Organic thin film-transistors – fundamentals and recent results
Field effect transistors (FETs) especially metal-oxide-semiconductor FETs (MOS-FETs) are the central type of devices in modern microelectronics. Since the 1970s organic transistors have been studied as an interesting alternative. Typically, they are realized as thin-film transistors (O-TFTs). The working principle of TFTs and therefore O-TFTs is different from that of MOS-FETs. In the first part of my talk, I will compare these two kinds of transistors. A very important parameter in this context is the so called threshold voltage Vth. The influence of space charges and dipoles located at the gate-dielectric/semiconductor interface on Vth can be understood from fundamental electrostatics and a more in-depth picture emerges from drift-diffusion based modeling. In this talk, I will present experimental results concerning approaches to control and shift Vth in OTFTs. One method to shift Vth is by depositing self-assembled monolayers or polymers including acidic functionalities at the gate/dielectric-semiconductor interface. Further I will present the impact of the dielectric thickness as well as the influence of the interface layer thickness on Vth. The methods used in these studies are x-ray reflectivity (XRR) to determine layer thicknesses, contact angle (CA) measurement to determine the surface energy of the modified gate-dielectric, an electrical characterization of the devices using a dual-source meter, and Raman spectroscopy to measure doping effects.