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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]

Large operating voltages are often required to switch organic field-effect transistors (OFETs) on and off because commonly used gate dielectric layers provide low capacitive coupling between the gate electrode and the semiconductor. We present here a pentacene OFET gated by a solution-deposited polymer electrolyte film in which the current was modulated over four orders of magnitude using gate voltages less than 2 V. A subthreshold slope of 180 mV per decade of current was observed during transistor turn on at a source-drain bias of -1 V; the estimated dielectric layer specific capacitance was 5 μF/cm2. Sweep rate-dependent hysteresis may be attributed to a combination of ion migration and charge carrier trapping effects. Strategies to improve switching speeds for polymer electrolyte-gated OFETs are also discussed. [ABSTRACT FROM AUTHOR]

Single-crystal organic field-effect transistors (SX-OFETs) with channel lengths of 1 and 100 μm have been fabricated by adhering thin crystals of tetracene to freshly ashed SiO2 substrates containing countersunk gold contacts. The intrinsic transport properties of the tetracene single crystals, corrected for potential contact effects by using a standard four-probe configuration, have been measured from room temperature down to 4.2 K. These OFETs exhibit mobilities as high as 0.1 cm2 V-1s-1, subthreshold swings of <500 mV/decade, and Ion/Ioff ratios in excess of 109. The larger devices (L=100 μm,W=1000 μm) show thermally activated mobilities over the temperature range 200 K105 V/cm. [ABSTRACT FROM AUTHOR]