Transient charge accumulation in pentacene field effect transistor with silver electrode

Time-resolved microscopic optical second harmonic generation (TRM-SHG) imaging was employed to study a transient charge accumulation in top-contact pentacene field effect transistor (FET) with Ag electrodes. It was demonstrated that the SHG signal at the edge of the Ag electrode decayed but remained in a steady state depending on biasing condition. An electric field formed in pentacene layer below Ag electrode activates the SHG, indicating the insufficient accumulation of injected carriers in the FET channel. By using the TRM-SHG technique transient change of the carrier density in the OFET is obtained.     

Organic field-effect transistors containing a SiO2 nanoparticle thin film as the gate dielectric

In this article, we report the fabrication of organic field-effect transistors using self-assembled SiO2 as a gate dielectric material and pentacene as a semiconductor. The dielectric layer was self-assembled with 10 layers of SiO2 nanoparticles 45 nm in diameter, and its breakdown field was larger than 0.57 MV/cm. Being a low-cost and low-temperature process, the layer-by-layer self-assembly is particularly suitable for organic field-effect transistor fabrication. The pentacene was thermally evaporated on the substrate under high vacuum at the room temperature. The fabricated transistor has a threshold voltage of 0.3 V, field-effect mobility of 0.05 cm2/Vs, and slope of 1.4 V/decade.     

Field-Effect Transistors Made by Functionalized Pentacene with Logic Gate Applications

Funtionalized pentacene, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), field-effect transistors (FET) were made by thermal evaporation or solution deposition methods and the temperature dependent mobility was measured. The field-effect mobility (μ_FET) activation energy is gate voltage dependent. At low gate voltage, activated conduction is dominant with Ea ~ 0.27 eV, slightly smaller than the bulk value, and the activation energy decreases with increasing gate voltage. This is ascribed to traps in the film. A non-monotonic temperature dependence is observed at high gate voltage (V_G < −30 V) with Ea ~ 60 - 170 meV at lower temperatures below the mobility maximum. Realization of simple logic gate circuits such as NOT (inverter), NOR, and NAND is demonstrated.     

A nanoparticulate indium tin oxide field-effect transistor with solid electrolyte gating

Reversible tuning of the transport properties of metallic conducting systems is not reported widely in the literature. Here, we report a junction field-effect transistor (FET) based on a transparent conducting oxide (TCO) nanoparticle channel and a solid polymer electrolyte as a gate. The device principle is based on the variation of the drain current induced by the capacitive double layer charging at the electrolyte/nanoparticle interfaces. A device with a metallic conducting channel made of indium tin oxide (ITO) nanoparticles exhibits an on/off ratio of 2 × 10(3) even when the gate potential is limited within the electrochemical capacitive region to avoid redox reactions at the interface. An FET device with metal-like conductance is always favored for the low dimensions of the device and a high on-state current. The field-effect mobility is calculated to be 24.3?cm(2)?V(-1)?s(-1). A subthreshold swing between 230 and 425?mV?dec(-1) is observed.     

Structures and electronic properties of thin-films of polycyclic aromatic hydrocarbons

We report the fabrication and characterization of organic thin-film transistors (TFTs) using several polycyclic aromatic hydrocarbons (PAHs). Pentacene, ovalene, dibenzocoronene and hexabenzocoronene were deposited as organic semiconductors on silicon wafers with gold electrodes as the bottom-contact configuration of the TFTs. The pentacene TFT showed the highest field-effect mobility of more than 0.1 cm²/Vs in comparison with the other PAHs. The results clarified that the high field-effect mobility of the pentacene thin film is due to large grain size and intrinsic electronic properties.     

Electric Field Effect on the Magnetic Properties of III–V Ferromagnetic Semiconductor (In,Mn)As and ((Al),Ga,Mn)As

We have investigated the magnetotransport properties of field-effect transistors (FET) having a III–V ferromagnetic semiconductor channel layer. One can control not only the ferromagnetic transition temperature T _C but also the magnetization and the coercive force of (In,Mn)As channel layers isothermally and reversibly by gate electric fields. A small change of the magnetization upon application of gate electric fields is also observed in FETs with a (Ga,Mn)As channel. Results on a (Al,Ga,Mn)As channel FET are also presented.     

Organic field-effect transistors using perylene

Organic thin-film field-effect transistors using organic semiconductor, perylene are fabricated, and electrical measurements are performed. The field-effect mobility of the device using perylene shows only p-type behavior while the electron and hole mobilities of its single crystal form are 5.5 and 0.5 cm²/V s, respectively. Stacked layers of perlyene (a layer fabricated with low deposition rate followed by another layer with high deposition rate) are formed for the active layer. Furthermore, hexadecafluorocopperphthalocyanine (F₁₆CuPc) and pentacene buffer layers are also used to modify the interface. For all of these devices, perylene layers acts as p-type. Electron trapping at grain boundaries and interface is thought to be a crucial factor. Hole mobility of 3.9×10⁻⁴ cm²/V s is obtained for the perylene film field-effect transistor device.     

Simultaneous monitoring of protein adsorption kinetics using a quartz crystal microbalance and field-effect transistor integrated device

We developed an integrated device comprising a quartz crystal microbalance (QCM) and a field-effect transistor (FET) with a single common gold electrode in a flow chamber. An alternating current inducing oscillations in the piezoelectric quartz of the QCM sensor is electrically independent of the circuit for the FET output so that the two sensors in different detection mechanisms simultaneously record binding kinetics from a single protein solution on the same electrode. A conjunction of adsorbed mass from QCM with electric nature of bound protein from FET provided deeper understanding on a complex process of nonspecific protein adsorption and subsequent conformational changes at a solid/liquid interface. Lower apparent k(on) values obtained by FET than those obtained by QCM on hydrophobic surfaces are interpreted as preferred binding of protein molecules facing uncharged domains to the electrode surface, whereas higher k(off) values by FET than those by QCM imply active macromolecular rearrangements on the surfaces mainly driven by hydrophobic association in an aqueous medium. The advanced features of the combined sensor including in situ, label-free, and real-time monitoring provide information on structural dynamics, beyond measurements of affinities and kinetics in biological binding reactions.     

Substrate- and oxidation-induced roughness of individual terraces of pentacene thin films

The roughness at the surface of individual pentacene terraces on naturally oxidized silicon wafers was investigated with scanning force microscopy as function of film thickness (one to five layers) and sample exposure to ambient air. For pristine samples, the root-mean-square roughness on individual (001) pentacene terraces was 0.18 nm and varied by less than 0.02 nm between monolayer terraces and terraces in the fifth layer. Storing samples in air and ambient light led to a substantial increase of the roughness, which for terraces up to the third layer became 0.24 nm after four weeks. For fourth layer terraces, the roughness increased less, and terraces in the fifth layer exhibited no significant roughness increase. We explain the roughness increase by photo-oxidation of pentacene, particularly strong within the first layer, which is supported by the appearance of grain boundary widening with storage time. The observation that layers beyond the third one from the substrate are less affected by photo-oxidation (smaller terrace roughness) is likely due to better structural perfection in layers farther from the substrate, which reduces the effective cross-section of molecules for oxidation. These results indicate that native silicon oxide does not allow for the immediate formation of structurally perfect pentacene films in the range of one to three layers, which will reduce charge carrier mobility in pentacene thin film transistors. Thicker pentacene layers can protect underlying layers against oxidation.     

Analysis of threshold voltage shift of pentacene field effect transistor with ferroelectric gate insulator as a Maxwell-Wagner effect

The current-voltage (I-V) characteristics of pentacene filed effect transistor (FET) with ferroelectric gate insulator (P(VDF-TeFE)) is investigated to analyze the threshold voltage shift in terms of Maxwell-Wagner (MW) effect. The spontaneous polarization generated in ferroelectric gate insulator modulates the amount of accumulated charges which is injected from the source electrode, and causes threshold voltage shift. Two peaks observed in the I-V characteristics were analyzed based on a MW effect element. Results reveal that the movement of accumulated charges at the pentacene/P(VDF-TeFE) interface along the electric field in the FET, and the ferroelectric polarization of P(VDF-TeFE) are main origins of the peaks.     

Crystal structures and transistor properties of phenyl-substituted tetrathiafulvalene derivatives

The crystal structures, thin-film properties, and field-effect transistor (FET) characteristics of tetrathiafulvalene (TTF) derivatives with two phenyl groups are systematically investigated. The highest mobility, 0.11?cm(2)?V(-1)?s(-1), is observed in biphenyl-substituted TTF (1). The correlation between the crystal structures and the FET properties demonstrates that good transistor properties are associated with two-dimensional intermolecular interaction, which is achieved when the molecules are standing nearly perpendicular to the substrate. Since these TTF derivatives are strong electron donors, the use of a metallic charge-transfer salt (TTF)(TCNQ) as the source and drain electrodes has resulted in a considerable reduction of the off current (TCNQ: tetracyanoquinodimethane).     

Synthesis and characterization of polystyrene brushes for organic thin film transistors

We synthesized and characterized polystyrene brushes on a silicon wafer using surface-initiated atom transfer radical polymerization. The thickness of the polymer brush was controlled by adjusting the reaction time. We investigated monomer conversion as well as the molecular weight and density of the polymer brushes. When the monomer conversion reached 100%, the number-average molecular weight and film thickness reached 135,000 and 113 nm, respectively. The estimated densities of the synthesized polystyrene brushes were in the range 0.34-0.54 chains/nm2, high enough to be categorized in the "concentrated brush" regime. The synthesized polymer brush was used as an insulating layer in an organic thin-film transistor. Organic thin-film transistors were fabricated using pentacene as an active p-type organic semiconductor and a polystyrene brush on a SiO2 layer as a gate dielectric. The pentacene based organic thin-film transistor with the polystyrene brush exhibited a field-effect mobility microFET of 0.099 cm2/(V x s).     

High-performance triisopropylsilylethynyl pentacene transistors via spin coating with a crystallization-assisting layer

The effects of spin speed and an amorphous fluoropolymer (CYTOP)-patterned substrate on the crystalline structures and device performance of triisopropylsilylethynyl pentacene (TIPS-PEN) organic field-effect transistors (OFETs) were investigated. The crystallinity of the TIPS-PEN film was enhanced by decreasing the spin speed, because slow evaporation of the solvent provided a sufficient time for the formation of thermodynamically stable crystalline structures. In addition, the adoption of a CYTOP-patterned substrate induced the three-dimensional (3D) growth of the TIPS-PEN crystals, because the patterned substrate confined the TIPS-PEN molecules and allowed sufficient time for the self-organization of TIPS-PEN. TIPS-PEN OFETs fabricated at a spin speed of 300 rpm with a CYTOP-patterned substrate showed a field-effect mobility of 0.131 cm(2) V(-1) s(-1), which is a remarkable improvement over previous spin-coated TIPS-PEN OFETs.     

Evaluation of molecular orientation and alignment of poly(3-hexylthiophene) on Au (111) and on poly(4-vinylphenol) surfaces

The orientation and alignment of regioregular poly(3-hexylthiophene) (P3HT) molecules on Au (111) surface and on poly(4-vinylphenol) (PVP) thin film were investigated. The P3HT molecules on the smooth Au (111) are oriented with both the backbones and the side chains parallel to the substrate (plane-on orientation) as revealed by the scanning tunneling microscope (STM) images. However, the P3HT molecules on the PVP thin films are preferably oriented with side chains perpendicular to the surface (edge-on orientation). Surface modification of the PVP by hexamethyldisilazane (HMDS) can increase the crystalline size in the P3HT semicrystalline films. The performance of an all-polymer organic field-effect transistor (OFET) with the drop-cast P3HT semiconductor layer and the crosslinked PVP gate insulator on poly(ethylene naphthalate) (PEN) substrate was evaluated.     

Organic field-effect transistor/memory devices with pentacene/polydiacetylene composite film as active channel material: a morphology dependence study

Composite films of pentacene and poly(10,12-pentacosadiynoic acid) were prepared and used as the active channel material in a top-contact, bottom-gate field-effect transistor. The transistors exhibited high field-effect mobility as well as large I-V hysteresis as a function of gate bias history. The polydiacetylenic moieties incorporated in the pentacene film served as charge storage vehicles, which affected the threshold voltage shifts and created the electric bistability needed in a memory device. The memory window, response, and retention highly depend on the morphology of the polydiacetylene film buried under. Detailed film structure analyses and correlation with the transistor/memory property are provided.     

Improved performance of ZnO nanowire field-effect transistors via focused ion beam treatment

A seven orders of magnitude increase in the current on/off ratio of ZnO nanowire field-effect transistors (FETs) after Ga( + ) irradiation was observed. Transmission electron microscopy characterization revealed that the surface crystal quality of the ZnO nanowire was improved via the Ga( + ) treatment. The Ga( + ) irradiation efficiently reduces chemisorption effects and decreases oxygen vacancies in the surface layer. The enhanced performance of the nanowire FET was attributed to the decrease of surface trapped electrons and the decrease in carrier concentration.     

Thermionic emission model for contact resistance in organic field-effect transistor

Injection mechanism of top-contact pentacene field-effect transistor (OFET) was investigated in respect to the internal field. The contact resistance was evaluated by the transmission line method for various applied external voltages as well as various pentacene film thicknesses. The behaviour of contact resistance was described in terms of the thermionic emission model (Schottky injection) and internal electric field generated by excess charges accumulated on pentacene–gate insulator interface. It was shown that pentacene film thickness changes the internal electric field affecting the carrier injection barrier. It was concluded that the space-charge field effect made a significant contribution for smaller pentacene film thicknesses and therefore in accordance to the thermionic model was able to decrease contact resistance representing the potential drop.     

Synthesis and electronic properties of ferrocene-filled double-walled carbon nanotubes

Double-walled carbon nanotubes (DWNTs) are filled with ferrocene molecules by a vapour diffusion method for the first time. The as-synthesized ferrocene-filled DWNTs are characterized by transmission electron microscopy (TEM), energy-dispersive x-ray spectrometry (EDX) and Raman spectroscopy. Electronic properties of double-walled carbon nanotubes (DWNTs) filled with ferrocene molecules are studied by fabricating them as the channels of field-effect transistor (FET) devices. Our results reveal that electronic properties of ferrocene-filled DWNTs are greatly modified due to the charge transfer between ferrocene molecules and DWNTs. In addition, after ferrocene molecules are decomposed inside DWNTs, electronic properties of DWNTs exhibit a further change due to Fe encapsulation, and unipolar n-type semiconducting DWNTs are consequently obtained.     

Low-concentration NO/sub 2/ detection with an adsorption porous silicon FET

Adsorption porous silicon FET (APSFET) is a porous silicon (PS)-based device constituted of a FET structure with a porous adsorbing layer between drain and source. Adsorbed gas molecules in the porous layer induce an inverted channel in the crystalline silicon under the PS itself. The mobile charge per unit area in the channel depends on the molecular gas concentrations in the sensing layer so that adsorbed gas molecules play a role similar to the charge on the gate of a FET. In this work, NO/sub 2/ detection by using the APSFET is demonstrated for the first time. NO/sub 2/ concentration as low as 100 ppb was detected. Devices with both as-grown and oxidized PS layers were fabricated and compared in order to investigate the effect of a low-temperature thermal oxidation on the electrical performances of the sensor. Nonoxidized sensors show a high sensitivity only for fresh devices, which reduces with the aging of the sample. Oxidation of the PS layer improves the electrical performance of sensors, in terms of stability, recovery time, and interference with the relative humidity level, keeping the high sensitivity to nitrogen dioxide.     

Parallel core-shell metal-dielectric-semiconductor germanium nanowires for high-current surround-gate field-effect transistors

Core-shell germanium nanowires (GeNW) are formed with a single-crystalline Ge core and concentric shells of nitride and silicon passivation layer by chemical vapor deposition (CVD), an Al2O3 gate dielectric layer by atomic layer deposition (ALD), and an Al metal surround-gate (SG) shell by isotropic magnetron sputter deposition. Surround-gate nanowire field-effect transistors (FETs) are then constructed using a novel self-aligned fabrication approach. Individual SG GeNW FETs show improved switching over GeNW FETs with planar gate stacks owing to improved electrostatics. FET devices comprised of multiple quasi-aligned SG GeNWs in parallel are also constructed. Collectively, tens of SG GeNWs afford on-currents exceeding 0.1 mA at low source-drain bias voltages. The self-aligned surround-gate scheme can be generalized to various semiconductor nanowire materials.