Organic transistors: Improved performance and fast response

Organic field-effect transistors (OFETs) have been proposed for a number of applications including displays, electronic barcodes and sensors. The attractions of low cost processes, large-area circuits and the chemically active nature of organic materials are the chief driving forces to make OFETs important in various applications. FETs based on p-type semiconductors such as pentacene or oligothiophenes in which holes are the majority carriers, have received most of the attention. In contrast to the extensively studied p-type materials, the number of organic compounds with good n-type characteristics is still limited. This is an important problem since n-channel transistors are required for the fabrication of complimentary circuits. Increasing electron affinity of molecules can be achieved by introduction of electron-withdrawing functional groups and the Marks group has successfully synthesized families of carbonyl-substituted oligothiophenes (DFHCO-4T and others) which have achieved high electron mobilities. We describe techniques to form small channel length (4 μm) bottom-contact transistors that will be useful for a number of applications including complementary logic circuits and display drivers. It is very important to pay attention to the details of surface preparation and the interface between the contact metal and the organic semiconductor [1]. The transient response of organic transistors is crucially important in determining the speeds of circuits. One important technical hurdle that has to be overcome for using organic transistors in RFID tags is for these devices to operate at RF frequencies (typically 13.56 MHz) in the front end. It was long thought that organic transistors are too slow for this. In recent work [2], we have shown that organic transistor based full-wave rectifier circuits utilizing pentacene, a p-channel organic semiconductor, can operate at this frequency with a useful efficiency. In order to achieve such high-frequency operation, we make use of the non-quasi static (NQS) state of the transistors. We will review the transport phenomena in pentacane transistors and present a model of how fast rectifier circuits will work. Finally, the characteristics of nanoscale organic and polymer transistors are discussed. At small geometries, contacts play an increasingly dominant role.