Ingo Hörselmann

Organic field-effect transistors with nonlithographically defined submicrometer channel length

We developed an underetching technique to define submicrometer channel length polymer field-effect transistors. Short-channel effects are avoided by using thin silicon dioxide as gate insulator. The transistors with 1 and 0.74 mum channel length operate at a voltage as low as 5 V with a low inverse subthreshold slope of 0.4-0.5 V/dec, on-off ratio of 10(4), and without short-channel effects. The poly(3-alcylthiophene)s still suffer from a low mobility and hysteresis does occur, but it is negligible for the drain voltage variation. With our underetching technique also device structures with self-aligned buried gate and channel length below 0.4 mum are fabricated on polymer substrates.

Accurate Capacitance Modeling and Characterization of Organic Thin-Film Transistors

This paper presents analysis of the charge storage behavior in organic thin-film transistors (OTFTs) by means of admittance characterization, compact modeling, and 2-D device simulation. The measurements are performed for frequencies ranging from 100 Hz to 1 MHz and bias potentials from zero to -3 V on top-contact OTFTs that employ air-stable and high-mobility dinaphtho-thieno-thiophene as the organic semiconductor. It is demonstrated that the dependence of the intrinsic OTFT gate-source and gate-drain capacitances on the applied voltages agrees very well with Meyers capacitance model. Furthermore, the impact of parasitic elements, including fringe current and contact impedance, is investigated. The parameters used for the simulation and modeling of all the dynamic characteristics correspond closely to those extracted from static measurements. Finally, the implications of the admittance measurements are also discussed relating to the OTFTs dynamic performance, particularly the cutoff frequency and the charge response time.

Influence of traps on top and bottom contact field-effect transistors based on modified poly(phenylene-vinylene)

We report on the investigations of the differences in the measured current characteristics of source/drain top contact (TOC) and bottom contact (BOC) organic field-effect transistors (OFETs). The active layer was made from poly[1,4-phenylene-(4-methylphenyl)imino-4,48-diphenylene-(4-methylphenyl)imino-1,4-phenylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] and a highly doped silicon wafer with a thermal oxide was used for the gate and the insulator, respectively. Both transistors show a good subthreshold behavior characterized by a threshold voltage of Vth≈−5 V and an inverse subthreshold increase of S≈0.25 V∕dec. The estimated mobility is in the range of 10−3 cm2∕V s. The main difference between the TOC and the BOC transistors is a nonlinear increase of the drain current at small drain voltages in the output characteristics of the TOC OFETs. Numerical two-dimensional simulations show that a high concentration of donor-like traps with a Gaussian or exponential distribution are the reas...

Contact characterization by photoemission and device performance in P3HT based organic transistors

The operation of organic devices as organic field-effect transistors (OFET) depends critically on the contact between the organic layer and the material for source/drain electrodes. Small barriers for carrier injection are required for efficient operation. In order to support the understanding of organic devices, photoemission spectroscopy has been used to determine the properties of metal/organic interfaces. Values for the hole injection barrier determined in the last decade by different groups are frequently of the order of 0.5–1 eV. It is not clear whether barrier lowering due to the image charge is sufficient to make contacts with such barriers efficient for carrier injection. Indeed, no results have been reported where the preparation of the samples for the photoemission study and for the devices are the same. Here we present results of such an investigation for OFETs with gold source/drain contacts. The measured hole barrier at the gold contact of 0.6 eV results from the Au work function of 4.6 eV. ...

Low-Cost Submicrometer Organic Field-Effect Transistors

Low-cost fabrication of circuits with organic field-effect transistors (OFETs) as basic devices requires solution-based technologies. However, then the carrier mobility values can hardly exceed 0.01–0.1 cm2 V−1 s−1. For a cut-off frequency above 100 kHz { to} 1 MHz, relevant for broader applications, and an operation voltage below 10 V, the channel length of the transistors should be smaller than 1–10 μm. Considering inevitable parasitic capacitances one must envisage submicrometer channel lengths. Demonstrations of different patterning have so far realized neither resolution nor alignment accuracy desired, and photolithography is too costly for the submicrometer regime. Here, an overview is given of short-channel OFETs based on the definition of the submicrometer structures by undercutting, and preparation of the devices using additional simple steps of well established microelectronics technology without any of the costly steps such as high temperature treatments, high resolution lithography, and ion implantation.

Organic [6,6]-phenyl-C61-butyric-acid-methyl-ester field effect transistors: Analysis of the contact properties by combined photoemission spectroscopy and electrical measurements

Carrier injection barriers determined by photoemission spectroscopy for organic/metal interfaces are widely accepted to determine the performance of organic field-effect transistors (OFET), which strongly depends on this interface at the source/drain contacts. This assumption is checked here in detail, and a more sophisticated connection is presented. According to the preparation process described in our recently published article [S. Scheinert, J. Appl. Phys. 111, 064502 (2012)], we prepared PCBM/Au and PCBM/Al samples to characterize the interface by photoemission and electrical measurements of PCBM based OFETs with bottom and top (TOC) contacts, respectively. The larger drain currents for TOC OFETs indicate the presence of Schottky contacts at source/drain for both metals. The hole injection barrier as determined by photoemission is 1.8 eV for both Al and Au. Therefore, the electron injection barriers are also the same. In contrast, the drain currents are orders of magnitude larger for the transistors ...

Analyzing the influence of negative gate bias stress on the transconductance of solution-processed, organic thin-film transistors

We have investigated the bias stress stability of poly(3-hexylthiophene) (P3HT)-based organic field-effect transistors (OFETs), prepared from chloroform or trichlorobenzene solutions on a thin SiO2 dielectric (bottom gate) with an additional self-assembled monolayer. Three different processes of gate oxide treatment were compared with regard to their reaction to prolonged constant gate bias. Furthermore, we analyzed the transconductance gm during gate bias stress, as well as the contact resistance Rc using the transmission line method and found that the bias stress has no influence on the contact resistance of P3HT-based OFETs, but with the addition that Rc strongly affects gm. The most stable characteristics were achieved for transistors with a dielectric coated with hexamethyldisilazane over 22 h, which exhibited a pronounced reduction of the threshold voltage shift and contact resistance in comparison with the other variants of gate dielectric treatment. Based on two-dimensional simulations, we demonst...