential for large-scale production of electronic devices like flat panel LCD displays or image sensors.

References

  1. 1 2 Kanicki, Jerzy (1992). Amorphous & Microcystalline Semiconductor Devices Volume II: Materials and Device Physics. Artech House, Inc. ISBN   0-89006-379-6.
  2. Brody, T. Peter (November 1984). "The Thin Film Transistor - A Late Flowering Bloom". IEEE Transactions on Electron Devices. 31 (11): 1614–1628. doi:10.1109/T-ED.1984.21762.
  3. Brody, T. Peter (1996). "The birth and early childhood of active matrix - a personal memoir". Journal of the SID. 4/3: 113–127.
  4. 1 2 Wager, John. OSU Engineers Create World's First Transparent Transistor Archived 2007-09-15 at the Wayback Machine .. College of Engineering, Oregon State University, Corvallis, OR: OSU News & Communication, 2003. 29 July 2007.
  5. Chun, Yoon Soo; Chang, Seongpil; Lee, Sang Yeol (2011). "Effects of gate insulators on the performance of a-IGZO TFT fabricated at room-temperature". Microelectronic Engineering. 88 (7): 1590–1593. doi:10.1016/j.mee.2011.01.076. ISSN   0167-9317.
  6. Fortunato, E. M. C.; Barquinha, P. M. C.; Pimentel, A. C. M. B. G.; Gonçalves, A. M. F.; Marques, A. J. S.; Pereira, L. M. N.; Martins, R. F. P. (March 2005). "Fully Transparent ZnO Thin-Film Transistor Produced at Room Temperature". Advanced Materials. 17 (5): 590–594. doi:10.1002/adma.200400368.
  7. Fortunato, E.; Correia, N.; Barquinha, P.; Pereira, L.; Goncalves, G.; Martins, R. (September 2008). "High-Performance Flexible Hybrid Field-Effect Transistors Based on Cellulose Fiber Paper" (PDF). IEEE Electron Device Letters. 29 (9): 988–990. doi:10.1109/LED.2008.2001549.