AN ANALYTICAL MODEL FOR THE ORGANIC FIELD-EFFECT TRANSISTOR IN THE DEPLETION MODE - APPLICATION TO SEXITHIOPHENE FILMS AND SINGLE-CRYSTALS

Organic field-effect transistors (OFETs) usually operate in the accumu lation mode, where the biasing of the gate induces the charging of the insulator-semiconductor interface; the bias is negative in the most c ommon case of p-type semiconductors. We show here that the application of a positive gate bias leads to the formation of a depletion layer. and a subsequent decrease of the drain current. We develop an analytic al model for this depletion mode of the OFET. It is shown that measure ments in the depletion mode give access to parameters such as the dopi ng level and the density of trap levels. The model is applied to data obtained on sexithiophene (6T) film and single crystal OFETs. A substa ntial amount of traps is found in unsubstituted 6T, whereas the dihexy l substituted 6T is practically trap free. 6T single crystals are char acterized by a very low doping level, which can be related to their ve ry high purity. The possible use of the depletion mode to increase the on-off current ratio of OFETs is discussed.