An oxide thin-film transistor (oxide TFT) or metal oxide thin film transistor is a type of thin film transistor where the semiconductor is a metal oxide compound. An oxide TFT is distinct from a metal oxide field effect transistor (MOSFET) where the word "oxide" refers to the insulating gate dielectric (normally silicon dioxide). In an oxide TFT, the word oxide refers to the semiconductor. Oxide TFTs have applications as amplifiers to deliver current to emitters in display backplanes.
The first transistor employing a metal oxide as the semiconductor was reported in 1964 by Klasens and Koelmans at Philips Research Laboratories. [1] However, oxide TFTs were seldom considered again for several decades after this. It wasn't until the early 2000's that Hideo Hosono, who was studying transparent conducting oxides, [2] discovered that oxysulfides [3] and indium gallium zinc oxide [4] [5] could be used as semiconductors in TFTs. Soon after, John Wager at Oregon State University reported oxide TFTs employing the binary oxide zinc oxide as the semiconductor. [6]
Oxides have several properties which make them desirable over hydrogenated amorphous silicon (a-Si:H), which was the incumbent TFT technology in the early 2000's. [7] Firstly, the electron mobility is roughly 100 times higher in oxide TFTs. [8] Because the source-drain current in transistors is linearly proportional to electron mobility, [9] so too are the amplification properties. The result of this is that smaller transistors can be used to provide the same current. In a display this means that a higher resolution and switching speed is possible.
a-Si:H additionally suffers from issues with environmental stability, such as the Staebler-Wronski Effect. [10] As oxides are already oxidized, they are generally more environmentally stable, however they do experience a phenomenon called Negative Bias Illumination Stress (NBIS) where the threshold voltage changes under constant illumination. [11]
Most n-type (electron transporting) oxide TFTs employ semiconductors that have a wide bandgap; generally greater than 3 eV. For this reason they are attractive for use in fully transparent electronics. Their wide bandgap also means they have a low off-current, and hence a high on/off ratio; a desirable property for well-defined on- and off-states.
One significant drawback with oxide TFTs is that there are very few p-type (hole transporting) metal oxide semiconductors. [12] While not a significant problem when providing amplification to emitters, this does mean oxide semiconductors are less suitable for complementary logic, and hence information processing.
Metal oxide semiconductors are typically deposited using sputtering, a vacuum-based growth technique resulting in an amorphous or polycrystalline layer. Oxides can also be deposited from solution, such as via spin-coating or spray coating. [13]
Several companies have adopted oxide TFTs as a platform for display drivers. Notably Sharp in 2012, [14] and Apple in 2013. [15]
Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation.
A thin-film transistor (TFT) is a special type of field-effect transistor (FET) where the transistor is made by thin film deposition. TFTs are grown on a supporting substrate. A common substrate is glass, because the traditional application of TFTs is in liquid-crystal displays (LCDs). This differs from the conventional bulk metal oxide field effect transistor (MOSFET), where the semiconductor material typically is the substrate, such as a silicon wafer.
A flat-panel display (FPD) is an electronic display used to display visual content such as text or images. It is present in consumer, medical, transportation, and industrial equipment.
Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy. Indium tin oxide is typically encountered as an oxygen-saturated composition with a formulation of 74% In, 8% Sn, and 18% O 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 gray. In the infrared region of the spectrum it acts as a metal-like mirror.
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement, lubricants, paints, sunscreens, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants, semi conductors, and first-aid tapes. Although it occurs naturally as the mineral zincite, most zinc oxide is produced synthetically.
A thin-film-transistor liquid-crystal display is a variant of a liquid-crystal display that uses thin-film-transistor technology to improve image qualities such as addressability and contrast. A TFT LCD is an active matrix LCD, in contrast to passive matrix LCDs or simple, direct-driven LCDs with a few segments.
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. One of the benefits of OFETs, especially compared with inorganic TFTs, is their unprecedented physical flexibility, which leads to biocompatible applications, for instance in the future health care industry of personalized biomedicines and bioelectronics.
Magnetic semiconductors are semiconductor materials that exhibit both ferromagnetism and useful semiconductor properties. If implemented in devices, these materials could provide a new type of control of conduction. Whereas traditional electronics are based on control of charge carriers, practical magnetic semiconductors would also allow control of quantum spin state. This would theoretically provide near-total spin polarization, which is an important property for spintronics applications, e.g. spin transistors.
Indium(III) oxide (In2O3) is a chemical compound, an amphoteric oxide of indium.
A degenerate semiconductor is a semiconductor with such a high level of doping that the material starts to act more like a metal than as a semiconductor. Unlike non-degenerate semiconductors, these kinds of semiconductor do not obey the law of mass action, which relates intrinsic carrier concentration with temperature and bandgap.
Indium gallium zinc oxide (IGZO) is a semiconducting material, consisting of indium (In), gallium (Ga), zinc (Zn) and oxygen (O). IGZO thin-film transistors (TFT) are 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 currently used as the thin-film transistors for use in organic light-emitting diode (OLED) TV displays.
In field-effect transistors (FETs), depletion mode and enhancement mode are two major transistor types, corresponding to whether the transistor is in an on state or an off state at zero gate–source voltage.
Hideo Hosono is a Japanese material scientist most known for the discovery of iron-based superconductors.
John Robertson FRS is a Professor of Electronics, in the Department of Engineering at the University of Cambridge. He is a leading specialist in the theory of amorphous carbon and related materials.
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.
Douglas A. Keszler is a distinguished professor in the Department of Chemistry at Oregon State University, adjunct professor in the Physics Department at OSU and adjunct professor in the Department of Chemistry at University of Oregon. He is also the director of the Center for Sustainable Materials Chemistry, and a member of the Oregon Nanoscience and Microtechnologies Institute (ONAMI) leadership team.
Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs.
Oxyphosphides are chemical compounds formally containing the group PO, with one phosphorus and one oxygen atom. The phosphorus and oxygen are not bound together as in phosphates or phosphine oxides, instead they are bound separately to the cations (metals), and could be considered as a mixed phosphide-oxide compound. So a compound with OmPn requires cations to balance a negative charge of 2m+3n. The cations will have charges of +2 or +3. The trications are often rare earth elements or actinides. They are in the category of oxy-pnictide compounds.
The Urbach Energy, or Urbach Edge, is a parameter typically denoted , with dimensions of energy, used to quantify energetic disorder in the band edges of a semiconductor. It is evaluated by fitting the absorption coefficient as a function of energy to an exponential function. It is often used to describe electron transport in structurally disordered semiconductors such a hydrogenated amorphous silicon.