0
Want this question answered?
Be notified when an answer is posted
Add your answer:
Earn +20 pts
What is a TFTs blood test?
Thyroid function blood test
What is the difference between a smooth rat terrier and a rat terrier?
Rat Terriers and Toy Fox Terriers are DEFINITELY not the same, although they have a common background and some Rat Terrier's have Toy Fox Terrier's background in their pedigrees. Here are some of the differences: # Color. TFTs may only be white and black with tan trim, white and black, or white and tan. RTs come in a much wider variety of colors. # Size. TFTs may not be over 7 pounds. Although the RT standard does not specify weight, it allows dogs up to 19 inches in height, measured at the withers. # Background. The TFTs were bred down from the Smooth Fox Terrier. The RT's background includes Smooth Fox Terriers, Beagles, Whippets, and Manchesters. # Ears. The TFTs must have erect ears. RTs may have erect, tipped, or button ears. # Tail. TFTs have docked or natural bob tails, with a high tail set. RTs may have docked, natural bob or natural tails, set on at the end of the croup. # Eyes. TFTs may only have dark eyes. RTs may have any shade of brown from dark to amber, blue, or hazel eyes. # Croup. TFTs have a flat croup. RTs have a slighly sloping croup. Rat Terriers and Toy Fox Terriers are DEFINITELY not the same, although they have a common background and some Rat Terrier's have Toy Fox Terrier's background in their pedigrees. Here are some of the differences: # Color. TFTs may only be white and black with tan trim, white and black, or white and tan. RTs come in a much wider variety of colors. # Size. TFTs may not be over 7 pounds. Although the RT standard does not specify weight, it allows dogs up to 19 inches in height, measured at the withers. # Background. The TFTs were bred down from the Smooth Fox Terrier. The RT's background includes Smooth Fox Terriers, Beagles, Whippets, and Manchesters. # Ears. The TFTs must have erect ears. RTs may have erect, tipped, or button ears. # Tail. TFTs have docked or natural bob tails, with a high tail set. RTs may have docked, natural bob or natural tails, set on at the end of the croup. # Eyes. TFTs may only have dark eyes. RTs may have any shade of brown from dark to amber, blue, or hazel eyes. # Croup. TFTs have a flat croup. RTs have a slighly sloping croup.
What is used in the diagnosis of thyroid disease?
Thyroid function tests (TFTs) are the main diagnostic criteria for a thyroid disease. These are however done in the face of other clinical findings and patients symptoms (differs for hypo and hyperthyroidism)
What is skin cell process?
Cell process is the process of joining the back and the front substrate and filling the space with the liquid crystal material that is in between the both substrates. The back substrate contains the TFTs while the front substrate contains the color filters.
How much is Mark Zuckerberge?
According to Forbes, Zuckerberg is now worth a note worthy $6.9 billion thanks to his 24% stake in Facebook.Read: Facebook's Mark Zuckerberg Now Worth $6.9 Billion, Richer Than Steve Jobs [Facebook Founder Mark Zuckerberg Surpasses Steve Jobs On Forbes 400 List, Shows No Signs of Stopping] » TFTS -- Technology, Gadgets & Curiositiesright now he is worth around $13.5billion !
Can lack of affection make you physically feel cold?
No, but an underactive thyroid gland can. Ask your doctor to run some thyroid function tests (TFTs) on you. No, only coldness in the heart, which is worse. But reduced activity brought on by depression could. Exercise is the first recommended treatment. Get out and go for a brisk walk in the fall air. That will both warm you up and help the depression. If it's too cold out, do a couple laps around the mall -- then window shop. Walk first. Call a friend and talk it over. Maybe the friend would like some exercise too. You can't make someone feel affection, but it's up to you how you deal with it. If the lack of affection seems to be due to alcohol or drugs, hit an Alanon or CODA meeting. Google either one for meetings in your area. If the coldness continues, by all means speak with your physician, but the most obvious is usually the place to start.
How much is Mark Zuckerberg worth?
The creator of facebook is called Mark ZuckerbergNow 2011 he has$2,147,483,647 a year$178,956,971 per month$44,739,248 per week$8,947,848.53 per day OR
How does a screensaver function?
Right click your mouse in the wallpaper click properties select screensaver then choose one and type what minute will it appear then click OK. Screensaver is a anti virus protection when you use it
What are the various types of transistors?
TransistorsThe two main types of transistors are the bipolar junction transistor (BJT) and the field-effect transistor (FET). Bipolar Junction TransistorsBJTs can have two different polarities, NPN and PNP. An NPN BJT is one where a positively-doped (P-type) semiconductor is sandwiched between two negatively-doped (N-type) semiconductors. A PNP BJT is, obviously, one where an N-type semiconductor is sandwiched between two P-types. Both types of BJTs have an exponential dependence between the input voltage and the current output. For the record, I should state that a semiconductor is basically a material with conductance between that of an insulator and a conductor. Silicon and germanium are the two most well-known semiconductors. Also, doping just means the addition of impurities into a semiconducting material in order for it to either: increase its electron acceptance (P-type) or increase its electron conductance (N-type). Some specific types of BJTs:HBT - heterojunction bipolar transistor - These types of transistors are very similar to BJTs except that the two P-type semiconductors in the PNP polarity, or the two N-type semiconductors in the NPN polarity, are doped differently relative to each other. The reason for doing this, simply stated, is to make it more difficult for a transistor to operate in the reverse direction from which is was intended.Grown-junction transistor - This was the first type of BJT and is self-explanatory. The PN or NP junctions, depending on whether it's of NPN or PNP polarity, respectively, are grown onto a single, solid crystal of semiconductor material. Grown, in this case, means slowly attached, chemically.Alloy-junction transistor - Similar to a grown-junction transistor except the semiconducting material onto which the PN or NP junctions are grown is specifically germanium.MAT - micro-alloy transistor - An improved, speedier version of the alloy-junction transistor. The materials of the PN or NP junctions of a MAT are metal-semiconductor, as opposed to semiconductor-semiconductor.MADT - micro-alloy diffused transistor - An improved, speedier version of the MAT. The dopant material of a MADT is diffused (thinly spread) accross the entire germanium crystal prior to PN or NP growth, as opposed to a MAT where the doping material is only on the metallic side of the PN or NP junction.PADT - post-alloy diffused transistor - An improved, speedier version of the MADT. A thin, diffused dopant layer of germanium is grown onto the germanium crystal, as opposed to the entire germanium crystal being diffused, which allows the germanium crystal to be as thick as necessary for mechanical strength purposes. The PN or NP junctions are then grown onto this thin layer.Schottky transistor - These are alloy-junction transistors with a Schottky barrier between the metal-semiconductor junction. All metal-semiconductor junctions act sort of like capacitors with a voltage between the junctions. Often, you'd like to minimize this voltage in order to minimize the saturation (the amount of the germanium crystal) needed for the transistor to work. Minimizing the saturation effectively speeds up the transistor's performance, which is great for things like switches. Schottky barriers use various materials to do exactly this.Surface-barrier transistor - These are just like Schottky transistors except that both junctions are metal-semiconductor as opposed to only one.Drift-field transistor - The doping agent of these transistors is engineered to produce a specific electric field. This effectually reduces the electrons' transit time between the junctions of the transistor, thereby making it work faster.Avalanche transistor - These transistors can operate in the breakdown voltage region of a transistor's junctions. The breakdown voltage is simply the minimum voltage in which an insulator starts acting like a conductor. Thus, these transistors allow for higher currents to be applied to them than their normal counterparts.Darlington transistor - These are simply two BJTs connected together to further increase the gain of the current output.IGBT - insulated-gate bipolar transistor - These transistors combine the use of BJTs as switches with an isolated-gate FET (see below) as the input. IGBTs provide much more efficient and faster switching than regular BJTs and are thus some of the most common transistors found in modern appliances.Photo transistor - These transistors convert electromagnetic radiation in the form of visible light, UV-rays, or X-rays into current or voltage. As opposed to the normal PN junctions found in many transistors, photo transistors use PIN junctions. PIN junctions are similar to PN junctions except that they have an additional intrinsic semiconductor between the P-type and N-type semiconducting regions. This intrinsic semiconductor is a very lightly doped semiconductor which exists, at least for the purposes of photo transistors, to supply a region within the junction where a photon (a particle of electromagnetic radiation with a specific energy) can ionize (knock an electron out of via the photoelectric effect) an atom of this semiconducting material. Because of the electric field caused from the surrounding P-type and N-type semiconducting regions, this ionization causes the photoelectron to move toward one end of the junction, thereby producing what's known as a photocurrent, which is then amplified in the same manner as all other BJTs. I promise that the rest of my answer won't get more complicated than this.Field-Effect TransistorsFETs use electric fields to control only one-type of charge carrier, as opposed to BJTs which control both types. Now's as good a time as any to introduce the concept of electron holes. Intuitively, electrons carry negative charge and are thus referred to as negative charge carriers. Well, the absence of an electron where one used to be is called an electron hole. These holes act exactly as electrons do in transistors except that they carry positive charge, in the form of missing negative charge, and are thus called positive charge carriers. FETs are designed to control either positive or negative charge carriers, in the form of holes or electrons, but not both. The flow of positive or negative charge carriers occurs through what's called the channel of an FET. FET channels are created within the bulk material of the FET, which is usually silicon. If you find this idea more complicated than what I wrote about photo transistors, that's only because you haven't looked up the physics behind the photoelectric effect yet. Some specific types of FETs:CNTFET - carbon nanotube field-effect transistor - These FETs use carbon nanotubes instead of silicon as their channel material. Carbon nanotubes are needed as FETs continue to get smaller in size. They help reduce effects, such as quantum tunneling and overheating, which are beginning to become real problems in small, silicon-based FETs.JFET - junction gate field-effect transistor - This FET supplies a voltage accross the charge-carrying channel that can pinch it shut, effectively stopping the current through the channel.MESFET - metal semiconductor field-effect transistor - Similar to, but faster than, JFETs, MESFETs use a Schottky barrier (see above) instead of a PN junction.HEMT - high electron mobility transistor - The FET version of an HBT (see above). Faster than a MESFET, the charge-carrying channel is between two different materials instead of within a single, doped region. Also known as a heterostructure FET (HFET) or a modulation-doped FET (MODFET).MOSFET - metal-oxide-semiconductor field-effect transistor - This is the most basic, and most common, type of FET, analogous to the standard BJT (see above). Instead of pinching its charge-carrying channel shut as in a JFET, a MOSFET has an insulator attached to its input electrode which can be turned on or off depending on whether a voltage is supplied accross it. The channel can be N-type (nMOS) or P-type (pMOS), as explained above under the "bipolar junction transistors" heading.ITFET - inverted-T field-effect transistor - This is simply any type of FET that extends vertically out from the horizontal plane in a T-shape, hence the name.MuGFET - multiple gate field-effect transistor - A MOSFET where more than one input shares the bulk material of the FET. The idea is to use the same FET, thus the same sized object, for multiple things. This concept came about due to the ever shrinking sizes of transistors.MIGFET - multiple independent gate field-effect transistor - A MuGFET where the multiple inputs are independently controlled.Flexfet - A MIGFET with two inputs, one on a JFET and the other on a MOSFET. The JFET and MOSFET are then "stacked" on top of each other. Due to its design, the JFET and MOSFET are coupled to each other; i.e. the channel through one effects the channel through the other and vice versa.FinFET - A MuGFET where the charge-carrying channel is wrapped around a piece of silicon, called a fin. The reason for doing this is similar to that of a PADT (see above); i.e. mechanical strength.FREDFET - fast-recovery (or reverse) epitaxial diode field-effect transistor - A cute name for a transistor which is basically designed to quickly turn off when no more voltage is being supplied to it.TFT - thin-film transistor - An FET where the semiconducting material is placed via thin films over the bulk of the device. This is opposed to the bulk of the device being the semiconductor itself, as in most FETs. The bulk material used in TFTs is often glass. The reason being so that the transistors can work behind a clear display in applications like liquid crystal display (LCD) monitors.OFET - organic field-effect transistor - An FET with an organic polymer semiconductor as its channel. These are like TFTs except the bulk of the device is plastic, allowing for very cool, flexible LCD monitors.FGMOS - floating gate MOSFET - A MOSFET with a "floating gate" input; i.e. an electrically isolated input that can store charge, like a capacitor, to be used later. These are the transistors behind flash drives.ISFET - ion-sensitive field-effect transistor - An FET that changes its current depending on the ion concentration of a solution. The solution itself is used as the input electrode in an ISFET.EOSFET - electrolyte-oxide-semiconductor field-effect transistor - A MOSFET with the metal replaced by an electrolyte solution. EOSFETs are used to in neurochips to detect brain activity.DNAFET - Deoxyribonucleic acid (DNA) field-effect transistor - A MOSFET with its input electrode being a layer of immobilized, single-stranded DNA. The current through the MOSFET is modulated by the varying charge distributions that occur when complimentary DNA strands hybridize to the layer of single-stranded DNA on the input electrode. DNAFETs are used, not surprisingly, in DNA sequencing.My sources all stem from the link below which is also a great place to learn more about transistors.
What does TFT mean?
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. 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 Si/SiO2 oxide as gate dielectric. Organic polymers, such as poly(methyl-methacrylate) (PMMA), can be used as dielectric, too.In May 2007, Sony reported the first full-color, video-rate, flexible, all plastic display, in which both the thin film transistors and the light emitting pixels were made of organic materials.History of OFETsThe field-effect transistor (FET) was first proposed by J.E. Lilienfeld, who received a patent for his idea in 1930.[3] He proposed that a field-effect transistor behaves as a capacitor with a conducting channel between a source and a drain electrode. Applied voltage on the gate electrode controls the amount of charge carriers flowing through the system.The first field-effect transistor was designed and prepared in 1960 by Kahng and Atalla using a metal-oxide-semiconductor. However, rising costs of materials and manufacturing, as well as public interest in more environmentally friendly electronics materials have supported development of organic based electronics in more recent years. In 1987, Koezuka and co-workers reported the first organic field-effect transistor based on a polymer of thiophene molecules. The thiophene polymer is a type of conjungated polymer that is able to conduct charge, eliminating the need to use expensive metal oxide semiconductors. Additionally, other conjugated polymers have been shown to have semi-conducting properties. OFET design has also improved in the past few decades. Many OFETs are now designed based on the thin film transistor (TFT) model, which allows the devices to use less conductive materials in their design. Improvement on these models in the past few years have been made to field-effect mobility and on-off current ratios.MaterialsOne common feature of OFET materials is the inclusion of an aromatic or otherwise conjugated π-electron system, facilitating the delocalization of orbital wavefunctions. Electron withdrawing groups or donating groups can be attached that facilitate hole or electron transport.OFETs employing many aromatic and conjugated materials as the active semiconducting layer have been reported, including small molecules such as rubrene, tetracene, pentacene, diindenoperylene, perylenediimides, tetracyanoquinodimethane (TCNQ), and polymers such as polythiophenes (especially poly 3-hexylthiophene (P3HT)), polyfluorene, polydiacetylene, poly 2,5-thienylene vinylene, poly p-phenylene vinylene (PPV).The field is very active, with newly synthesized and tested compounds reported weekly in prominent research journals. Many review articles exist documenting the development of these materials.Rubrene-based OFETs show the highest carrier mobility 20-40 cm2/(V·s). Another popular OFET material is pentacene, which has been used since 1980s, but resulted in about 10 times lower mobilities than rubrene. The major problem with pentacene, as well as many other organic conductors, is its rapid oxidation in air to form pentacene-quinone. However if the pentacene is preoxidized, and the thus formed pentacene-quinone is used as the gate insulator, then the mobility can approach the rubrene values. This pentacene oxidation technique is akin to the silicon oxidation used in the silicon electronics.Polycrystalline tetrathiafulvalene and its analogues result in mobilities in the range 0.1-1.4 cm2/(V·s). However, the mobility exceeds 10 cm2/(V·s) in solution-grown or vapor-transport-grown single crystalline hexamethylene-tetrathiafulvalene (HMTTF). The ON/OFF voltage is different for devices grown by those two techniques, presumably due to the higher processing temperatures using in the vapor transport grows.All the above-mentioned devices are based on p-type conductivity. N-type organic OFETs are yet poorly developed. They are usually based on perylenediimides or fullerenes or their derivatives, and show electron mobilities below 2 cm2/(V·s).Device design of organic field-effect transistorsThree essential components of field-effect transistors are the source, the drain and the gate. Field-effect transistors usually operate as a capacitor. They are composed of two plates. One plate works as a conducting channel between two ohmic contacts, which are called the source and the drain contacts. The other plate works to control the charge induced into the channel, and it is called the gate. The direction of the movement of the carriers in the channel is from the source to the drain. Hence the relationship between these three components is that the gate controls the carrier movement from the source to the drain.When this capacitor concept is applied to the device design, various devices can be built up based on the difference in the controller-the gate. This can be the gate material, the location of the gate with respect to the channel, how the gate is isolated from the channel, and what type of carrier is induced by the gate voltage into channel (such as electrons in an n-channel device, holes in a p-channel device, and both electrons and holes in a double injection device).MISFETThe most prominent and widely used FET in modern microelectronics is the MOSFET. There are different kinds in this category, such as MISFET (Metal Insulator Semiconductor Field-Effect Transistor), and IGFET (Insulator Gate Field-Effect Transistor). The scheme of a MISFET is shown in Figure 1a. The source and the drain are connected by a semiconductor and the gate is separated from the channel by a layer of insulator. If there is no bias (potential difference) applied on the gate, the band bending is induced due to the energy difference of metal conducting band and the semiconductor Fermi-level. Therefore a higher concentration of holes is formed on the interface of the semiconductor and the insulator. When an enough positive bias is applied on the gate contact, the bended band becomes flat. If a larger positive bias is applied, the band bending in the opposite direction occurs and the region close to the insulator-semiconductor interface becomes depleted of holes. Then the depleted region is formed. At an even larger positive bias, the band bending becomes so large that the Fermi-level at the interface of the semiconductor and the insulator becomes closer to the bottom of the conduction band than to the top of the valence band, therefore, it forms an inversion layer of electrons, providing the conducting channel. Finally, it turns the device on.MESFETThe second type of device is described in Fig.1b. The only difference of this one from the MISFET is that the n-type source and drain are connected by an n-type region. In this case, the depletion region extends all over the n-type channel at zero gate voltage in a normally "off" device (it is similar to the larger positive bias in MISFET case). In the normally "on" device, a portion of the channel is not depleted, and thus leads to passage of a current at zero gate voltage.TFTThe concept of TFT was first proposed by Paul Weimer in 1962. This is illustrated in Fig. 1c. Here the source and drain electrodes are directly deposited onto the conducting channel (a thin layer of semiconductor) then a thin film of insulator is deposited between the semiconductor and the metal gate contact. This structure suggests that there is no depletion region to separate the device from the substrate. If there is zero bias, the electrons are expelled from the surface due to the Fermi-level energy difference of the semiconductor and the metal. This leads to band bending of semiconductor. In this case, there is no carrier movement between the source and drain. When the positive charge is applied, the accumulation of electrons on the interface leads to the bending of the semiconductor in an opposite way and leads to the lowering of the conduction band with regards to the Fermi-level of the semiconductor. Then a highly conductive channel forms at the interface.OFETOFETs adopt the architecture of TFT. With the development of the conducting polymer, the semiconducting properties of small conjugated molecules have been recognized. The interest in OFETs has grown enormously in the past ten years. The reasons for this surge of interest are manifold. The performance of OFETs, which can compete with that of amorphous silicon (a-Si) TFTs with field-effect mobilities of 0.5−1 cm2 V−1 s−1 and ON/OFF current ratios (which indicate the ability of the device to shut down) of 106-108, has improved significantly. Currently, the mobility values for thin-film OFETs are 5 cm2V−1s−1 in the case of vacuum-deposited small molecules [13] and 0.6 cm2 V−1 s−1 for solution-processed polymers[14] have been reported. As a result, there is now a greater industrial interest in using OFETs for applications that are currently incompatible with the use of a-Si or other inorganic transistor technologies. One of their main technological attractions is that all the layers of an OFET can be deposited and patterned at room temperature by a combination of low-cost solution-processing and direct-write printing, which makes them ideally suited for realization of low-cost, large-area electronic functions on flexible substrates. [15]Device preparationThermally oxidized silicon is a traditional substrate for OFETs where the silicon dioxide serves as the gate insulator. The active FET layer is usually deposited onto this substrate using either (i) thermal evaporation, (ii) coating from organic solution, or (iii) electrostatic lamination. The first two techniques result in polycrystalline active layers; they are much easier to produce, but result in relatively poor transistor performance. Numerous variations of the solution coating technique (ii) are known, including dip-coating, spin-coating, inkjet printing and screen printing. The electrostatic lamination technique is based on manual peeling of a thin layer off a single organic crystal; it results in a superior single-crystalline active layer, yet it is more tedious. The thickness of the gate oxide and the active layer is below one micrometer.[5]Evolution of carrier mobility in organic field-effect transistor.[5]The carrier transport in OFET is specific for two-dimensional (2D) carrier propagation through the device. Various experimental techniques were used for this study, such as Haynes - Shockley experiment on the transit times of injected carriers, time-of-flight (TOF) experiment for the determination of carrier mobility, pressure-wave propagation experiment for probing electric-field distribution in insulators, organic monolayer experiment for probing orientational dipolar changes, optical time-resolved second harmonic generation (TRM-SHG), etc. Whereas carriers propagate through polycrystalline OFETs in a diffusion-like (trap-limited) manner,[16] they move through the conduction band in the best single-crystalline OFETs.[5]The most important parameter of OFET carrier transport is carrier mobility. Its evolution over the years of OFET research is shown in the graph for polycrystalline and single crystalline OFETs. The horizontal lines indicate the comparison guides to the main OFET competitors - amorphous (a-Si) and polycrystalline silicon. The graph reveals that the mobility in polycrystalline OFETs is comparable to that of a-Si whereas mobility in rubrene-based OFETs (20-40 cm2/(V·s)) approaches that of best poly-silicon devices.[5]Light-emitting OFETsBecause an electric current flows through such a transistor, it can be used as a light-emitting device, thus integrating current modulation and light emission. In 2003, a German group reported the first organic light-emitting field-effect transistor (OLET)[17]. The device structure comprises interdigitated gold source- and drain electrodes and a polycrystalline tetracene thin film. Both, positive charges (holes) as well as negative charges (electrons) are injected from the gold contacts into this layer leading to electroluminescence from the tetracene.
What is Organic TFT?
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. 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 Si/SiO2 oxide as gate dielectric. Organic polymers, such as poly(methyl-methacrylate) (PMMA), can be used as dielectric, too.In May 2007, Sony reported the first full-color, video-rate, flexible, all plastic display, in which both the thin film transistors and the light emitting pixels were made of organic materials.History of OFETsThe field-effect transistor (FET) was first proposed by J.E. Lilienfeld, who received a patent for his idea in 1930.[3] He proposed that a field-effect transistor behaves as a capacitor with a conducting channel between a source and a drain electrode. Applied voltage on the gate electrode controls the amount of charge carriers flowing through the system.The first field-effect transistor was designed and prepared in 1960 by Kahng and Atalla using a metal-oxide-semiconductor. However, rising costs of materials and manufacturing, as well as public interest in more environmentally friendly electronics materials have supported development of organic based electronics in more recent years. In 1987, Koezuka and co-workers reported the first organic field-effect transistor based on a polymer of thiophene molecules. The thiophene polymer is a type of conjungated polymer that is able to conduct charge, eliminating the need to use expensive metal oxide semiconductors. Additionally, other conjugated polymers have been shown to have semi-conducting properties. OFET design has also improved in the past few decades. Many OFETs are now designed based on the thin film transistor (TFT) model, which allows the devices to use less conductive materials in their design. Improvement on these models in the past few years have been made to field-effect mobility and on-off current ratios.MaterialsOne common feature of OFET materials is the inclusion of an aromatic or otherwise conjugated π-electron system, facilitating the delocalization of orbital wavefunctions. Electron withdrawing groups or donating groups can be attached that facilitate hole or electron transport.OFETs employing many aromatic and conjugated materials as the active semiconducting layer have been reported, including small molecules such as rubrene, tetracene, pentacene, diindenoperylene, perylenediimides, tetracyanoquinodimethane (TCNQ), and polymers such as polythiophenes (especially poly 3-hexylthiophene (P3HT)), polyfluorene, polydiacetylene, poly 2,5-thienylene vinylene, poly p-phenylene vinylene (PPV).The field is very active, with newly synthesized and tested compounds reported weekly in prominent research journals. Many review articles exist documenting the development of these materials.Rubrene-based OFETs show the highest carrier mobility 20-40 cm2/(V·s). Another popular OFET material is pentacene, which has been used since 1980s, but resulted in about 10 times lower mobilities than rubrene. The major problem with pentacene, as well as many other organic conductors, is its rapid oxidation in air to form pentacene-quinone. However if the pentacene is preoxidized, and the thus formed pentacene-quinone is used as the gate insulator, then the mobility can approach the rubrene values. This pentacene oxidation technique is akin to the silicon oxidation used in the silicon electronics.Polycrystalline tetrathiafulvalene and its analogues result in mobilities in the range 0.1-1.4 cm2/(V·s). However, the mobility exceeds 10 cm2/(V·s) in solution-grown or vapor-transport-grown single crystalline hexamethylene-tetrathiafulvalene (HMTTF). The ON/OFF voltage is different for devices grown by those two techniques, presumably due to the higher processing temperatures using in the vapor transport grows.All the above-mentioned devices are based on p-type conductivity. N-type organic OFETs are yet poorly developed. They are usually based on perylenediimides or fullerenes or their derivatives, and show electron mobilities below 2 cm2/(V·s).Device design of organic field-effect transistorsThree essential components of field-effect transistors are the source, the drain and the gate. Field-effect transistors usually operate as a capacitor. They are composed of two plates. One plate works as a conducting channel between two ohmic contacts, which are called the source and the drain contacts. The other plate works to control the charge induced into the channel, and it is called the gate. The direction of the movement of the carriers in the channel is from the source to the drain. Hence the relationship between these three components is that the gate controls the carrier movement from the source to the drain.When this capacitor concept is applied to the device design, various devices can be built up based on the difference in the controller-the gate. This can be the gate material, the location of the gate with respect to the channel, how the gate is isolated from the channel, and what type of carrier is induced by the gate voltage into channel (such as electrons in an n-channel device, holes in a p-channel device, and both electrons and holes in a double injection device).MISFETThe most prominent and widely used FET in modern microelectronics is the MOSFET. There are different kinds in this category, such as MISFET (Metal Insulator Semiconductor Field-Effect Transistor), and IGFET (Insulator Gate Field-Effect Transistor). The scheme of a MISFET is shown in Figure 1a. The source and the drain are connected by a semiconductor and the gate is separated from the channel by a layer of insulator. If there is no bias (potential difference) applied on the gate, the band bending is induced due to the energy difference of metal conducting band and the semiconductor Fermi-level. Therefore a higher concentration of holes is formed on the interface of the semiconductor and the insulator. When an enough positive bias is applied on the gate contact, the bended band becomes flat. If a larger positive bias is applied, the band bending in the opposite direction occurs and the region close to the insulator-semiconductor interface becomes depleted of holes. Then the depleted region is formed. At an even larger positive bias, the band bending becomes so large that the Fermi-level at the interface of the semiconductor and the insulator becomes closer to the bottom of the conduction band than to the top of the valence band, therefore, it forms an inversion layer of electrons, providing the conducting channel. Finally, it turns the device on.MESFETThe second type of device is described in Fig.1b. The only difference of this one from the MISFET is that the n-type source and drain are connected by an n-type region. In this case, the depletion region extends all over the n-type channel at zero gate voltage in a normally "off" device (it is similar to the larger positive bias in MISFET case). In the normally "on" device, a portion of the channel is not depleted, and thus leads to passage of a current at zero gate voltage.TFTThe concept of TFT was first proposed by Paul Weimer in 1962. This is illustrated in Fig. 1c. Here the source and drain electrodes are directly deposited onto the conducting channel (a thin layer of semiconductor) then a thin film of insulator is deposited between the semiconductor and the metal gate contact. This structure suggests that there is no depletion region to separate the device from the substrate. If there is zero bias, the electrons are expelled from the surface due to the Fermi-level energy difference of the semiconductor and the metal. This leads to band bending of semiconductor. In this case, there is no carrier movement between the source and drain. When the positive charge is applied, the accumulation of electrons on the interface leads to the bending of the semiconductor in an opposite way and leads to the lowering of the conduction band with regards to the Fermi-level of the semiconductor. Then a highly conductive channel forms at the interface.OFETOFETs adopt the architecture of TFT. With the development of the conducting polymer, the semiconducting properties of small conjugated molecules have been recognized. The interest in OFETs has grown enormously in the past ten years. The reasons for this surge of interest are manifold. The performance of OFETs, which can compete with that of amorphous silicon (a-Si) TFTs with field-effect mobilities of 0.5−1 cm2 V−1 s−1 and ON/OFF current ratios (which indicate the ability of the device to shut down) of 106-108, has improved significantly. Currently, the mobility values for thin-film OFETs are 5 cm2V−1s−1 in the case of vacuum-deposited small molecules [13] and 0.6 cm2 V−1 s−1 for solution-processed polymers[14] have been reported. As a result, there is now a greater industrial interest in using OFETs for applications that are currently incompatible with the use of a-Si or other inorganic transistor technologies. One of their main technological attractions is that all the layers of an OFET can be deposited and patterned at room temperature by a combination of low-cost solution-processing and direct-write printing, which makes them ideally suited for realization of low-cost, large-area electronic functions on flexible substrates. [15]Device preparationThermally oxidized silicon is a traditional substrate for OFETs where the silicon dioxide serves as the gate insulator. The active FET layer is usually deposited onto this substrate using either (i) thermal evaporation, (ii) coating from organic solution, or (iii) electrostatic lamination. The first two techniques result in polycrystalline active layers; they are much easier to produce, but result in relatively poor transistor performance. Numerous variations of the solution coating technique (ii) are known, including dip-coating, spin-coating, inkjet printing and screen printing. The electrostatic lamination technique is based on manual peeling of a thin layer off a single organic crystal; it results in a superior single-crystalline active layer, yet it is more tedious. The thickness of the gate oxide and the active layer is below one micrometer.[5]Evolution of carrier mobility in organic field-effect transistor.[5]The carrier transport in OFET is specific for two-dimensional (2D) carrier propagation through the device. Various experimental techniques were used for this study, such as Haynes - Shockley experiment on the transit times of injected carriers, time-of-flight (TOF) experiment for the determination of carrier mobility, pressure-wave propagation experiment for probing electric-field distribution in insulators, organic monolayer experiment for probing orientational dipolar changes, optical time-resolved second harmonic generation (TRM-SHG), etc. Whereas carriers propagate through polycrystalline OFETs in a diffusion-like (trap-limited) manner,[16] they move through the conduction band in the best single-crystalline OFETs.[5]The most important parameter of OFET carrier transport is carrier mobility. Its evolution over the years of OFET research is shown in the graph for polycrystalline and single crystalline OFETs. The horizontal lines indicate the comparison guides to the main OFET competitors - amorphous (a-Si) and polycrystalline silicon. The graph reveals that the mobility in polycrystalline OFETs is comparable to that of a-Si whereas mobility in rubrene-based OFETs (20-40 cm2/(V·s)) approaches that of best poly-silicon devices.[5]Light-emitting OFETsBecause an electric current flows through such a transistor, it can be used as a light-emitting device, thus integrating current modulation and light emission. In 2003, a German group reported the first organic light-emitting field-effect transistor (OLET)[17]. The device structure comprises interdigitated gold source- and drain electrodes and a polycrystalline tetracene thin film. Both, positive charges (holes) as well as negative charges (electrons) are injected from the gold contacts into this layer leading to electroluminescence from the tetracene.
