Organic Field Effect Transistor
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Organic field-effect transistors (OFETs) use organic semiconductors like pentacene that can be deposited through low-cost solution processing. OFETs have the potential for applications requiring flexibility and large-area coverage. Pentacene has shown high carrier mobility, with mobilities on par with amorphous silicon in the best OFETs. While progress has been made, understanding charge transport and developing n-type and ambipolar materials remains an area of ongoing research to further improve organic electronics.
Organic Field Effect Transistor
- 1. Organic Field-Effect Transistors Submitted by: Santosh Kumar Meena B10130
- 2. Organic Field-Effect Transistors Introduction Our daily life involves the continuous use of electronic devices (e.g. TV, bank cards, computer screens, etc.). Since the invention of the first transistor in 1947 by John Bardeen, William Shockley and Walter Brattain, the vast majority of these devices have mainly been based on inorganic semiconductors and, in particular, on silicon. However, due to technological limitations associated with the use of silicon, substantial efforts are currently devoted to developing organic electronics. The processing characteristics of organic semiconductors make them potentially useful for electronic applications where low cost, large area coverage and structural flexibility are required. Organic field-effect transistors (OFETs) also sometimes known as organic thin-film transistors are attracting widespread interest today because of the possibility to fabricate these with acceptable performances over large areas and on flexible substrates using cost-effective and materials-efficient fabrication methods. These include the well-known spin-casting technique, and also newer methods of ink-jet or other forms of printing. Its rapid growth has been spurred primarily by the remarkable development of new materials with improved characteristics and also advancement in the understanding of their structure–morphology–property relations. Field-effect transistors are the main logic units in electronic circuits, where they usually function as either a switch or an amplifier. Organic field-effect transistors (OFETs) have been mainly based on two types of semiconductors conjugated polymers and small conjugated molecules. The first OFET was reported in 1986 and was based on a film of electrochemically grown polythiophene.2 Four years later, the first OFET employing a small conjugated molecule (sexithiophene) was fabricated. The performance of OFETs in the last 20 years has improved enormously. Nowadays, charge carrier motilities of the same order as amorphous silicon (0.1–1𝑐𝑚2 𝑉 −1 𝑠 −1 ) are achieved in the best OFETs. Thiophene and, especially, acene derivatives are Considered to be the benchmark in OFETs and most of the best motilities have been found in these two families of compounds. However, devices prepared with these molecules are typically prepared by evaporation of the organic materials due to their low solubility in common organic solvents. The main goal of this review is to give a general overview about the current standing in the area of OFETs focusing on the new process able small molecules that have been recently reported for their use as organic semiconductors.
- 3. Basic device structure and physics Field-effect transistors are three-terminal devices comprising gate, source and drain electrodes. In an OFET, an organic semiconductor is deposited to bridge the source and drain electrodes, and is itself spaced from the gate electrode by an insulating gate dielectric layer. The organic semiconductor can be a pi-conjugated polymer or oligomer. Important examples include oligothiophenes and polythiophenes, and substituted pentacenes, which can be deposited by solution-processing. This solution capability is central to the objective to fabricate large arrays of transistors on large format and potentially flexible substrates, for example, as required in electronic paper and posters. Two voltages are applied relative to the source electrode which is kept at common (0 V): the drain voltage (Vds) is applied to the drain electrode, while the gate voltage (Vgs) is applied to the gate electrode. This gate voltage provides an electric field that leads to the accumulation of charge carriers at the semiconductor/ dielectric interface which modulates the source-todrain conductance. Unlike traditional silicon and other doped inorganic semiconductors which can operate in both the inversion and accumulation modes, OFETs at present typically operate in the accumulation mode due to the difficulty to impose a stable controlled level of background doping. Depending on whether p-channel or n-channel FET characteristics are being measured, the Vds and Vgs are swept in the negative or positive voltages to accumulate the appropriate sign of charge carriers at the semiconductor/ dielectric interface, i.e., holes or electrons. The measured field-effect characteristics of these devices can then be classified as transfer characteristics (Fig. 1a) and output characteristics (Fig. 1b) depending on whether Vgs is varied while Vds is kept constant, or Vds is varied while Vgs is kept constant. These characteristics are typically measured on a semiconductor parameter analyser, and sometimes as a function of temperature to understand the underlying physics. For research devices fabricated on 200-nm-thick thermal oxide on doped silicon substrate as the gate electrode, Vgs can be routinely applied up to 60 V without gate dielectric breakdown. These
- 4. voltages can be downscaled to a few volts by using thinner insulators and by operating at lower currents. In the transfer curves (a) as the magnitude of Vgs increases for a given Vds, the source–drain current at first increases quadratically with Vgs beyond a threshold voltage, and then quasilinearly with Vgs as Vgs becomes larger than Vds. In this Vgs > Vds regime, the linear-regime field-effect mobility (mFET) can be evaluated using a standard equation from silicon metaloxide-semiconductor FET theory. In the output curves (b) as the magnitude of Vds increases for a given Vgs, the source–drain current at first increases linearly with Vds, and then levels off, i.e. saturates, as Vds becomes larger than Vgs. In this Vds > Vgs regime, the saturation mFET can similarly be evaluated.
- 5. These two mFET values can differ somewhat depending on the quality of the device characteristics. Furthermore, it is well appreciated now that these values can also depend strongly on the dielectric interface and the organic semiconductor morphology at this interface (varying by more than a factor of ten). Furthermore, they can vary with Vgs and temperature, and can be degraded by contact resistance. Nevertheless, these mFET values are still useful indicators of the performance of the organic semiconductor in particular device configurations. There are two common device configurations used in OFETs 1. Top contact 2. Bottom contact Top contact: When the source and drain electrodes are evaporated on the top of the organic material. Bottom contact: When the source and drain electrodes are evaporated on the dielectric before depositing the organic semiconductor.
- 6. Organic semiconductors As mentioned before, polymers and small conjugated molecules are the two families of organic semiconductors that have been studied. Typically, solution processed polymers form complex microstructures, where microcrystalline domains are embedded in an amorphous matrix. The disordered matrix limits the charge transport resulting in low field-effect nobilities. The most studied polymer for OFETs is poly (3-hexylthiophene) (P3HT), which has reported to have an OFET mobility of 0.1 𝑐𝑚2 𝑉 −1 𝑠 −1 this high mobility is related to structural order in the polymer film induced by the regioregular head-to-tail coupling of the hexyl side chains. P-Type semiconductors P-Type semiconductors are materials in which the majority of carriers are holes. As stated before, the most studied materials for OFETs have been acene and thiophene derivatives. Field effect hole motilities of 1.1 and 3 𝑐𝑚2 𝑉 −1 𝑠 −1 have been reported for thin films of alkylsubstituted oligothiophene and pentacene respectively. Also, promising organic semiconductors based on p-extended heteroarenes have resulted in high nobilities between 2.1 and 2.9𝑐𝑚2 𝑉 −1 𝑠 −1 . Examples:
- 7. N-Type semiconductors In n-type semiconductors the majority of carriers are electrons. The development in devices using these semiconductors is still far from the performance achieved with p-type materials due to the fact that the transport in n-channel conductors is degraded easily by air, which acts as electron traps together with the dielectric surface trapping sites, and that most of the known organic materials tend to conduct holes better than electrons. An added difficulty in progressing on the fabrication of electron conducting OFETs is finding suitable metals for contacts. Typically, the metal used in OFETs is gold, which has a work function of 5.1 eV close to the ionization potential suitable metal for injecting holes into the HOMO of most organic materials that behave as p-semiconductors. However, to inject electrons to the LUMO of ntype organic semiconductors it is more appropriate to employ metals with low work functions, such as Al, Ca or Mg. The drawback is that these metals tend to oxidize easily. Examples:
- 8. Best Example of Organic Material Organic Molecule – Pentacene Pentacene Molecule Organic semiconductor Composed of five Benzene rings Polycyclic Aromatic hydrocarbon Easy to synthesize High carrier mobility Pentacene mobility v/s other organics
- 9. Conclusions OFET performances of the same order as amorphous silicon are currently achieved. However, there are still many questions, such as understanding the charge transport mechanisms and the relation structure–mobility, that remain open and which should be addressed to further progress in the field. Finding materials for n-type and ambipolar devices will be also fundamental for the future design of complementary circuits. Finally, if low-cost, flexible and large area-coverage devices are to be realized with organic semiconductors, it is imperative to search for materials with high charge carrier mobility that can be soluble processed, as well as solution-based techniques which allow the preparation of reproducible devices. In conclusion, although there is a lot of work to be done, OFETs promise to be tremendously important for future applications in areas where electronics meets with information technology, biomedicine or optics. References 1. 2. 3. 4. 5. 6. P. T. Herwig and K. Mu¨ llen, Adv. Mater., 1999, 11, 480. G. Horowitz, D. Fichou, X. Z. Peng, Z. G. Xu and F. Garnier,Solid State Commun., 1989. S. Aramaki, Y. Sakai and N. Ono, Appl. Phys. Lett., 2004, 84,2085. D. A. da Silva Filho, E.-G. Kim, J.-L. Bredas, Adv. Mater. 2005, 17, 1072. D. Natali, L. Fumagalli, M. Sampietro, J. Appl. Phys. 2007, 101, 014501. J. Veres, S. Ogier, G. Lloyd, Chem. Mat. 2004, 16, 4543.