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LATERAL AND VERTICAL ORGANIC TRANSISTORS

AL-SHADEEDI, AKRAM
http://orcid.org/0000-0002-5592-1531
http://rave.ohiolink.edu/etdc/view?acc_num=kent1492441683969202

Abstract Details

2017, PHD, Kent State University, College of Arts and Sciences / Department of Physics.
An extensive study has been performed to provide a better understanding of the operation principles of doped organic field-effect transistors (OFETs), organic pin diodes,Schottky diodes, and organic permeable base transistors (OPBTs). This has been accomplished by a combination of electrical and structural characterization of these devices. The discussion of doped OFETs focuses on the shift of the threshold voltage due to increased doping concentrations and the generation and transport of minority charge carriers. Doping of pentacene OFETs is achieved by co-evaporation of pentacene with the n-dopant W2(hpp)4. It is found that pentacene thin film are efficiently doped and that a conductivity in the range of 2.6 x 10-6 S cm-1 for 1 wt% to 2.5 x 10-4 S cm-1 for 16 wt% is reached. It is shown that n-doped OFET consisting of an n-doped channel and n-doped contacts are ambipolar. This behavior is surprising, as n-doping the contacts should suppress direct injection of minority charge carriers (holes). It was proposed that minority charge carrier injection and hence the ambipolar characteristic of n-doped OFETs can be explained by Zener tunneling inside the intrinsic pentacene layer underneath the drain electrode. It is shown that the electric field in this layer is indeed in the range of the breakdown field of pentacene based p-i-n Zener homodiodes. Doping the channel has a profound influence on the onset voltage of minority (hole) conduction. The onset voltage can be shifted by lightly n-doping the channel. The shift of onset voltage can be explained by two mechanisms: first, due to a larger voltage that has to be applied to the gate in order to fully deplete the n-doped layer. Second, it can be attributed to an increase in hole trapping by inactive dopants. Moreover, it has been 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 at high doping concentrations. In subsequent chapters, the working mechanisms of OPBTs are discussed. OPBTs consist of two Schottky diodes (top and bottom diode), and the charge transport in these C60-based Schottky diodes is studied first. Two transport regimes can be distinguished in forward direction - injection limited currents (ILCs) and space charge limited currents (SCLCs). It is found that the current increases exponentially with applied voltage in the ILC regime and depends quadratically on the applied voltage in the SCLC regime. Furthermore, it is observed that the forward and backward currents of the Schottky diode are increased by decreasing the C60 layer thickness, increasing the active area, and increasing the temperature. Furthermore, in order to reach a high performance, various treatments have been applied. Air exposure, a variation of the thickness of the top electrode, as well as annealing of the diodes are used to optimize the diodes. OPBTs are processed by using the semiconductor C60 due its high charge carrier mobility and good film-forming properties. Again, the working mechanism of OPBTs is studied by electrical characterization (base-sweep measurements and output characteristics). To achieve a high performance of OPBTs, various treatments and techniques have been applied. The annealing of the OPBTs after fabrication changes the morphology of the base electrode. Thus, openings (pinholes) are formed in the base electrode, which enables a high current transfer from the upper to lower semiconductor layer. The formation of openings is proved by analyzing SEM and TEM image of the base electrode. Adding a doped layer at the emitter is another process to optimize the OPBTs. The doped layer ensures a high charge carrier injection at the emitter, leading to a high transmission and current gain. Furthermore, it has been observed that the ON/OFF ratio and transconductance of OPBTs increases by decreasing their active area. A very high transconductance gm of 37 S/cm2 is reached, which has the potential to boost the switching speed of organic transistors to 5 MHz. Furthermore, it is shown that the base electrode thickness is an essential parameter for OPBTs. The current gain ß decreases by increasing thickness of base electrode, whereas the ON/OFF ratio increases for thicker base electrodes.
Bjorn LUSSEM (Advisor)
Bos Philip (Committee Chair)
Mann Elizabeth (Committee Member)
Bunge Scott (Committee Member)
Balci Hamza (Committee Member)
126 p.
Electrical Engineering; Molecular Physics; Organic Chemistry; Physics
Organic Field Effect Transistor, Doped Organic Field Effect Transistor, Vertical Transistors, Organic Permeable Base Transistor, Zener Tunneling, and Pentacene p i n diode

Recommended Citations

Citations

  • AL-SHADEEDI, A. (2017). LATERAL AND VERTICAL ORGANIC TRANSISTORS [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1492441683969202

    APA Style (7th edition)

  • AL-SHADEEDI, AKRAM. LATERAL AND VERTICAL ORGANIC TRANSISTORS. 2017. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1492441683969202.

    MLA Style (8th edition)

  • AL-SHADEEDI, AKRAM. "LATERAL AND VERTICAL ORGANIC TRANSISTORS." Doctoral dissertation, Kent State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=kent1492441683969202

    Chicago Manual of Style (17th edition)

kent1492441683969202
351
© 2017, some rights reserved.Creative Commons License LATERAL AND VERTICAL ORGANIC TRANSISTORS by AKRAM AL-SHADEEDI is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by Kent State University and OhioLINK.