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A study of carrier transport in top-gate and bottom-contact TIPS-pentacene organic field-effect transistors (OFETs) based on mobility is presented. Among three mobilities extracted by different methods, the low-field mobility obtained by the Y function exhibits the best reliability and ease for use, whereas the widely applied field-effect mobility is not reliable, particularly in short-channel transistors and at low temperatures. A detailed study of contact transport reveals its strong impact on short-channel transistors, suggesting that a more intrinsic transport analysis is better implemented in relatively longer-channel devices. The observed temperature dependences of mobility are well explained by a transport model with Gaussian-like diffusivity band tails, different from diffusion in localized states band tails. This model explicitly interprets the non-zero constant mobility at low temperatures and clearly demonstrates the effects of disorder and hopping transport on temperature and carrier density dependences of mobility in organic transistors. [ABSTRACT FROM AUTHOR]

Fully printed electronics on plastic have attracted considerable interest owing to their high compatibility and ease of integration. Here, an ultra-high-resolution printing technique based on parallel vacuum ultraviolet patterning that can produce high-contrast wettability regions on flexible substrates was developed. This technique was used to selectively deposit a functional ink with a 1µm feature size, thereby allowing the large-scale fabrication of organic thin-film transistors with channels as short as 1µm under an ambient atmosphere. Moreover, in short-channel devices, hole injection barriers can be tuned by printing the optimum gate overlaps associated with selectively doping semiconductor/electrode interfaces, resulting in a marked reduction in contact resistance from 20 to 1.5 kΩcm, and an elevation of the charge carrier mobility to a record high of 0.3 cm2 V-1 s-1 in a 1-µm-channel device. The results indicate that the developed technique is promising for the fabrication of large-area, high-resolution, low-cost electronics. [ABSTRACT FROM AUTHOR]

We investigate the effect of the parasitic access resistance on device parameters, particularly the threshold voltage (VT) and the contact resistance (RC), of staggered organic field-effect transistors by varying the active layer thickness (ds). At low gate voltages, RC decreases as ds increases due to the free carrier density increasing in the semiconductor film. At high gate voltages, RC increases as ds increases due to increasing access resistance. These factors degrade the device parameters with increasing ds. The contribution of the change in ds on the VT shift is assessed by subtracting the contact effect from the apparent VT. [ABSTRACT FROM AUTHOR]

The charge injection efficiency of organic field-effect transistors (OFETs) is found to be a critical factor determining the subthreshold characteristics of these devices. OFETs fabricated using a wide band gap organic semiconductor and gold source/drain contacts display large threshold voltage and poor subthreshold characteristics. Insertion of a metal-oxide charge injection layer at the contact/semiconductor interface lower the injection barrier height, resulting in marked improvements in threshold voltage and subthreshold slope and strong suppression of the short-channel effect. The improved subthreshold characteristics are attributed to enhanced charge injection and the consequent promotion of charge accumulation. [ABSTRACT FROM AUTHOR]

A bottom-up fabrication technique for the preparation of self-organized organic field-effect transistors (OFETs) on flexible plastic substrates is presented. Solution-based self-organization of OFETs is achieved by patterning the insulator surface with solution-wettable and unwettable regions. The proposed method satisfies several important requirements of printable electronics, including reduction in energy consumption, minimization of facilities, and the on-demand use of molecular materials. Self-organized OFETs display an average mobility of 0.53 cm2/(V s), on/off ratio of 109, and subthreshold slope of 0.18 V/dec, with near-zero and narrowly distributed threshold voltage. An inverter circuit prepared using these devices is demonstrated with high signal gain. [ABSTRACT FROM AUTHOR]

Semiconductor channels of organic field-effect transistors have been directly self-organized from a solution phase. The alkyl-modified surface was locally patterned by using a phenyl self-assembled monolayer (SAM) for the channels. Drop-cast small organic molecules were selectively crystallized on the phenyl SAM region. The self-organized process allows the simultaneous formation of polycrystalline transistor arrays from the patterned channels. The phenyl SAM under the channel is critical for the improvement of device stability. Further optimization of the deposition process realized direct growth of a single crystal channel from solution between prefabricated electrodes, and the single-crystal transistors exhibited excellent performance. [ABSTRACT FROM AUTHOR]

The effect of semiconductor crystal structure on the performance of organic field-effect transistors is investigated through the fabrication and characterization of devices based on single porphyrin crystals with different center metals. The field-effect mobility of the transistors is found to increase with decreasing intermolecular distance, attributable to greater overlap of π orbitals among close-packed molecules and the consequent promotion of charge transport. [ABSTRACT FROM AUTHOR]

Revealing the intrinsic electrical properties is the basis of understanding new functional materials and developing their applications. However, in nonideal field‐effect transistors (FETs), conventional current–voltage characterizations do not accurately probe charge transport, particularly for newly developed semiconductors. Here, a generalized gated four‐probe (G‐GFP) technique is developed, which detects dynamic changes in carrier accumulation and transport. The technique is suitable for exploring the intrinsic properties of semiconductors in FETs with arbitrary contacts and in any operational regimes above the threshold. Application to simulated transistors confirms its accuracy in probing the evolution of channel potential, drift field, and gate‐dependent carrier mobility for devices with a contact‐limited operation and disordered semiconductors. Comparative experiments are performed based on FETs with various materials, device structures, and operational temperatures. The G‐GFP technique proves to exclude the various injection properties, to detect in situ how carriers are accumulated, and to clarify carrier mobility of the semiconductors. In particular, the well‐known "double‐slope" features in the current–voltage relations are controllably generated and their origins are identified. The approach could be used to explore electronic properties of newly developed materials such as organic, oxide, or 2D semiconductors. [ABSTRACT FROM AUTHOR]

For newly developed semiconductors, obtaining high‐performance transistors and identifying carrier mobility have been hot and important issues. Here, large‐area fabrications and thorough analysis of InGaZnO transistors with enhanced current by simple encapsulations are reported. The enhancement in the drain current and on–off ratio is remarkable in the long‐channel devices (e.g., 40 times in 200 µm long transistors) but becomes much less pronounced in short‐channel devices (e.g., 2 times in 5 µm long transistors), which limits its application to the display industry. Combining gated four‐probe measurements, scanning Kelvin‐probe microscopy, secondary ion mass spectrometry, X‐ray photoelectron spectroscopy, and device simulations, it is revealed that the enhanced apparent mobility up to several tens of times is attributed to the stabilized hydrogens in the middle area forming a degenerated channel area while that near the source‐drain contacts are merely doped, which causes artifact in mobility extraction. The studies demonstrate the use of hydrogens to remarkably enhance performance of oxide transistors by inducing a new mode of device operation. Also, this study shows clearly that a thorough analysis is necessary to understand the origin of very high apparent mobilities in thin‐film transistors or field‐effect transistors with advanced semiconductors. With simple and optimized encapsulations, InGaZnO thin‐film‐transistors (TFT) achieve significantly enhanced apparent mobility (up to 40 times) as compared with pristine TFT. To understand that, the current–voltage characteristics, capacitance–voltage relations, gated four‐probe measurement for potentials, scanning Kelvin probe microscope for surface, 2D technology computer aided design device simulations, depth‐profiling of elements, photoelectron spectroscopy for ions, density functional theory calculations, and temperature‐dependent measurements are investigated. [ABSTRACT FROM AUTHOR]

Abstract: We report one-step formation of the gate dielectric and conduction channel for enhancing the performance of organic field effect transistors (OFETs). The resulting OFET with the semiconductor/dielectric bi-layers spun in ambient conditions exhibits μ FET up to 1.6cm2/Vs and on–off ratio higher than 106, no additional treatment needed. Contact angle measurements and absorption spectra reveals that a well-defined semiconductor-top and dielectric-bottom film form after spin-coating the mixture of the two components, which is due to the surface induced self-organized phase separation. Compared to the single layer semiconductor film, the staggered film exhibits over 5 times higher mobility and nearly 90% reduced hysteresis in OFET. The higher performance is attributed to the simultaneous optimization in the dielectric interface and semiconductor crystallization. The approach is significant for the fabrication of low cost, easy processed and high performance OFETs. [Copyright &y& Elsevier]

Abstract: Large surface roughness is a major obstacle for electronic systems fabricated on paper substrates. Here, a mixture solution of organic semiconductor and polymer dielectric was spin-coated on paper substrate with a patterned wettability. This spin-coating process produced organic crystals and a very smooth semiconductor/dielectric interface with a low trap density in well-confined patterns. Despite the large roughness of the paper substrate, the fabricated transistor arrays exhibited high performance with a field-effect mobility reaching 1.3cm2/Vs and an on/off ratio of 108. The presented results offer a simple fabrication method for the current rapidly developing technology of paper electronics. [Copyright &y& Elsevier]