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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]

Complementary organic inverters were fabricated by inkjet patterning of both the metal contacts and the semiconductors. Bottom-gate, bottom-contact organic thin-film transistors with Ta2O5-polymer bilayer dielectrics, inkjet-printed silver electrodes, and inkjet-printed organic semiconductors exhibit hole and electron mobilities as high as ∼10-2 cm2/V s. Complementary inverters based on these transistors operate in ambient and exhibit a gain of -4.4 with supply voltage VDD=+20 V and -3 dB cutoff at 100 kHz with a load of 0.02 pF. The electrical stability of the inverters was evaluated for analog and digital operation, and a noise margin ≥1.1 V at VDD=+15 V was measured with bias-stress effects included. [ABSTRACT FROM AUTHOR]

The effects of x-ray irradiation on the electrical performance of organic thin film transistors (OTFTs) have been examined up to equivalent lifetime doses for medical digital x-ray imaging backplanes. Transistor performance metrics have been assessed before and after exposures up to 500 Gy for bottom-gate, bottom-contact OTFTs of p-channel polytriarylamine and polyfluorene semiconductors. There is no discernible degradation in device performance for either material and no indication of the creation of additional trap states. In situ measurements indicate that x-ray exposure tends to reverse undesirable shifts in the threshold voltage due to prolonged atmospheric exposure and bias stress. [ABSTRACT FROM AUTHOR]

Hydrostatic pressure was applied to single-crystal rubrene photoconductors and p channel field-effect transistors. Under illumination from a GaInN light-emitting diode, we observed linear increases in photoconductivity, by up to a factor of 2.1 at 0.43 GPa. We also measured increases in the drain current of the single-crystal rubrene organic field-effect transistors (OFETs) with increasing pressure up to 0.52 GPa. Analyzing the transfer characteristics of the OFETs, we extracted the pressure dependence of the field-effect hole mobility. The different OFETs examined showed similar hole mobility increase ratios with pressure, although their atmospheric pressure mobilities varied by more than a factor of two. Threshold voltages shifts with pressure were small. All results were reversible, i.e., the measured currents returned to their atmospheric pressure values upon release of pressure unless complete device failure occurred at the highest pressure. [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]

In this letter we report the use of roughly polished aluminum substrates with spin-coated polymer-smoothing layers for the fabrication of pentacene field-effect transistors. Transistors with spin-coated poly(methylmethacrylate) gate insulator layers were fabricated and showed good performance. On the gate insulator surface, the root-mean-square roughness was found to be 0.18 nm, significantly smaller than the aluminum surface roughness, which is on the scale of tens of nanometers. Field-effect carrier mobilities extracted from the device data reached 0.75 cm2 V-1 s-1; the maximum on/off current ratio was near 5×106. [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]