Scaling behavior and parasitic series resistance in disordered organic field-effect transistors

Abstract

The scaling behavior of the transfer characteristics of solution-processed disordered organic thin-film transistors with channel length is investigated. This is done for a variety of organic semiconductors in combination with gold injecting electrodes. From the channel-length dependence of the transistor resistance in the conducting ON-state, we determine the field-effect mobility and the parasitic series resistance. The extracted parasitic resistance, typically in the M⍀ range, depends on the applied gate voltage, and we find experimentally that the parasitic resistance decreases with increasing field-effect mobility. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1581389͔The interest in organic thin-film transistors ͑TFTs͒ has grown rapidly due to envisaged applications, such as integrated circuits 1 and active-matrix displays. 2 The switching speed of organic integrated circuits can be estimated from the performance of the individual transistors and is roughly proportional to ϳ FE /L 2 , 3 where L is the channel length of the transistor, and FE is the field-effect mobility. To reach higher switching speeds, the search for higher mobility materials is therefore important, but it is also of great interest to downsize the transistor geometries. In this work, the scaling behavior of the transfer characteristics with transistor channel length is investigated for a variety of solutionprocessable organic TFTs.In the experiments, we use heavily doped Si wafers as the gate electrode, with a 200-nm-thick layer of thermally oxidized SiO 2 as the gate-insulating layer. Using conventional lithography, gold source and drain contacts 100 nm thick are defined, with channel widths ranging from 1 mm to 1 cm and channel lengths between 0.75 and 40 m. The structures typically have an underetch of 0.5 m, which we neglect in the following analysis. A 10-nm layer of titanium acts as an adhesion layer for the gold on the SiO 2 . The SiO 2 layer is treated with the primer hexamethyldisilazane to make the surface hydrophobic. No special care is taken to clean the gold surface prior to deposition of the semiconductor. Poly͑2,5-thienylene vinylene͒ ͑PTV͒ films as semiconductor layer are deposited using a precursor-route process. 3 We systematically varied the processing conditions for the conversion from precursor to PTV, and we determined the average degree of conversion using the method described by Fuchigami et al. 4 This enabled us to systematically study PTV transistors at various degrees of conversion ranging from 60% to 100%, and consequently over a range of field-effect mobilities, between 10 Ϫ4 and 10 Ϫ3 cm 2 /V s. Poly͑3-hexyl thiophene͒ ͑P3HT͒ is spin-coated from a 1 wt % chloroform solution. Films of poly͓͑2-methoxy-5-(3Ј,7Ј-dimethyloctyloxy͔͒-p-phenylene vinylene͒ (OC 1 C 10 -PPV) and poly͓͑2,5-di-(3Ј,7Ј-dimethyloctyloxy͔͒-p-phenylene vinylene͒ (OC 10 C 10 -PPV), are spun from a 0.5 wt % toluene solution. Pentacene thin films are deposited using a precursor-route process. 3 The measurements are performed on freshly...

Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers

“…We observe that the saturation mobility of different MW generally increases as channel length decreases, which is opposite to the reduction of mobility caused by severe contact limited short-channel transistors with Cr-Au or Ti-Au electrodes. 23,24 The mobility is found to be more strongly channel-length dependent in low-mobility transistors, but little dependence is seen in highly crystalline, high-MW transistors. Figure 3 shows the output characteristics of 2 m-and 20 m-channel TCB spin-cast transistors plotted on a current scale normalized by the channel length L. The 20 m-channel transistors for both low ͑a͒ and high MW ͑b͒ polymers exhibit nearly ideal scaling characteristics.…”

Soluble fullerene derivatives: The effect of electronic structure on transistor performance and air stability

“…When using Au contacts, qualitatively, equivalent behavior is observed. The observation of Ohmic contacts with respect to the lateral field and the absence of long-range crystallinity allows the use of the transmission line method 41,42 to quantify the contact resistance (R Contact ¼ R Source þ R Drain ). Assuming the resistance of the contacts is in series with that of the OFET channel (R Channel ), the transmission line method allows extraction of R Contact using a linear extrapolation of the total device resistance of a series of transistors of varying channel length (but fixed width, W ¼ 1500 lm) to L ¼ 0 lm (i.e., R Channel ¼ 0 X).…”

Bias-dependent contact resistance in rubrene single-crystal field-effect transistors

“…Irreproducibility makes the situation even worse: for gold-contacted pentacene thin-film FETs, for instance, the spread in contact resistance values was recently observed to exceed three orders of magnitude ͑from 2 k⍀ cm to more than 1 M⍀ cm͒. 5 The current lack of understanding of the microscopic carrier injection 6 processes from a metal electrode into an organic semiconductor does not help us in determining the causes of the observed irreproducibility and more systematic experiments are needed.Here we report systematic transport measurements of rubrene ͑C 42 H 28 ͒ single-crystal FETs with electrodes made of five different metals ͑Ni, Co, Cu, Au, and Pt͒. All the transistors have been fabricated with a sufficiently short channel length, so that the total device resistance is entirely dominated by the contacts.…”

“…Irreproducibility makes the situation even worse: for gold-contacted pentacene thin-film FETs, for instance, the spread in contact resistance values was recently observed to exceed three orders of magnitude ͑from 2 k⍀ cm to more than 1 M⍀ cm͒. 5 The current lack of understanding of the microscopic carrier injection 6 processes from a metal electrode into an organic semiconductor does not help us in determining the causes of the observed irreproducibility and more systematic experiments are needed.…”