Hetemjunction Organic Thin-Film Transistors

Organic transistors have some characteristics which make them attractive in applications such as large area electronics. These include low-cost deposition processes, compatibility with plastic substrates, and performance levels that are steadily improving. Thin-film transistors (TFTs) with active materials consisting of oligomers of thiophene have been pioneered by Gamier et al. [l]. Such TFTs are p-channel transistors which work in the accumulation and depletion modes, but never in the inversion mode. We have recently shown that the field-effect mobilities of 1-5 x lo-’ cm2N-s, and odoflcurrent ratios more than lo6 are achievable with such materials [2]. Such high odoflratios have previously never been achieved in organic transistors. We have also found that the field-induced conductance is independent of the thickness of the organic active material. The switching speeds of a-6T thin-film transistors, which depend on bias conditions and device dimensions, have been measured. Switching times of - 10 ps are typical and indicate that such transistors are fast enough for use in display applications. A few organic materials such as c60 have been used to make n-channel TFTs. These devices operate in the accumulation mode and, as with the oligothiophenes, can never be operated in the inversion mode. We have developed a unique organic transistor structure which enables us to realize both p-channel and n-channel operation in a single device. This device also convincingly demonstrates the applicability of heterojunction concepts to organic semiconductors. The active material consists of two layers: the .first (adjacent to the gate dielectric) layer is made up of a-hexathienylene (a-6T), a thiophene oligomer which exhibits good p-channel operation. This layer is typically 10-20 nm thick. The second active material is c60 and is about 20-30 m thick. The energy levels of the occupied and unoccupied molecular orbitals of a-6T and e60 are such that when the gate is biased negatively with respect to the source, the p-channel material is filled with holes and when the gate is biased positively, the n-channel material (&) is filled with electrons. If the a-6T layer is very thick (> 40 nm), n-channel operation is affected, and for this reason is kept below 20 nm. Thus, with two active materials, the same transistor can be used as either an n-channel or a p-channel device. This device structure convincingly demonstrates the applicability of heterojunction concepts and design techniques to organic/polymeric materials. A detailed analysis of the transistor characteristics provides important insights into transport in organic materials and wiU also be discussed.