Device physics of organic field-effect transistors

PhD ceremony: Mr. J.J. Brondijk, 16.15 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Device physics of organic field-effect transistors

Promotor(s): prof. D.M. de Leeuw, prof. P.W.M. Blom

Faculty: Mathematics and Natural Sciences

Conventional electronics is based on inorganic semiconductors, mainly silicon. Silicon has excellent performance but is also brittle and expensive. Organic semiconductors on the other hand are flexible and soluble, which allows processing by established printing techniques. Therefore, these carbon-based materials are interesting for application in low-cost flexible electronics on large-area. Examples include (disposable) electronic barcodes and flexible displays.

The basic building block of any electronic circuit is the field-effect transistor. To successfully realize applications, a thorough understanding of this element is essential. In this thesis, the device physics of organic field-effect transistors is studied. The work focuses on three essential aspects of the device physics of organic transistors: the charge injection from the metal contacts into the semiconductor, the electrical transport inside the semiconductor, and the influence of the transistor geometry on the electrical transport.

The first main result focuses on memory. Memory is crucial to for example electronic barcodes, which need to store and send information. An organic ferroelectric field-effect transistor (FeFET) can be used as a memory element. In this type of transistor, a ferroelectric layer which can be polarized is integrated, resulting in a rewritable memory with two stable states: on or off. Despite recent technological advances, the fundamental understanding of organic FeFETs is still in its infancy. In this thesis, for the first time, a model is developed which describes the polarization-dependent charge transport in organic FeFETs.

The second main result follows from investigating the effect of carrier confinement on the charge-transport properties of organic transistors. Confinement is achieved experimentally by the use of a semiconductor layer which is only one molecule thick. The two-dimensional confinement of charge carriers provides access to a previously unexplored charge-transport regime and is reflected by a reduced temperature dependence of the transfer curves of organic monolayer transistors.