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An organic field-effect transistor (OFET) uses organic molecules as a conducting material between two ohmic contacts: the source and drain electrodes. The amount of charge being carried across the plate is regulated by the voltage applied to the gate electrode. These devices are composed of an organic semiconductor layer, a dielectric layer, and three conducting electrodes: gate, source and drain.

Organic field-effect transistors (OFETs) were first reported using polymerized polythiophene in 1986 by Tsumura, Koezuka and Ando at Mitsubishi Chemical. Interest in this field grew in the late 1980s and early 1990s with the development of organic heterojunction solar cells by Tang and co-workers in 1986 and the organic light emitting diode in 1987. Improvements in materials and processing have made organic field-effect transistors a more promising material, giving the best OFETs charge carrier mobilities comparable to silicon.

These types of devices are important because of their low manufacturing cost, large area, and structural flexibility can be used for new organic electronics, radio-frequency identification tags and light-emitting displays. They can be deposited on plastic substrates at low temperatures using techniques such as solution-based printing and spin-coating, which reduces manufacturing costs compared to current technologies for semiconducting materials like amorphous silicon.

Advances in OFETs have been significant in the past 20 years, making them competitive with inorganic thin film transistors; however, there is vast amount research that can be done in the future. Among these are developing the knowledge of ways to improve purity, using solution-based manufacturing methods, understanding the charge transport mechanisms, and employing small molecules in place of polymers.

History of OFETs
The field-effect transistor (FET) was first proposed by J.E. Lilienfeld, who received a patent for his idea in 1930. 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.

Device Design of Organic Field-Effect Transistors
Three 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).

Classified by the properties of the carrier, three types of FETs are shown schematically in Figure 1. They are MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), MESFET (MEtal Semiconductor Field-Effect Transistor) and TFT (Thin Film Transistor).

MISFET
The 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.

MESFET
The 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.

TFT
The 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 (shown in Figure 2).



OFET
OFETs 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 and 0.6 cm2 V–1 s–1 for solution-processed polymers 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.

OFET Conduction Mechanism
Modeling the conduction mechanism of OFETs is a difficult task due to a limited understanding of the physical origin of high mobility in organic semiconductors. For this reason, an exact description of the conduction mechanism of organic semiconductors has not yet been established. However, much theoretical work has been done in an attempt to gain insight into this open-ended question.

It is generally accepted that the forces in a semiconductor play a significant role in determining its conduction mechanism. In inorganic semiconductors, the atoms are held tightly together via strong covalent forces which have energies as high as 76 kcal mol-1. This arrangement causes the discrete energy levels of the atoms to widen into bands and allows charges to move freely with high mobilities. The mobility of the charge is then limited by thermally induced lattice vibrations, known as phonons. These vibrations increase with temperature and scatter the carriers, lowering the charge mobility of the inorganic semiconductor.

In contrast, organic semiconductors are held together mainly by weak van der Waals forces with energies usually smaller than 10 kcal mol-1. The carbons in these materials are sp2 hybridized, which results in a conjugated structure. Each carbon is bonded to its neighbors by three sigma-bonds (2s, 2px, 2py) and one pi-bond (2pz). The pi-bond offers significantly less overlap than the sigma-bond, therefore, reducing the energy distance between the HOMO-LUMO levels. This allows the organic material to behave as a semiconductor at nonzero temperatures. Because charge transport occurs through delocalized pi-bonds, it is mostly limited by localized states induced by defects and unwanted impurities. Consequently, charge mobility and conduction mechanism can largely vary from sample to sample because the transport properties of these materials are highly dependent on their chain packing, degree of order in the solid state, density of impurities, and structural defects. If defects of the structure are too great, the material may exhibit very low charge mobility, rendering it uncompetitive with inorganic field-effect transistors.

Because charge mobility is highly important for determining OFET performance, much research has focused on this parameter. Many models have been suggested by theoretical studies which predict the mobility is dependent upon electric field, temperature, and charge carrier density. Recently, a review by Horowitz highlighted four of the most widely accepted conduction mechanisms, which are described below:

Hopping


This mechanism (Fig. 3) involves charge transport by the hopping of charges between localized states and is valid for amorphous or organic semiconductors. This is different from the mechanism found in inorganic semiconductors because thermally activated phonons actually assist charge transport. Mobility, therefore, increases with increasing temperature.

Small Polaron
This mechanism is a one-dimensional, one-electron model developed by Holstein. It involves self-trapping of the charge in a localized state in the gap between the valence and conduction bands. This localization results from the formation of a polaron, which is a deformation of the conjugated chain due to the presence of the charge. The polaron then causes hopping of the charge to neighboring sites, which Holstein assumed were uncorrelated. This results in the mobility varying in a simple thermal activation mechanism down to some critical temperature. Below this temperature band transport, like what is found in inorganic semiconductors, dominates.

Field-Dependent Mobility
At high electric field (excess of ~105 V cm-1), mobility in organic materials becomes field dependent. This occurs via a Poole-Frenkle mechanism. In this mechanism, the applied field modifies the coulombic potential near the localized states. This results in an increase of the tunnel transfer rate between localized states, thereby increasing mobility.

Multiple Trapping and Release


This mechanism suggests that narrow delocalized bands interact with a large number of localized states. These localized states act as traps that charge carriers can fall into as they move through the delocalized states {Fig. 4]. It is assumed the probability of a charge carrier being instantly trapped after arriving at the localized state is very near one. It is also assumed the release of the charge carrier is controlled by a thermally activated process. Therefore, the mobility increases with increasing temperature.

P-type vs. N-type Conduction
The conduction mechanism of an OFET can also depend on whether the transistor functions as a p-type or an n-type. For p-type transistors, the major charge carriers are holes, while in n-type transistors the major carriers are electrons. Materials that are n-type are characterized by high electron affinity, while p-type materials are characterized by a low ionization potential. Thus far, the majority of research has been done on p-type OFETs. This is mainly due to their stability in air and relatively high mobility. Most n-type transistors are air and moisture sensitive. This is due to the organic anions found in their structures, which react with oxygen and water under normal operating conditions. The electron mobility degrades as a result and there is often no field effect when the device is exposed to air.

p-Type transistors also show promise because it has been found that most organic materials tend to transport holes better than electrons. Pentacene has received much attention in this area as a potential p-type OFET and it has the highest recorded field-effect mobilities (0.3-0.7 cm2V-1s-1 on SiO2/Si substrates, 1.5 cm2V-1s-1 on chemically modified SiO2/Si substrates, and 3 cm2V-1s-1 on polymer gate dielectrics). Oligomers made from conjugated oligothiophene and polymers also show promise as p-type OFETs. These structures are easily modified through synthetic techniques, which allows for fine-tuning of their electronic properties. The charge carrier mobility of these structures can easily be improved by simple addition of alkyl chains to the end of oligothiophene rings. This is likely due to enhanced π-orbital overlap.

Methods of Preparation
One of the main features of field effect transistors, especially OFETs, is the deposition of the semiconducting film. In FETs this film is made from inorganic materials, while in OFETs the semiconducting film is made from an organic polymer. The semiconducting film is the substance that conducts all the electrons through the device, making sure that they reach both the source and the drain. There are several ways to deposit the semiconducting film, as outline below.

Electropolymerization
Electropolymerization uses an electrical current to form a polymer from a monomer. A current is passed through a solvent containing the dissolved monomer. As the current is passed through the solution, the monomer is assembled into a polymer and deposited on the anode. The product of electrochemical polymerization is a film that adheres well to the electrode surface.

Advantages of electropolymerization include it being a rapid and relatively simple technique. The only equipment necessary for electropolymerization includes a solution containing the monomer and a source of electricity (a simple battery would suffice). Additionally electropolymerization allows for even film thickness over irregular surfaces.

There are several disadvantages in using electropolymerization to deposit semiconducting films. Electropolymerization occurs only on conducting substrates. This limits the types of surfaces that can be electropolymerized. For example, this method works for metal surfaces but not for rubber or non-conductive plastics. Additionally, the semiconducting film, usually a conducting polymer, is obtained in its oxidized form as a result of electropolymerization, when the reduced form is desired. This means the film needs to first be reduced in order to achieve semiconducting capabilities. This reduction step adds additional cost and time in the manufacturing of OFETs. Furthermore, reducing the polymer from its oxidized form can introduce an additional source of disorder. These defects limit the device performance. Because of these drawbacks, electropolymerization is generally not used in the fabrication of OFETs.

Solution-Processed Deposition
Solution-processed deposition, also known as chemical deposition, employs a starting solution containing a solvent and substrate that undergoes a physical change at a solid surface leaving a solid layer behind. Spin coating is a specific type of solution-processed deposition and involves the solution of interest being dropped onto the surface of the device, with the whole complex being spun to evenly disperse the solution and remove the liquid components.

Advantages of solution-processed deposition are that it allows for the production of a very homogeneous film and offers excellent control over the film thickness. Film thickness can be controlled through the deposition of subsequent layers on the substrate.

The primary disadvantage in solution-processed deposition is that not all polymers are soluble. For solution-processed deposition to be useful for OFETs, the polymer must be soluble in a convenient or viable solvent so that it can be dropped onto the substrate. Many polymers are not soluble.

There are two main ways to create solution-processed deposition of insoluble polymers. The precursor approach introduces thermally labile, solubilizing groups that aid in the formation of thin-films from solution. After deposition these solubilizing groups can be removed to form the polymer film. In the solubilizing approach functional groups are incorporated in the semiconductor molecule to increase solubility. Generally these solubilizing functional groups are grafted to the polymer backbone. Attachment of additional functional groups can be used to tune the planar structures and subsequent crystal structure of the semiconductors. This method is more frequently used to deposit the polymer semiconducting film onto the substrate.

Vacuum Evaporation
Vacuum evaporation is a form of physical vapor deposition in which thin films of material are deposited onto surfaces. Vacuum evaporation consists of heating the material to be deposited under reduced pressure to produce a flux of vapor. The vapor then seeks out the cooler areas of the chamber, near the surface of the substrate where the deposited material is desired, and condenses.

Advantages of vacuum evaporation for deposition of polymer substrates include easy control of both the thickness and purity of film. Highly ordered films can be realized by monitoring the deposition rate of the polymer onto the substrate. High purity of films occur as a result of the different vaporizing temperatures necessary for different small molecules, allowing the deposition of the desired molecule by controlling the temperature.

There are however several disadvantages to this method. Generally, vacuum evaporation requires sophisticated instrumentation when compared to the simplicity and low cost of spin coating. Special chambers and pumps are required to produce a low vacuum, which may introduce additional expenses. Additionally, many polymers decompose upon heating. This presents a challenge in getting the right form of the polymer deposited onto the substrate of interest.

To date, vacuum evaporation has demonstrated the highest electrical performances in OFETs, especially compared to solution-processed polymer semiconductors. Despite their high performance, it has been very difficult to translate vacuum evaporation to large-scale production and solution-processed deposition still remains a favorite for manufacturers of OFETs.

Langmuir-Blodgett Technique


The Langmuir-Blodgett technique [Fig. 5] involves dipping the substrate surface through a layer of polymer deposited on the surface of a solvent. This technique was realized through the observation that when less than a monolayer of an amphiphilic species is deposited on water and then compressed, there is a rapid increase of surface pressure as molecules in the hydrophobic layer pack together. The amphiphilic species orient with their hydrophilic heads on the water surface, and their hydrophobic tails in the air to minimize interfacial free energy. Blodgett, a colleague of Langmuir transferred monolayers of fatty acids from the surface of water onto a solid surface by dipping a sheet of glass through the air-surfactant-water interface. Repetitive dipping produces Langmuir-Blodgett films many bilayers thick. Surfactants, hydrophobic, and colloidal particles can all be coated as monolayers and multilayers using this technique.

Advantages of the Langmuir-Blodgett technique include that it allows fine control of both the structure and thickness of the semiconductor film.

One disadvantage of this technique is that it is restricted to amphiphilic molecules, molecules with a hydrophilic head group and a hydrophobic chain. Only a handful of the molecules used as semiconducting layers for OFETs fall into this category. Additionally, mixing of electrically active and inactive compounds (hydrophobic vs. hydrophilic compounds) leads to a decrease in the mobility of charge as compared to vacuum evaporated films.

Substrate and Insulator
Historically OFETs are constructed on a silicon wafer, with SiO2 as the insulator. There are several reasons why it is desirable to move away from this in the future. Using silicon wafers in OFET construction makes the OFET not truly organic, and increases the cost of OFETs. To remedy this, work continues towards the use of plastic substrates to make the OFET fully organic.

Initially a glass substrate (generally ITO) instead of a metal substrate (Si) was used, with a spin-coated organic polymer as an insulator. Once OFETs were successfully produced on a glass substrate, the next step was to make the substrate fully organic by making the substrate out of a polymeric film (one example used polyamide). Once the substrate was made fully organic through the use of a polymeric film instead of a glass side, OFETs had the added benefit of being flexible.

Ease of processability was also pursued through the use of ink-jet printing of the electrodes. As the ink-jet printing technology was refined, OFETs were made from an all-printed device on an ITO coated polyester substrate.

Applications
OFETs have shown great potential in the construction of complementary circuits, flexible displays, radio-frequency identification (RFID) tags, and sensors due to their low cost and flexibility. These characteristics lead to their promising application in fabricating cheap, portable devices. To date, their performance has already been able to rival the performance of amorphous silicon-based FETs.

Organic Complementary Circuits
Complementary circuits [Fig. 6] are circuits composed of p-channel and n-channel transistors which require less power than unipolar circuits since one of the transistors is able to block current flow during signal input. Compared to p-channel circuits, complementary circuits are ten times faster under the same mobility. Since they require two types of transistors to be patterned, the deposition process of complementary circuits are more complicated. OFETs simplify the patterning since devices which function both as p-channel and n-channel transistors can be created by mixing two types of semiconductors or having two layers of thin film instead of a mixture.

Organic complementary circuits have two major disadvantages: their high power consumption and the fact that the electron transportation process tends to be more environmentally sensitive than hole transportation process. Scientists have been working to decrease the energy consumption by using self-organizing monolayers as thin as 3nm as the insulator layer. This lowers the operating voltage of the transistor, as operating voltage depends on the thickness of the insulator. Scientists have also been trying to improve the charge carrier mobility and the stability of n-channel OFET materials. One example is F16CuPc which has a charge mobility in excess of 10-2cm2V-1s-1 in well-ordered thin-film form.

RFID tags
Radio-frequency identification (RFID) tags are objects used for tracking purposes and communicate with the RFID reader using radio waves. RFID tags identify individual objects like barcodes do, which enable asset tracking. The major disadvantage for this technique is the high cost of Si-based tags. Several companies are now working on developing all-polymer RFID products.

One of the key components of an RFID tag is the front-end rectifier. This is a p-type circuit with two connected transistors as diodes and two as switches. In 2006, a flexible circuit which used pentacene-based bottom-gate bottom-contact structure transistors. It operated at 13.56MHz which is a non-quasi static regime for pentacene transistors.

Sensors
Applications of OFET as chemical and biological sensors are mainly based on the fact that the interaction between analyte and organic semiconductors will convert their chemical information to electronic information. During the several years, OFET sensors with bottom gate configurations (which expose the semiconductor layer to gaseous analyte) have shown to have a variety of applications in sensing gas-phase molecules. For example, α-sexithiophene (α-6T) based transistor for detecting alcohol in the gas phase with sensitivity in ppm range has been reported. Another example is NiPc-based FET sensor with sensitivity to ozone concentrations below 10 ppb. By utilizing regioregular poly(3-hexylthiophene) (P3HT) as a semiconducting material, OFETs also show potential as solution-phase pH sensors and enzyme based biosensors.

Flexible Displays
In 2001, Philips first demonstrated an active-matrix display using OFETs. OFET were utilized to drive polymer-dispersed liquid-crystals between light-scattering and transparent states. This active-matrix display was capable of displaying images, utilizing 256 gray levels.

Besides its application as switching devices in flexible displays, an OFET can act as a light-emitting device due to the fact that ambipolar operation of an OFET can induce carrier recombination, exciton formation and light emission, which can be termed as an organic light emitting transistor (OLET). In 2003, Hepp and coworkers reported light emission from a tetracene thin film OFETs. Light was emitted at 540nm, the typical wavelength of tetracene. In 2008, Kudo reported that by optimizing the layer thickness and the size of gate electrode, high light emission with very low gate voltage(~1V) can be achieved.

Future Work
Future work in the area of device design depends upon two key areas: reliability and electrical operating conditions. In order to avoid device degradation, it will be necessary to shift threshold voltage, reduce field-effect mobility and increase the OFF current. Trapping charge carriers in the gate dielectric or in localized areas of the semiconductor can be improved as well. As with most devices, structural defects and chemical impurities also impact device performance. Future device design may look at ways to tune these defects to maximize efficiency. Other areas for future work stems from understanding how structure affects properties of OFETs. With more theoretical and experimental studies, materials with higher performance could potentially be designed.