Tailoring Properties of Printed Field-Effect Transistors by Design and Material Changes

Due to the increasing digitalization in all areas of life and work, including digital signatures or even electronic feedbacks from single component parts or consumer articles, the question for new possibilities for quick and simple manufacturing of circuits arises. In this respect, also mass production of transistors, the core building units of electronic logics, has to be reconsidered in order to overcome the often complex production of silicon chips. In this scope, printing of electronic components has presented itself as a highly promising method within the recent years. Conductive paths, solar cells, or displays have already been integrated as printed electronics in manyfold industrial production processes. In contrast to this, so far, no reliable processes for printed transistors in an industrial scale have been established. Organic and inorganic semiconducting materials often have properties with diametral differences: while organic materials use to be p-type semiconductors and come up with simple processability and mechanical flexibility, inorganic systems tend to be n-type semiconductors, brittle and in need of high processing temperatures. ... mehrThese disadvantages of inorganic, especially oxidic semiconductors however are compensated by often severely better electronic properties and increased environmental stability. In order to achieve very low operation voltages while maintaining sufficiently high currents, it is recommended to realize channel polarization via electrolytes instead of dielectrics, as the formation of Helmholtz double layers allows for locally very high fields. By this, units can be operated at voltages typical for commercially available batteries. Within the scope of this work new and improved methods for the processing of printed field-effect transistors were successfully implemented, contributing to the development towards large-scale production of devices with predictable properties. Starting form planar field-effect transistors with displaced gates, three possibilities for improvements have been examined: a vertical geometry, doped channels and an alternative electrolyte. By changing from a planar to a vertical device geometry, channel lengths may become independent from the material printers’ resolutions and can be reduced to the thickness of the deposited films. By this lengths may be shrunk from a two-digit micron range to submicron values. As channel lengths are directly correlated with output currents, a severe improvement can be realized. However, in this case the channel must be porous in order to allow a large surface being covered by electrolyte. Within this work, a known system with an SnO2 channel could be improved by developing a simplified production method with quickly available materials, successfully resulting in a fully functional device. For reliably tailoring the central property of the threshold voltage, i.e., the gate voltage at which the channel changes from an insulating to a conducting state, on a per-device level, an In2O3 precursor ink with varying chromium doping has been developed. Through this doping, the threshold voltage could be changed linearly with the dopant concentration, however under severe loss in the output current. In a third experimental series, the usually applied composite solid polymer electrolyte has been replaced with Al2O3 . However, instead of a dielectric gating, a material with low density and many hydroxy functionalities acting as an electrolyte was created due to low temperatures during the atom layer deposition process. Depending on the humidity, the functionality of such transistors can be changed as the electrolytic properties are based on protons generated on the Al2O3 surface. Within this work, three alternatives for the production of inorganic field-effect transistors could be successfully shown and a contribution towards the further development of this technology was made.