Showing papers on "Field effect published in 2008"

Tunable electrical conductivity of individual graphene oxide sheets reduced at "low" temperatures.

Abstract: Step-by-step controllable thermal reduction of individual graphene oxide sheets, incorporated into multiterminal field effect devices, was carried out at low temperatures (125−240 °C) with simultaneous electrical measurements. Symmetric hysteresis-free ambipolar (electron- and hole-type) gate dependences were observed as soon as the first measurable resistance was reached. The conductivity of each of the fabricated devices depended on the level of reduction (was increased more than 106 times as reduction progressed), strength of the external electrical field, density of the transport current, and temperature.

Bias stress stability of indium gallium zinc oxide channel based transparent thin film transistors

Abstract: The effects of bias stress on transistor performance are important when considering nontraditional channel materials for thin film transistors. Applying a gate bias stress to indium gallium zinc oxide transparent thin film transistors was found to induce a parallel threshold voltage shift without changing the field effect mobility or the subthreshold gate voltage swing. The threshold voltage change is logarithmically dependent on the duration of the bias stress implying a charge tunneling mechanism resulting in trapped negative charge screening the applied gate voltage.

Effect of Water Vapor on Electrical Properties of Individual Reduced Graphene Oxide Sheets

Abstract: The electrical conductivity and gas-sensing characteristics of individual sheets of partially reduced graphene oxide are studied, and the results display a strong dependence on the chosen reduction method Three reduction procedures are considered here: thermal, chemical, and a combined chemical/thermal approach Samples treated by chemical/thermal reduction display the highest conductivity whereas thermally reduced samples display the fastest gas-sensing response times The chemo-resistive response to water vapor adsorption is well fit by a linear driving force model The conductivity upon exposure to water vapor and measured as a function of the gated electric field displays significant hysteresis These results illustrate how the chemical structure of graphene oxide may be tailored to optimize specific properties for applications such as field effect devices and gas sensors

Thin-Film Morphology Control in Naphthalene-Diimide-Based Semiconductors: High Mobility n-Type Semiconductor for Organic Thin-Film Transistors

Abstract: In organic thin film transistors (OTFT), the morphology and microstructure of an organic thin film has a strong impact on the charge carrier mobility and device characteristics. To have well-defined and predictable thin film morphology, it is necessary to adapt the basic structure of semiconducting molecules in a way that results in an optimum crystalline packing motif. Here we introduce a new molecular design feature for organic semiconductors that provides the optimized crystalline packing and thin film morphology that is essential for efficient charge-carrier transport. Thus, cyclohexyl end groups in naphthalene diimide assist in directing intermolecular stacking leading to a dramatic improvement in field effect mobility. Accordingly, OTFT devices prepared with vapor deposited N,N′-bis(cyclohexyl) naphthalene-1,4,5,8-bis(dicarboximide) (1) regularly exhibit field effect mobility near 6 cm2/(V s), which is one of the highest carrier mobilities reported for either n- or p-type organic semiconducting thin...

Nonvolatile Switching in Graphene Field-Effect Devices

Abstract: The absence of a band gap in graphene restricts its straightforward application as a channel material in field-effect transistors. In this letter, we report on a new approach to engineer a band gap in graphene field-effect devices (FEDs) by controlled structural modification of the graphene channel itself. The conductance in the FEDs is switched between a conductive ldquoon-staterdquo and an insulating ldquooff-staterdquo with more than six orders of magnitude difference in conductance. Above a critical value of an electric field applied to the FED gate under certain environmental conditions, a chemical modification takes place to form insulating graphene derivatives. The effect can be reversed by electrical fields of opposite polarity or short current pulses to recover the initial state. These reversible switches could potentially be applied to nonvolatile memories and novel neuromorphic processing concepts.

High speed Ge channel heterostructures for field effect devices

Abstract: A method and a layered heterostructure for forming high mobility Ge channel field effect transistors is described incorporating a plurality of semiconductor layers on a semiconductor substrate, and a channel structure of a compressively strained epitaxial Ge layer having a higher barrier or a deeper confining quantum well and having extremely high hole mobility for complementary MODFETs and MOSFETs. The invention overcomes the problem of a limited hole mobility due to alloy scattering for a p-channel device with only a single compressively strained SiGe channel layer. This invention further provides improvements in mobility and transconductance over deep submicron state-of-the art Si pMOSFETs in addition to having a broad temperature operation regime from above room temperature (425 K) down to cryogenic low temperatures (0.4 K) where at low temperatures even high device performances are achievable.

Optical conductivity of bilayer graphene with and without an asymmetry gap

Abstract: When a bilayer of graphene is placed in a suitably configured field effect device, an asymmetry gap can be generated and the carrier concentration made different in each layer. This provides a tunable semiconducting gap, and the valence and conductance bands no longer meet at the two Dirac points of the graphene Brillouin zone. We calculate the optical conductivity of such a semiconductor with particular emphasis on the optical spectral weight redistribution brought about by changes in gap and chemical potential due to charging. We derive an algebraic formula for an arbitrary value of the chemical potential for the case of the bilayer conductivity without a gap.

Non-volatile switching in graphene field effect devices

Abstract: The absence of a band gap in graphene restricts its straight forward application as a channel material in field effect transistors. In this letter, we report on a new approach to engineer a band gap in graphene field effect devices (FED) by controlled structural modification of the graphene channel itself. The conductance in the FEDs is switched between a conductive "on-state" to an insulating "off-state" with more than six orders of magnitude difference in conductance. Above a critical value of an electric field applied to the FED gate under certain environmental conditions, a chemical modification takes place to form insulating graphene derivatives. The effect can be reversed by electrical fields of opposite polarity or short current pulses to recover the initial state. These reversible switches could potentially be applied to non-volatile memories and novel neuromorphic processing concepts.

Oligothiophene Derivatives Functionalized with a Diketopyrrolopyrrolo Core for Solution-Processed Field Effect Transistors: Effect of Alkyl Substituents and Thermal Annealing

Abstract: Two new oligothiophene derivatives bearing a diketopyrrolopyrrole core, 2,5-di-n-hexyl-3,6-bis(5′′-n-hexyl[2,2′;5′,2′′]terthiophen-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (DHT6DPPC6) and 2,5-di-n-dodecyl-3,6-bis(5′′-n-hexyl[2,2′;5′,2′′]terthiophen-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (DHT6DPPC12), and their use in solution-processed organic field effect transistors are reported. Depending on the type of alkyl substituent and film annealing temperature, the crystal grain sizes and interlayer spacing vary as observed using atomic force microscopy and X-ray diffractometry, respectively. These changes in film morphology and interlayer spacing lead to an order of magnitude difference in the field effect mobilities. The field effect mobilities for annealed DHT6DPPC6 and DHT6DPPC12 films are 0.02 and 0.01 cm2/V s, respectively.

Thickness Dependence of the Crystalline Structure and Hole Mobility in Thin Films of Low Molecular Weight Poly(3-hexylthiophene)

Abstract: The morphology of thin films at the polymer-to-insulator interface is of great importance for OFET applications. In order to find a relation between the thickness dependence of structural order and the electrical parameters in low molecular weight (Mw ∼ 2.5 kDa) poly(3-hexylthiophene) (P3HT), we have performed grazing-incidence X-ray diffraction and field effect mobility measurements. The samples were prepared from solutions with different concentrations by spin-coating mainly onto HMDS-pretreated Si/SiO2 substrates, resulting in film thicknesses that vary between 10 and 200 nm. The X-ray diffraction curves display Bragg peaks of nanocrystallites diluted into an amorphous matrix where the orientational distribution of the crystallites changes significantly as a function of film thickness. The orientation of nanocrystals was found to be random for the thickest films. Reducing the film thickness, we found an increase in the alignment of the stacking direction of molecules along the surface normal. At same time the mean crystal size along the film normal decreases less than the decrease of film thickness. This is interpreted by a preferential pinning of nanocrystals at the film-to-insulator interface when the crystal size becomes in the order of the film thickness, i.e., below 25 nm. The model of pinning effect is supported by temperature-resolved X-ray measurements performed between room temperature and melting temperature. For films thicker than 25 nm the phase transition appears rather continuously with temperature, but it becomes sharp for thinner films. In contrast to X-ray measurements the field effect mobility is found to be constant within the whole investigated range. Our findings give evidence that the charge transport in low molecular weight P3HT is dominated by the ultrathin layer stabilized at the film-to-insulator interface. Despite the very uniform orientation of the crystallites within this layer, the field effect mobility remains low for all thicknesses. This is attributed to the presence of amorphous regions between highly crystalline domains, which ultimately limits the charge transport in the layer plane.

Field-Effect Semiconductor Device, and Method of Fabrication

Abstract: A HEMT-type field-effect semiconductor device has a main semiconductor region comprising two layers of dissimilar materials such that a two-dimensional electron gas layer is generated along the heterojunction between the two layers. A source and a drain electrode are placed in spaced positions on a major surface of the main semiconductor region. Between these electrodes, a gate electrode is received in a recess in the major surface of the main semiconductor region via a p-type metal oxide semiconductor film whereby a depletion zone is normally created in the electron gas layer, with a minimum of turn-on resistance and gate leak current.

Phthalocyanato Tin(IV) Dichloride: An Air‐Stable, High‐Performance, n‐Type Organic Semiconductor with a High Field‐Effect Electron Mobility

Abstract: Phthalocyanato tin(IV) dichloride, an axially dichloriniated MPc, is an air-stable high performance n-type organic semiconductor with a field-effect electron mobility of up to 0.30 cm(2) V-1 s(-1). This high mobility together with good device stability and commercial availability makes it a most suitable n-type material for future organic thin-film transistor applications.

Thin film field effect transistor and method of manufacturing the same

Abstract: An object of the present invention is to provide a TFT having a high field effect mobility, a high ON / OFF ratio, and an improved environmental temperature dependency. And a display device using the same. A thin film field effect transistor having a gate electrode, a gate insulating film, an active layer made of an amorphous oxide semiconductor, a source electrode, and a drain electrode on a substrate, wherein the dangling bond density of the gate insulating film is 5 A thin film field-effect transistor characterized in that it is × 10 16 cm −3 or less and the hydrogen concentration in the gate insulating film is 1 × 10 19 cm −3 or less. [Selection figure] None

Temperature and Electric Field Dependence of Conduction in Low-Density Polyethylene

Abstract: A traditional constant voltage conductivity test method was used to measure how the conductivity of highly insulating low-density polyethylene (LDPE) polymer films depends on applied electric field, repeated and prolonged electric field exposure, and sample temperature. The strength of the applied voltage was varied to determine the electric field dependence. At low electric field, the resistivity was measured from cryogenic temperatures to well above the glass transition temperature. Comparisons were made with a variety of models of the conduction mechanisms common in insulators, including transient polarization and diffusion and steady-state thermally activated hopping conductivity and variable range hopping conductivity, to determine which mechanisms were active for LDPE and to provide a better picture of its electrical behavior.

Growth related properties of pentacene thin film transistors with different gate dielectrics

Abstract: We report a study on the influence of dielectric surface properties on the growth, the morphology and ordering of pentacene layers and associated consequences on the performance of organic field effect transistors (OFETs). This work mainly aims at disentangling the respective influences of surface roughness and surface energy on pentacene growth. A range of inorganic high- k oxides as well as polymer dielectrics were compared: Ta 2 O 5 deposited by e-beam evaporation or grown by anodic oxidation, polymethylmethacrylate (PMMA) single layer or PMMA/Ta 2 O 5 bi-layer. Some complementary results on anodic HfO 2 were also added. Atomic force microscopy, X-ray diffraction and infrared absorption evidenced that hydrophobicity and surface roughness drastically influence pentacene growth mechanisms. Transistors realized with the different dielectrics show characteristics well correlated to pentacene structural properties. In particularly, we point out the relationship between the grain size and the field effect mobility with the surface energy of the dielectric substrate. The general trend is that the bigger the grains, the higher the mobility but that best electrical performances of OFETs are obtained with a dielectric surface energy close to that of pentacene. This work also bears out our former results on the benefit of a polymer/high- k oxide bilayer dielectric configuration to improve the field effect mobility while keeping a low operating voltage.

Sectional field effect devices and method of fabrication

Abstract: A field effect device is disclosed which has a body formed of a crystalline semiconductor material and has at least one vertically oriented section and at least one horizontally oriented section. The device is produced in SOI technology by fabricating first a formation of the device in masking insulators, and then transferring this formation through several etching steps into the SOI layer. The segmented field effect device combines FinFET, or fully depleted silicon-on-insulator FETs, type devices with fully depleted planar devices. This combination allows device width control with FinFET type devices. The segmented field effect device gives high current drive for a given layout area. The segmented field effect devices allow for the fabrication of high performance processors.

Solution processed large area field effect transistors from dielectrophoreticly aligned arrays of carbon nanotubes

Abstract: We demonstrate solution processable large area field effect transistors (FETs) from aligned arrays of carbon nanotubes (CNTs). Commercially available, surfactant free CNTs suspended in aqueous solution were aligned between source and drain electrodes using ac dielectrophoresis technique. After removing the metallic nanotubes using electrical breakdown, the devices displayed p-type behavior with on-off ratios up to ~ 2X10^4. The measured field effect mobilities are as high as 123 cm2/Vs, which is three orders of magnitude higher than typical solution processed organic FET devices.

Nanoscale thickness double-gated field effect silicon sensors for sensitive pH detection in fluid

Abstract: In this work, we report on the optimization of a double-gate silicon-on-insulator field effect device operation to maximize pH sensitivity. The operating point can be fine tuned by independently biasing the fluid and the back gate of the device. Choosing the bias points such that device is nearly depleted results in an exponential current response—in our case, 0.70decade per unit change in pH. This value is comparable to results obtained with devices that have been further scaled in width, reported at the forefront of the field, and close to the ideal value of 1decade∕pH. By using a thin active area, sensitivity is increased due to increased coupling between the two conducting surfaces of the devices.

An Organic Light‐Emitting Diode with Field‐Effect Electron Transport

Abstract: In Chapter 3, a novel two-electrode light-emitting device structure is proposed. The device is a hybrid structure between a diode and a field-effect transistor. Compared to conventional OLEDs, the cathode is displaced one to several micrometers from the light-emission zone. As the light emission zone is not covered by metal, the device can be used for top emission or even as waveguide. The micrometer-sized distance between the cathode and the active region can be bridged by electrons with an enhanced field-effect mobility. Owing to this high charge carrier mobility, large current densities are possible. The external quantum efficiency at these high current densities is as high as that of conventional OLEDs using the same materials. In contrast to organic light-emitting field-effect transistors, only two electrodes are used. Moreover, lightemission in the novel device structure always occurs at a fixed position, irrespective of the applied bias, in contrast to LEOFETs where the emission zone can move within the channel by varying the bias conditions. The first section of this chapter describes the technology and the materials used to fabricate the device. Next, the device operation and the device performance are discussed. The electrical characteristics as well as the opto-electronic performance are studied. Suggestions for further improvement of the device performance are given in the last section of this chapter.

MOSFET having a JFET embedded as a body diode

Abstract: A field effect transistor, in accordance with one embodiment, includes a metal-oxide-semiconductor field effect transistor (MOSFET) having a junction field effect transistor (JFET) embedded as a body diode.

Using thin film transistors to quantify carrier transport properties of amorphous organic semiconductors

Abstract: The hole transport properties of two phenylamine-based compounds were evaluated by thin film transistor (TFT) measurement and time-of-flight (TOF) technique. The compounds were N,N′-diphenyl-N,N′-bis(1-naphthyl) (1,1′biphenyl)-4,4′diamine (NPB) and 4,4′,4″-tris[n-(2-naphthyl)-n-phenyl-amino] triphenylamine (2TNATA). With tungsten oxide/gold as the charge injecting electrode, the field effect mobility of NPB was found to be 2.4×10−5cm2∕Vs at room temperature, which was about one order of magnitude smaller than that obtained from independent TOF experiments (3×10−4cm2∕Vs). Similar observations were found for 2TNATA. Temperature dependent measurements were carried out to study the energetic disorder of the materials. It was found that the energetic disorder was increased in the neighborhood of a gate dielectric layer.

Hybrid nanocrystals-organic-semiconductor light sensor

Abstract: We present a light sensing device based on nearly spherical, defect free colloidal nanocrystals (NCs) of InAs acting as a light activated gate for a GaAs∕AlGaAs field effect semiconductor transistor. We use self-assembled organic monolayer as linkers that attach the InAs NCs to the surface of the semiconductor device, instead of the gate that exists in common transistors. When the NCs absorb light, at a frequency corresponding to their resonance, a change in the current through the transistor takes place while no current flows through the NCs themselves.

Field-effect semiconductor device

Abstract: A HEMT-type field-effect semiconductor device has a main semiconductor region comprising two layers of dissimilar materials such that a two-dimensional electron gas layer is generated along the heterojunction between the two layers. A source and a drain electrode are placed in spaced positions on the main semiconductor region. Between these electrodes, with spacings therefrom, an insulator is provided with is made from a material capable of developing a stress to reduce carrier concentration in neighboring part of the two-dimensional electron gas layer, creating a discontinuity in this layer. A gate electrode overlies the insulator via a piezoelectric layer which is made from a material capable of developing, in response to a voltage applied to the gate electrode, a stress for canceling out the stress developed by the insulator. Thus the device is physically held off by the action of the insulator while no voltage is being impressed to the gate electrode and, upon voltage application thereto, piezoelectrically turns on by the action of the piezoelectric layer. The turn-on resistance of the device is relatively low as the insulator occupies only part of the source-drain spacing.

Organic semiconductor-carbon nanotube bundle bilayer field effect transistors with enhanced mobilities and high on/off ratios

Abstract: Organic field-effect transistors containing pentacene or α sexithiophene on random arrays of carbon nanotube bundles were fabricated. The impact of nanotube bundles on the performance of devices with different geometries and surface treatments was studied. Upon incorporation of an appropriate amount of nanotube bundles, we observed an increase in the “effective” field effect mobility as high as 20 times while maintaining the high on/off ratios. Furthermore, our preliminary results show that nanotube bundles might template the growth of organic crystals under certain conditions, resulting in the formation of organic nucleates with preferred orientations.

Design and optimization of the SOI field effect diode (FED) for ESD protection

Abstract: A thorough investigation is carried out by numerical simulations of the field effect diode (FED) with the aim to explore its potential for ESD protection applications in silicon on insulator (SOI) technologies. It is shown that the carrier lifetime value has an important impact on the device operation. By careful sizing and doping, FED devices with reasonable breakdown voltage values can be achieved but at rather high gate voltage values. Better results are achieved by modifying the doping profile to resemble a PNPN structure with two gates.

Design and optimization of the SOI field effect diode (FED) for ESD protection

Abstract: A thorough investigation is carried out by numerical simulations of the field effect diode (FED) with the aim to explore its potential for ESD protection applications in silicon on insulator (SOI) technologies. It is shown that the carrier lifetime value has an important impact on the device operation. By careful sizing and doping, FED devices with reasonable breakdown voltage values can be achieved but at rather high gate voltage values. Better results are achieved by modifying the doping profile to resemble a PNPN structure with two gates.

Gating of single molecule transistors: Combining field-effect and chemical control

Abstract: Previously we have demonstrated that several structural features are crucial for the functionality of molecular field-effect transistors. The effect of additional structural aspects of molecular wires is explored. These include the type of, the thiol binding location on, and the chemical substitutions of a conjugated system. Pentacene, porphyrin, and the Tour–Reed devices are utilized as model systems. The thiol binding location is shown to have a varied effect on the transmission of a system depending on the molecular orbitals involved. Substitution by electron withdrawing and donating groups is illustrated to have a substantial effect on the transmission of single molecule devices. The substitution effect is either a simple energy shifting effect or a more complicated resonance effect, and can be used to effectively tune the electronic behavior of a single molecule field effect transistor.

Semiconductor device and method for fabricating the same

Abstract: A semiconductor device having such a structure as a light emitting layer of an organic matter or the like is sandwiched between a work function control single layer carbon nanotube cathode including donor of low ionization potential and a work function control single layer carbon nanotube anode including accepter of large electron affinity. A semiconductor device represented by an organic field effect light emitting element which can reduce emission start voltage, enhance characteristics/function such as emission efficiency, enhance reliability such as the lifetime, and enhance productivity such as reduction in fabrication cost is provided along with its fabrication process.

Non-ohmic contact resistance and field-effect mobility in nanocrystalline silicon thin film transistors

Abstract: Contact resistance has a significant impact on the electrical characteristics of thin film transistors. It limits their maximum on-current and affects their subsequent behavior with bias. This distorts the extracted device parameters, in particular, the field-effect mobility. This letter presents a method capable of accounting for both the non-ohmic (nonlinear) and ohmic (linear) contact resistance effects solely based upon terminal I-V measurements. Applying our analysis to a nanocrystalline silicon thin film transistor, we demonstrate that contact resistance effects can lead to a twofold underestimation of the field-effect mobility.

Solution-processed trilayer inorganic dielectric for high performance flexible organic field effect transistors

Abstract: High performance organic field effect transistors using a solution-processable processed trilayer sol-gel silica gate dielectric architecture fabricated on plastic substrates exhibited low driving voltages of −3.0V, high saturation mobilities of ∼3.5cm2∕Vs, and on-off current ratio of 105. The enhancement in field effect mobility is attributed to improved dielectric-semiconductor interfacial morphology and increased capacitance of the tristratal dielectric. The pentacene devices displayed no signs of electrical degradation upon bending through a bending radius of 24mm, 2.27% strain. The slight increase in the drain currents upon bending strain was investigated using Raman spectroscopy, which revealed enhanced in-phase intermolecular coupling.