Organic Thin-Film Transistors - Essay Example

 The two electrodes are the source and drain electrodes. Between the channel and the metal gate contact, a film of insulator material is deposited. TFTs have been used in the manufacturing of TFT LCDs for example in LCD TVs and monitors. Digital radiography has seen drastic improvement since the adoption of TFTs in general radiography. This technology has also been used in the Active Matrix OLED (ALOMED) screens. 

Klauk, (2006, pp41-47) states that organic transistors are transistors that are made using thin films from an organic semiconductor instead of inorganic semiconductors in their channel. Small organic molecules may be evaporated in a vacuum to make the organic transistor. Alternatively, organic transistors are prepared by solution casting and transfer of organic single-crystalline layers on a suitable substrate. The principal idea is the use of organic material as the active semiconductor, to replace the initially used macromolecular organic materials (polymers) to increase efficiency. Electropolymerisation may still be used in case of the unavailability of suitable and commonly used organic semiconductors (Brazis, Gamota, Kalyanasundaram & Zhang, 2004, p9).

The Two Fabrication Structures
Optimization of structural design and devise interfaces led to several construction designs formulations but the most widely used are top and bottom contact Organic Thin-Film Transistors (OTFTs). The terms top and bottom refer to the position of the two electrodes in relation to the position of the semiconductor.
The Top Contact OTFTs
Top-contact OTFTs have their thin semiconductor film layer deposited first, then followed by the source and drain electrodes contacts onto the gate dielectric. A considerable reduction in contact resistance at the two electrodes; therefore they have little structural disorder at the interface.
The Bottom Contact OTFTs
Bottom contact OTFTs have their source and drain electrode contacts first deposited first onto the gate dielectric supporter then the organic semiconductor crystal structure deposited directly on top. Molecular layers in the crystal structure are laid in a less ordered manner during the deposition onto the source and drain electrodes.
While top contact OTFTs have reduced contact resistance and structural disorders at the interface, this is not the case with bottom contact which has a less ordered structure that has interface disorders. Bottom contact OTFTs have an easy fabrication procedure than top contact OTFTS. Top contact OTFT has the better performance of the two designs.
Advantages
• Top contact OTFTs have the highest performance due to reduced contact resistance when compared to bottom contact OTFTs. This relative advantage is due to the planar design that enables orderly morphology at the interface.
• Bottom contact OTFTs are relatively easy to fabricate than top contact OTFTs.
• Both techniques lead to electronic gadgets that require a little amount of charge to control, unlike other related technologies such as the Cathode Ray Tubes (CRTs) and Liquid Crystal Displays (LCDs).
• Both designs give better image tracking at high speed, which was not possible in earlier technologies such as LCD.
Disadvantages
Bottom contact OTFTs have higher contact structural disorders and reduced performance when compared to top contact OTFTs.
This topic is relevant in that it demonstrates the huge contribution that organic electronics and experiments can make towards improvements in technology (Brazis et al, 2004, p1).
Field Effect Mobility in Organic Transistors
Field-effect mobility µFE is the ratio of the measure of thin-film conductance inside a channel (organic semiconductor) to the field-induced per unit area.
Where g represents channel along with the conductance (Ω-1) and Qind represents the measure of induced charge (C cm-2) as reported in Klauk, (2006, p328).
Current Leakage in Transistors
When current flows through an insulator, it is referred to as leakage current. The penetration of current through an insulator happens when the outdoor insulator material gets exposed to conductive fog or when molecular salt ions form on the insulation material surface.
The most likely cause of current leakage in OTFTs is mainly due to defects sustained during threading dislocations. The size of the film thickness is important in determining the current leakages. The amount of current passing through the gate also determines whether there will be current leakages. The cause of leakages can also be traced from the output conductance in transistors that use field effects, such as OTFTs (Morkoc, 2008, p326).
To suppress current leakage at the gate, low temperature and atmospheric pressure are used in the deposition of the organic semiconductor with the help of an atmospheric pressure plasma jet. This procedure gives other desirable characteristics such as lower operation and threshold voltage as well as reduced threshold swing and high field electric mobility. The selected source of the film (semiconductor) is also important in determining the amount of leakage current density expected at the end of the deposition. The reduction of current leakages can also be enhanced by the adoption of suitable and quality gate insulator material (Joshi, Srinivasan & Todi, 2009, p27).
Further improvements can be affected by the improvement of device interfaces and architecture. The nature of contact between the semiconductor and the electrodes can be enhanced to improve the structural design with reduced resistance and defects.