Abstracts Category : Other

LATERAL AND VERTICAL ORGANIC TRANSISTORS

by AKRAM AL-SHADEEDI

An extensive study has been performed to provide abetter understanding of the operationprinciples of doped organicfield-effect transistors (OFETs), organic pin diodes,Schottkydiodes, and organic permeable base transistors (OPBTs). This hasbeen accomplished by a combination of electrical and structuralcharacterization of these devices.The discussion of doped OFETsfocuses on the shift of the threshold voltage due to increaseddoping concentrations and the generation and transport of minoritycharge carriers.Doping of pentacene OFETs is achieved byco-evaporation of pentacene with the n-dopant W2(hpp)4. It is foundthat pentacene thin film are efficiently doped and that aconductivity in the range of 2.6 x 10-6 S cm-1 for 1 wt% to 2.5 x10-4 S cm-1 for 16 wt% is reached. It is shown that n-doped OFETconsisting of an n-doped channel and n-doped contacts areambipolar. This behavior is surprising, as n-doping the contactsshould suppress direct injection of minority charge carriers(holes). It was proposed that minority charge carrier injection andhence the ambipolar characteristicof n-doped OFETs can be explainedby Zener tunneling inside the intrinsic pentacene layer underneaththe drain electrode. It is shown that the electric field in thislayer is indeed in the range of the breakdown field of pentacenebased p-i-n Zener homodiodes.Doping the channel has a profoundinfluence on the onset voltage of minority (hole) conduction. Theonset voltage can be shifted by lightly n-doping the channel. Theshift of onset voltage can be explained by two mechanisms: first,due to a larger voltage that has to be applied to the gate in orderto fully deplete the n-doped layer. Second, it can be attributed toan increase in hole trapping by inactive dopants. Moreover, it hasbeen shown that the threshold voltage of majority (electron)conduction is shifted by an increase in the doping concentration,and that the ambipolar OFETs can be turned into unipolar OFETs athigh doping concentrations.In subsequent chapters, the workingmechanisms of OPBTs are discussed. OPBTs consist of two Schottkydiodes (top and bottom diode), and the charge transport in theseC60-based Schottky diodes is studied first. Two transport regimescan be distinguished in forward direction - injection limitedcurrents (ILCs) and space charge limited currents (SCLCs). It isfound that the current increases exponentially with applied voltagein the ILC regime and depends quadratically on the applied voltagein the SCLC regime. Furthermore, it is observed that the forwardand backward currents of the Schottkydiode are increased bydecreasing the C60 layer thickness, increasing the active area, andincreasing the temperature. Furthermore, in order to reach a highperformance, various treatments have been applied. Air exposure, avariation of the thickness of the top electrode, as well asannealing of the diodes are used to optimize the diodes.OPBTs areprocessed by using the semiconductor C60 due its high chargecarrier mobility and good film-formingAdvisors/Committee Members: LUSSEM, Bjorn (Advisor), Philip, Bos (Committee Chair).