Anita Risteska

• Thin-film transistors (TFTs) are key building blocks for flat panel displays with active-matrix addressing. TFTs based on different material systems, ranging from inorganic to organic semiconductors, have been extensively researched for electronic applications requiring flexibility, low cost, large area and transparency. In the following thesis, different limiting aspects of the thin-film technology have been addressed and discussed. The ability of some materials to accumulate both, electrons and holes, demonstrating an ambipolar behavior, has enabled realization of ambipolar transistors and digital circuits. An analytical model was developed describing their electrical behavior. A comparison between unipolar and ambipolar devices is presented with good experimental agreement. The results showed that the operation of ambipolar circuitry is limited by their low static noise margin and increased power consumption. Increasing the device’s frequency range operation is usually achieved by scaling. A model describing the static and dynamic behavior was developed for CMOS inverters. The influence of device scaling on the performance was studied with good agreement to experiments. Scaling the lateral dimensions of the transistors down to few µm limits the circuit performance due to contact effects. Reducing the contact resistance (RC) has been proven to be an effective way for achieving faster circuit operation. A method of using self-assembled monolayers treatment for RC reduction is discussed and a model describing its effect on the RC was developed. The results revealed that the RC is mainly determined by the molecular ordering, rather than the tuning of the work function. The stability of TFTs is an important aspect for reliable circuit operation and high yield. A density-of-states model was used to study the environmental effects on the current/voltage characteristics of organic transistors. The results showed that the threshold voltage shift observed while biasing the devices in oxygen/moisture is caused by formation of acceptor- and/or donor-like states.