Submicron Channel Research Articles

Stimuli-Responsive Polymers in Nanotechnology: Deposition and Possible Effect on Drug Release

Stimuli-responsive polymers result in on-demand regulation of properties and function- ing of various nanoscale systems. In particular, they allow stimuli-responsive control of flow rates through membranes and nanofluidic devices with submicron channel sizes. They also allow regu- lation of drug release from nanoparticles and nanofibers in response to temperature or pH variation in the surrounding medium. In the present work two relevant mathematical models are introduced to address precipitation-driven deposition of surface layers of stimuli-responsive polymers and describe a possible effect of swelling-shrinkage transition of such polymers on drug release.

Current transport in short channel top-contact pentacene field-effect transistors investigated with the selective molecular doping technique

The contact doping profile is controlled in the top-contact configuration to clarify a transistor operation based on a current injection process from the metal contact to the organic channel in a submicron channel pentacene field-effect transistor. The molecular doping in the pentacene film underneath the metal contact, in which a thin layer of iron (III) chloride was introduced, drastically changes transistor characteristics. The doping profile control directly revealed the resistive part for current injection. A model to explain the saturation behavior of the top-contact short channel organic transistor is presented.

Crossover from diffusive to quasi-ballistic transport

We present a detailed study of quasi-ballistic transport in submicron semiconductor channels. The electron distribution in such channels differs significantly from a near-equilibrium, shifted Maxwell-Boltzmann distribution function, and displays a large broadening, as well as pronounced features, peaks and shoulders, in the high-velocity tail of the distribution associated with the nonequilibrium and quasi-ballistic nature of the electron transport. For submicron channel lengths, analyses of the spatial dependence of the velocity distribution function show that scattering in the entire channel is important and that the scattering efficiency has a strong spatial dependence. In this article, we particularly study the crossover between the diffusive and quasi-ballistic regimes of transport and find characteristic signatures in (i) the electrostatics, which manifests as a redistribution of the voltage drop from the channel to the contact regions; (ii) the electron density, where exponential and linear spatial dependences of the source-injected and channel- and drain-backscattered electron densities are signatures of diffusive and quasi-ballistic transport, respectively, and (iii) the electron distribution function, where the source-injected ballistic peak observed in the quasi-ballistic regime disappears at the onset of diffusive transport.

Megahertz operation of organic field-effect transistors based on poly(3-hexylthiopene)

Switching speed is crucial for many applications in organic electronics. The possibility to achieve higher frequencies will enable new fields of applications. The authors demonstrate high-frequency organic thin film transistors based on poly(3-hexylthiophene). Transistors with submicron channel lengths show unity-gain bandwidth of 2MHz in air at low supply voltages of 10V. For channel lengths L below 500nm deviations from ideal L scaling law are observed experimentally, which are attributed to contact effects. They present a model beyond the ideal scaling law to predict the maximum operational frequency based on transistor parameters, geometry, and contact resistance.

Formulation of monodisperse emulsions using submicron-channel arrays

Microchannel (MC) emulsification is a promising technique for formulating monodisperse emulsions using a microfabricated channel array. This study used submicron-channel (SMC) arrays with an equivalent diameter as small as 0.57 μm microfabricated on a single-crystal silicon plate to formulate soybean oil-in-water (O/W) emulsions with fine droplets. Monodisperse O/W emulsions with average droplet diameters of 1.4–3.5 μm and minimum coefficients of variation (CV) of approximately 9% were successfully formulated using the SMC arrays. The droplet production rate of an SMC plate reached up to 2.2 × 10 4 droplets/s.

Interface modification of a pentacene field-effect transistor with a submicron channel

Effective interface modification of a pentacene field-effect transistor with a submicron channel is performed. The two interfaces between the pentacene channel and the substrate and between the channel and the metallic electrode are markedly changed by nanoscale modification. At the substrate interface, thin polyparaxylylene film was formed, resulting in higher field-effect mobility of the short-channel transistor. A further increase in the field-effect mobility is achieved when multiple layers of tetracyanoquinodimethane are introduced to the pentacene film beneath the metallic electrodes.

Hydrodynamic Self-Consistent Field Theory for Inhomogeneous Polymer Melts

We introduce a mesoscale technique for simulating the structure and rheology of block-copolymer melts and blends in hydrodynamic flows. The technique couples dynamic self-consistent field theory with continuum hydrodynamics and flow penalization to simulate polymeric fluid flows in channels of arbitrary geometry. We demonstrate the method by studying phase separation of an ABC triblock copolymer melt in a submicron channel with neutral wall wetting conditions. We find that surface wetting effects and shear effects compete, producing wall-perpendicular lamellae in the absence of flow and wall-parallel lamellae in cases where the shear rate exceeds some critical Weissenberg number.

Characterization of functionalized pentacene field-effect transistors and its logic gate application

Functionalized pentacene, 6,13-bis(tri-isopropylsilylethynyl)pentacene (TIPS-pentacene), field-effect transistors (FETs) were made by both thermal evaporation and solution deposition methods, and the mobility was measured as a function of temperature and intensity of incident illumination. The field-effect mobility (μFET) has a gate-voltage dependent activation energy. A nonmonotonic temperature dependence was observed at high gate voltage (VG<−30V) with an activation energy of Ea∼60–170meV, depending on the fabrication procedure. The gate-voltage dependent mobility and nonmonotonic temperature dependence indicate that shallow traps play important role in the transport of TIPS-pentacene films. The current in the saturation regime as well as the mobility increase upon light illumination in proportion to the light intensity, mainly due to the photoconductive response. Transistors with submicron channel length showed unsaturating current-voltage characteristics due to the short channel effect. Realization of simple circuits such as NOT (inverter), NOR, and NAND logic gates are demonstrated for thin film TIPS-pentacene transistors.

Numerical study on the scaling of a-Si:H thin film transistors

In this article we discuss the impact of channel length scaling on the above threshold current characteristics of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs). MEDICI simulation results of the interface surface potential show a lowering of the potential barrier for shorter channel lengths. This suggests a decrease in threshold voltage and increase in subthreshold slope with drain voltage particularly in submicron channel lengths, causing a nonsaturating output current. Simulation results of the above threshold current-voltage characteristics for short channel TFTs corroborate with measurement data of fabricated devices.

Low-cost fabrication of submicron all polymer field effect transistors

All polymer field effect transistors have been fabricated combining nanoimprint lithography and inkjet printing. Trenches with hydrophilic bottoms confined by hydrophobic walls with considerable height are patterned by nanoimprint lithography. Conducting polymer solutions were then delivered into these trench liquid containers by inkjet printing. Dried conducting polymer in nearby trenches forms source-drain electrodes with the channel length accurately defined by the gap in between the designed two trenches. Top-gate all polymer field effect transistors with submicron channel lengths were successfully realized by such low-cost process.

Short-channel polymer field-effect-transistor fabrication using spin-coating-induced edge template and ink-jet printing

A method to fabricate polymer field-effect transistors with submicron channel lengths is described. A thin polymer film is spin coated on a prepatterned resist with a low resolution to create a thickness contrast in the overcoated polymer layer. After plasma and solvent etching, a submicron-sized line structure, which templates the contour of the prepattern, is obtained. A further lift-off process is applied to define source-drain electrodes of transistors. With a combination of ink-jet printing, transistors with channel length down to 400 nm have been fabricated by this method. We show that drive current density increases as expected, while the on/off current ratio 106 is achieved.

Modulation Effects of Photocarriers on the Terahertz Plasma-Wave Resonance in High-Electron-Mobility Transistors under Interband Photoexcitation

Two-dimensional (2-D) electron plasma in a submicron channel of a high-electron mobility transistor (HEMT) is excited by interband photoexcitation, resulting in performing the photomixing function. The injected photoelectrons modulate the total 2-D electron density, affecting the plasma resonant properties. The modulation depth of the density of 2-D electrons by the photoelectrons deeply relates to the resonant intensity and fr. This effect was modeled analytically in the 2-D plasma hydrodynamic equation. In order to validate the analytical calculation, the plasma-wave resonance was experimentally observed for a 0.15-µm gate-length InGaP/InGaAs/GaAs pseudomorphic HEMT in the terahertz range. At the modulation depth of 30%, the resonance was clearly observed with a double peak (the peak at 1.9/5.8 THz corresponding to the fundamental/third harmonic resonance). The resonant frequencies slightly shifted downward and the intensity attenuated with decreasing the modulation depth. Observed resonant frequencies support the analytical calculation.

Temperature dependence of the tangential momentum accommodation coefficient for gases

The temperature dependence of the tangential momentum accommodation coefficient (TMAC) is investigated by examining gas flows in a submicron channel using molecular dynamics simulations. The results show that the TMAC decreases with the increasing temperature following an exponential decay law, and is more sensitive to lower temperatures than to higher ones. The molecular trapping-desorption behaviors near the channel surface are found to be responsible for this dependence.

Quasiballistic transport of hot holes in GaAs submicron channels

We investigate the ballistic motion of hot holes in a submicron channel fabricated from a modulation-doped p-type GaAs∕(AlGa)As quantum well heterostructure. We observe peaks in the low temperature differential conductance, which we attribute to ballistic motion of holes limited by the onset of longitudinal-optical (LO) phonon emission when the voltage drop along the channel corresponds to an integer multiple of the LO-phonon energy.

Acoustic Transport of Electrons in Parallel Quantum Wires

Over the last few years we have developed a new method to control single-electrons by isolating and moving them through a submicron width channel formed in a GaAs/AlGaAs heterostructure using a surface acoustic wave. The acoustic wave acts to push electrons through the depleted submicron channel in packets each containing an integer number of electrons. Our primary motivation for studying this system has been to develop a new standard of dc current for metrological purposes, but our recent focus has widened to investigate the possibility of single-photon emission. Here we show new experimental results which demonstrate acoustoelectric current flow in adjacent 1D wires. These results have relevance both to the use of the system in a single-photon emission scheme, as well as in the creation of a proposed acoustoelectric quantum computer.

Rarefied Gas Flow in Rough Microchannels by Molecular DynamicsSimulation

The molecular dynamics simulation method is applied to investigate therarefied gas flow in a submicron channel with surface roughness whichis modelled by an array of triangle modules. The boundary conditions arefound to be determined not only by the Knudsen number but also theroughness, which implies that the breakdown of the Maxwell slip model underthe conditions that the surface roughness is comparable to themolecular mean free path. The effects of the rarefaction and thesurface roughness on the boundary conditions and the flowcharacteristics are strongly coupled. The flow friction increases with increasing roughness and with decreasing Knudsen number.

Study of asymmetrical effects of silicon submicron transistors

We studied the asymmetrical effect of submicron channel length NMOS silicon transistors. The threshold voltage of transistor was determined by transconductance (gm) extraction method and constant-current (CC) method. The effective channel length (Leff) was determined by ‘shift and ratio’ methods. The short channel and reverse short channel effect were observed from the threshold voltage (Vto) versus channel width (W) curve. The I–V curves were not shown significant asymmetry of drain and source. The results showed that the asymmetry of drain and source increased with reducing the channel length. The standard deviation of threshold voltage and effective channel length were increased with decreasing channel length.

Analytical Drain Thermal Noise Current Model Valid for Deep Submicron MOSFETs

In this paper, a physics-based MOSFET drain thermal noise current model valid for deep submicron channel lengths was derived and verified with experiments. It is found that the well-known /spl mu/Q/sub inv//L/sup 2/ formula, previously derived for long channels, remains valid for short channels. Carrier heating in the gradual channel region was taken into account implicitly with the form of diffusion noise source and then impedance field method taking velocity saturation effect was used to calculate the external drain thermal noise current. The derived model was verified by experimental noise for devices with channel lengths down to 0.18 /spl mu/m. Excellent agreement between measured and modeled drain thermal noise was obtained for the entire V/sub GS/ and V/sub DS/ bias regions.

Application of 2DMD to gaseous microflows

A two-dimensional molecular dynamics (2DMD) simulation is applied to gaseous microflows. Based on a velocity distribution function in equilibrium, the mean molecular speed, mean collision frequency, mean free path, and the dynamical viscosity are deduced theoretically. A Maxwell-type-like boundary condition for two-dimensional (2D) systems, which reveals a linear relationship between the slip length and the mean free path, is also derived. These expressions are consequently employed to investigate the rarefied gas flow in a submicron channel. The results show reasonable agreements with those by 3D simulations, and indicate that the 2DMD scheme can be very promising for the microflow researches because of its high efficiency in computation.

Milling of submicron channels on gold layer using double charged arsenic ion beam

The capability of using a focused ion beam (FIB) for milling of submicron channel structures on a gold layer is investigated. A double-charged arsenic (As2+) FIB is adopted to assess the effect of the dwell time on the final profiles of the milled structures. A single-pass milling, which creates relatively shallow microchannels, is conducted in order to estimate the corresponding milling yields. The condition to provide a uniform ion flux in milling is first studied. The procedure on conducting the milling experiment is then presented. The atomic force microscope (AFM) is applied for measuring the profiles of the milled channels. Based on the AFM measurements, the milling yields have been estimated and compared with the sputtering yields predicted by a more sophisticated numerical simulation. The milling yield for the relatively shallow microchannels presently considered has been discovered to be roughly equal to the predicted normal-incidence sputtering yield. Consistence has also been found as the present findings have been compared with other channel milling studies, which had used different ion beams and target materials. FIB milling has been shown to be an effective tool for making submicron channels in gold layers.