Transistors are “transfer resistors”, as their input resistance is high and output resistance is low, hence they “transfer” the resistance. This transfer of resistance allows the transistor to amplify signals, making it an indispensible component in modern day electronics.Field-Effect Transistors have three main parts, the source, where current enters; the gate, which controls the output; and the drain, where the current leaves the transistor.The input current, or source current (IS), enters at the source. The voltage applied at the gate will then control the output current, or the drain current (ID).
In n-channel depletion mode devices, when a negative voltage is applied across the gate to the source, the depletion region widens and when the voltage reaches the pinch-off voltage, the resistance is very high and it effectively closes the channel between the source and the drain, causing it to act as an open switch.In n-channel enhancement mode devices, a positive gate to source voltage is required to form the channel. This positive voltage, or threshold voltage, attracts enough electrons from the body to form a depletion region in the body for a conductive channel to be formed.
For both the enhancement mode and depletion mode devices, when the drain-to-source voltage is much lower than the gate-to-source voltage, the transistor will have a resistance that varies according to the gate-to-source voltage and it can be said that it is in the ohmic mode.
When the drain-to-source voltage is higher, it will be in the saturation (or active) mode. This is when the drain voltage increases with supply voltage and hence ID does not vary with the supply voltage. This is when the transistor acts as a voltage amplifier.
There are different types of FETs, including the most common Metal-Oxide-Semiconductor FET (MOSFET), Junction FET (JFET) and the Thin-Film Transistor (TFT).
The most commonly used geometry of the OFET is similar to that of the Thin-Film Transistor (TFT). A thin film is deposited onto a semicondctor, which lies on a non-conducting substrate, which varies according to the application for which the transistor is being used.
Instead of using expensive semiconductor materials such as silicon or metal oxides, lower cost organic materials are used in OFETs, allowing them to be used in electronics with large areas. They can also be biodegradeable and even in flexible screens. OLEDs can be combined with OFETs to create a fully flexible display made of organic materials.
Conduction of current through the OFET is dependent on the charge carriers at the organic semiconductor and dielectric interface. When a small negative gate to source voltage is applied, some holes start to accumulate in the organic semiconductor. As this voltage gets more negative, more of these holes are formed and hence increasing the conductivity of the organic semiconductor. Similar to the n-type transistors, the p-type organic transistor can operate in 2 regions, either in the saturation (active) or the linear region.
When in the saturation region, the drain current is given by
Where W is the channel width, L is the channel length, µ is the mobility, C is the capacitance of the dielectric, VG is the gate voltage and VT is the threshold voltage. It can be seen from this equation that the drain current does not depend on the source voltage once the source voltage is above the threshold.
In the linear or ohmic mode, the drain current increases linearly with the gate voltage according to the equation:
Where W is the channel width, L is the channel length, µ is the mobility, C is the capacitance of the dielectric, VG is the gate voltage, VT is the threshold voltage and VD is the drain voltage.
In conventional semiconductors, the charges travel in the delocalized states. However in OFETs, the low conductivity of the material prevents this from occurring. Instead, ‘hopping’ occurs. This hopping of the charge carriers varies with the energy gap between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbitals (LUMO) levels of the molecules.
In inorganic conductors such as metals, the “sea” of delocalized electrons function as the charge carriers, allowing for the transport of current through the material. This “sea” of delocalized electrons is not present in organic compounds. Instead, the delocalized electrons come from overlapping p-orbitals which form π bonds. The movement of these delocalized electrons allows the organic semiconductors to conduct electricity.
There are two types of charge carrier transport in organic semiconductors, band-like and hopping. In highly ordered crystals, the transport of charge carriers is similar to that of inorganic semiconductors. This form of transport is inversely proportional to temperature and is mostly due to the delocalized electrons in each molecule.
In more disorganized or less pure crystals, the charge carriers hop between molecules. This type of transport increases in efficiency with temperature. However, speeds tend to be much slower as compared to band-like transport.
Common materials used in OFETs are aromatic molecules (usually with multiple aromatic rings) or those with conjugated π systems. Electron donating or withdrawing groups are often added to these molecules to facilitate the transport of holes and electrons. Another area of interest for OFETs are polymers such as poly-thiophenes.
The most well-known molecule used in OFET production is probably rubrene, with the highest carrier mobility among the rest of the organic materials that have been characterized. However, its 20 – 40 cm2/V-1s-1 is still much lower than that of silicon, which is near 1400cm2/V-1s-1 for electron mobility and 450cm2/V-1s-1 for hole mobility. Other small molecules used in OFETs include tetracene (tetra = 4, acenes = fused aromatic rings), pentacene and picene.
Besides Rubrene’s high carrier mobility, also has high photoconductivity and large exciton diffusion length. However, the main problem with rubrene is that production of thin films of rubrene is the inability to form thin films despite the ease of formation of small crystals. Rubrene also oxidizes easily under light.
Acenes are linearly fused aromatic rings. Pentacene is made up of five benzene rings. It has a carrier mobility of about 5.5cm2/V-1s-1, but when it is pre-exposed to air to form pentacene-quinone, it can have carrier mobilities that approaches that of rubrene.
Picene is a "sister" of pentacene, with the same number of benzene rings but arranged in a W. Similar to pentacene, it shows high carrier mobilities when exposed to oxygen. However, one key difference between pentacene and picene is that picene does not interact with metals strongly. This allows picene to retain its chemical and physical properties and hence would not require stabilizing materials between the metal and itself, allowing for high-quality contact with the metal.
Polythiophenes can be doped to allow them to be conducting. Electrons can be added (n-doping) or removed (p-doping) from their conjugated p systems to give them either a negative or positive charge respectively. P-doping is generally favoured as it is more efficient.
Rubrene is synthesized by the addition of thionyl chloride (SOCl2) to 1,1,3-triphenylprop-2-yne-1-ol via an SN2 mechanism, releasing HCl, SO2 and forming 3-chloro-1,1,3-triphenylprop-1,2-diene.
This molecule then undergoes dimerization and then dehydrochlorination to form rubrene.
Pentacene is formed first by dehydration, then dehydrogenation with a copper catalyst. Both steps require temperatures of about 400°C.
As Picene is a relatively new molecule, its synthesis is still highly researched and it’s synthesis and characterization is still being carried out.
OFET devices have a number of potential applications which can take advantage of their properties. Because organic semiconductors are fully satisfied van der Waals solids which do not require any templating for deposition, most OFET processes have a low thermal budget, simple manufacturing processes, and are compatible with a range of substrates. It is these applications which drive interest in OFETs, and the technology’s longevity will be determined by its ability to address the challenges of applications of interest.
Direct view displays are currently the leading application for large area electronic devices. Amorphous silicon (a-Si) FETs dominate active matrix liquid crystal display (AMLCD) backplane architectures because they have a low enough process temperature to allow for economical fabrication on large glass substrates while delivering adequate performance for LCD field driving. Improvements in processing and panel architecture have allowed the commercialization of large (meter-scale) panels and the development of highly efficient manufacturing operations using motherglass sizes of extraordinary size.
Field-driven display elements such as liquid crystals and electrophoretic display materials (of which e-Ink is the leading example) are typically matrixed to create large panel sizes with a manageable number of external contacts. OFETs are an excellent candidate for this class of backplane applications. OFETs share many of the properties of a-Si LCDs, with an even more generous thermal budget. This reduced thermal budget allows fabrication on glass, metal foils, and plastic sheet.
In addition to driving charge controlled devices such as LCDs, OFETs can also be used in current drive configurations. These architecture can be used to drive OLEDs, and mechanically flexible AMOLEDs have been demonstrated using OFET backplanes.
In addition to the potential cost advantage due to easier processing via printing or evaporation, OFETs potentially offer reduced bias stress in current drive applications over a-Si transistors fabricated at less than 200°C. At these temperatures, transistors can be fabricated on a range of transparent flexible substrates and are particularly applicable to flexible OLED displays. There are also circuit and architecture advantages to using PFETS for bottom emission OLED displays
OFETs, because of their mechanical flexibility and large size, are well suited to switch and amplify mechanical actuations.
The first large scale application of OFETs in a mechanical sensor was demonstrated by Someya, T of University of Tokyo. A flexible OFET backplane was laminated together with a flexible conductor loaded elastomer whose resistance changed in response to an applied pressure. By switching through the transistor matrix and observing the current flow from a common power supply through the variable resistor, it is possible to create a force map for use as a flexible sensor skin.
Another application of OFETs to measuring mechanical stimuli is the buffering of charge signals from large piezoelectric polymer sheet materials, such as polyvinylidene difluoride. Extracting spatially localized information about the stimuli applied to the sheet can be challenging because the charge signal is dissipated across the parasitic capacitances between the stimulus and the location of the sensing circuitry. OFETs can serve as local transimpedance amplifiers which convert the charge signal into a current signal that can overcome this capacitance. This amplification can be achieved across heterogeneous substrates or by building the OFET directly onto the piezoelectric polymer sheet.
The large and flexible backplanes available via OFET technologies can also benefit image sensing applications, many of which can benefit from large area (e.g. X-ray sensors), mechanical flexibility (e.g. contact scanners for non-planar objects) or both characteristics.
Large-area, flexible, and lightweight sheet image scanner have been successfully manufactured on plastic films, for the first time, integrating high-quality organic transistors and organic photodetectors. Since this area-type image-capturing device does not require any optics or any mechanical scanning devices, it is innovatively light to carry, shock resistant and inexpensive to manufacture.
Another potentially interesting application of OFETs is in ultra low cost RFID tags and logic elements. If transistors, passive elements, and the tag antenna can be produced simultaneously using continuous printing processes, it is conceivable that economies similar to those achieved in paper printing can be achieved.
Fabrication of OFETs on substrates can be performed at temperatures below 100°C, allowing for flexible plastic substrates, eliminating the need for fragile silicon wafers while at the same time introducing flexibility in integrated circuits. The OFET will dramatically revolutionize the current RFID market with all-printed continuous roll to roll and flexible printed circuits and individual tags, replacing barcode technology and having applications as security tags for bank notes and passports.
The applications OFETs are applied to will continue to define the requirements for material, circuit, and device performance. It is ultimately the appeal of compelling applications which will drive continuing research, product development, and commercialization of OFET technology. It is therefore essential to understand the demands of these applications and the value that OFET technologies can bring to them.
The organic semiconductors are easily oxidized and reduced by water, which contains H+ and OH- ions, and oxygen (O2). As such, they are not very stable under ambient conditions. In order to prevent such reactions from consuming the semiconductors and rendering them useless, certain materials have been found to be more resistant. Currently, there are a variety of p-type materials that are stable enough to be considered. However, n-type materials generally have lower stability against oxygen, and the synthesis of materials that can be incorporated into OFETs is possible, but challenging.
Another challenge in the production of OFETs is the reactions due to the high temperatures used for the deposition of the materials. One of the reactions is that of between the gases in air and the organic semiconductor. While some synthesis methods intentionally make use of such gases to dope the material, this may reduce the performance of the transistors that are produced if unwanted doping occurs. Another reaction would be between aluminum and the foil boats used to vaporize it. At such high temperatures, molten aluminum forms alloys with the materials of the foil boat, slowly wearing it out and causing it to be increasingly brittle and hence requiring constant replacement.
The glass substrate used in OFETs need to be very clean to prevent shorting due to incomplete coating with dielectric layers. This can be done with harsh chemicals, but more often it would be easier for manufacturers to switch glass suppliers if the quality of the glass is not met. However, even high quality cut glass needs to be cleaned again before they are used for OFETs.