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A common device geometry for measuring the electrical characteristics of organic semiconductors is the thin-film organic field-effect transistor (OTFT). Mostly for reasons of cost, convenience, and availability, this usually involves depositing the organic material on a prefabricated gate structure such as Si/SiO2, the surface chemistry of which is often modified with self-assembled monolayers. The interactions between these surfaces and the deposited organic can have a profound effect on thin-film growth and the resulting electrical characteristics since most of the charge transport in these structures occurs near the organic-insulator interface. An alternative to this traditional technique is to assemble the transistor on top of the organic semiconductor. We have used chemical-vapor deposition of the polymeric dielectric material parylene to create pentacene OTFTs with gate electrodes both on top of and below the semiconductor film, with field-effect mobilities as high as 0.1 cm2/V s and Ion/Ioff ratios greater than 106 in the top-gated OTFTs. Comparing the electronic properties of top- and bottom-gate structures yields insight into the charge transport characteristics as well as the effects of device geometry, contacts, and surface roughness of the organic thin film. [ABSTRACT FROM AUTHOR]

We describe gated four-probe measurements designed to measure contact resistance in pentacene-based organic thin-film transistors (OTFTs). The devices consisted of metal source and drain electrodes contacting a 300-Å-thick pentacene film thermally deposited on Al2O3 or SiO2 dielectrics with a p-doped Si substrate serving as the gate electrode. Voltage-sensing leads extending into the source-drain channel were used to monitor potentials in the pentacene film while passing current during drain voltage (VD) or gate voltage (VG) sweeps. We investigated the potential profiles as a function of contact metallurgy (Pt, Au, Ag, and Ca), substrate chemistry, VG, and temperature. The contact-corrected linear hole mobilities were as high as 1.75 cm2/V s and the film sheet resistance and specific contact resistance were as low as 600 kΩ/□ and 1.3 kΩ-cm, respectively, at high gate voltages. In the temperature range of 50–200 K, the pentacene OTFTs displayed an activated behavior with activation energies of 15–30 meV. Importantly, the activation energy associated with the contact resistance showed no dependence on contact metal type at high gate voltage. Also, the activation energies of the contact resistance and film resistance were approximately the same. Above approximately 200 K and below 50 K, the mobility was essentially temperature independent. [ABSTRACT FROM AUTHOR]

We report structural and electrical properties in thin films of an n-channel organic semiconductor, N,N′-dipentyl-3,4,9,10-perylene tetracarboxylic dimide (PTCDI–C5). The structure of polycrystalline thin films of PTCDI–C5 was studied using x-ray diffraction and atomic force microscopy. Films order with single crystal-like packing, and the direction of π-π overlap is in the substrate plane. Organic thin film transistors (OTFTs) based on PTCDI–C5 were fabricated on hydrophobic and hydrophilic substrates. OTFTs showed effective mobility as high as 0.1 cm2/V s. Contact resistance of operating OTFTs was studied using resistance versus length plots and a four-probe method for three different contact metals (Au, Ag, Ca). Typical OTFTs had a specific contact resistance of 8×104 Ω cm at high gate voltage. There was no dependence of contact resistance with contact metal. Variable temperature measurements revealed that film resistance in the OTFT was activated in the temperature range 100–300 K, with typical activation energies of 60–80 meV. Contact resistance showed similar activated behavior, implying that the Schottky barrier at the contact is not the limiting resistance for the contact. Film resistance data showed a Meyer–Neldel relationship with characteristic energy EMN=20–25 meV, for various samples. The common TFT instability of threshold voltage shift (TVS) was observed in PTCDI–C5 OTFTs. A model is proposed to explain positive TVS in gate bias stress and oxygen exposure experiments. The model is based on the formation of a metastable complex between PTCDI–C5 and oxygen, which creates a deep acceptor-like trap state. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]