Movable Gate Research Articles

Fabrication and characterisation of a double-clamped beam structure as a control gate for a high-speed non-volatile memory device

We report the fabrication and characterisation of a suspended double-clamped beam structure that is implemented as a movable control gate within a hybrid high-speed non-volatile memory device. This structure features a foundation of SiO2/Si layers over which a poly-Si layer (sacrificial) is deposited by using a low-pressure chemical vapour deposition (LPCVD) and as a movable control gate, an aluminium (Al) layer is deposited by using an e-beam evaporator. The Al layer is patterned with double-clamped beam structures by using photolithography and through a combination of wet and dry etching processes, the structures are successfully suspended and characterised by using a C–V meter. From the structure characterisation, the pull-in curve is successfully obtained and due to unexpected large short-range forces such as the van der Waals forces, the pull-out curve is not observed. In order to clarify this issue, a numerical analysis is performed in which the structural materials under test shown the influence of such short-range forces on the structure and a solution to override them is proposed.

An Analytical Study on the Structural Performance Evaluation of the Multistage Overturing Movable Gate

Numerous water gates have been constructed and are under construction for the control of water level because, due to the topographical characteristics of mountainous area, many small size rivers are developed in Korean peninsula. Among the water gates, movable water gates are more efficient to control water level than the fixed water gates. By the field application of the movable water gates, many problems associated with the fixed water gates have been mitigated. The problems include the bottom water pollution, the change of habitats for the riparian organisms, etc. This is the reason to increase the field application of movable water gates. In the paper we present the result of analytical investigation pertaining to the structural behavior of multistage overturning movable water gate which is one of the movable water gates. In the analytical investigations, the finite element analysis on the constructed water gate has been conducted for illustration purposes and it was found that the multistage movable water gate is much better than the fixed and/or existing movable water gates in the point of structural and environmental performances.

Modal Analysis and Design of a Vertically Movable Gate Field Effect Transistor (VMGFET) Proposed for Low-Frequency Sensing Applications

The objective of this study is to design and optimize a vertically movable gate field effect transistor (VMGFET) - suitable for low-frequency, high-sensitivity applications - with an emphasis on modal analysis of the suspended gate structure, optimization of mesh density within the employed finite element analysis software, and optimization of the moveable gate dimensions given its relationship with fabrication complexity and the structure’s resonant frequencies. The methods of design, optimization, and analysis were carried out with COMSOL Multiphysics 4.2a under the assumption of no damping with free vibrations. The results indicate optimal dimensions of the suspended gate structure - given constraints on size, resonance, and fabrication complexity - which suggest a beam thickness of 3 µm and a beam width of 15 µm, yielding an upper limit of input force frequencies near 2 kHz.

Electrical modelling and design insight of a vertically movable gate field effect transistor for physical sensor applications

A novel electrical modelling for a vertically movable gate field effect transistor (VMGFET) for sensing applications is conducted. Presented is the manipulation of threshold voltage and drain current of the VMGFET as a function of the effective charge density and the air gap between the gate and the substrate body. The usage of the VMGFET in depletion mode, while operating in the saturation region, is proposed to prevent an electrostatic attraction between the suspended gate and substrate and to have a linear response of drain current over a varying air gap. The sensitivity of the VMGFET is simulated and shows a constant slope by changing the air gap under the operating conditions, such as working in depletion mode and operating in the saturation region. The sensitivity independence of the gate position is desirable when the VMGFET is applied for physical sensor applications.

Extracting the density profile of an electronic wave function in a quantum dot

We use a model of a one-dimensional nanowire quantum dot to demonstrate the feasibility of a scanning probe microscope (SPM) imaging technique that can extract both the energy of an electron state and the amplitude of its wave function using a single instrument. This imaging technique can probe electrons that are buried beneath the surface of a low-dimensional semiconductor structure and provide valuable information for the design of quantum devices. A conducting SPM tip, acting as a movable gate, measures the energy of an electron state using Coulomb blockade spectroscopy. When the tip is close to the nanowire dot, it dents the wave function \ensuremath{\Psi}($x$) of the quantum state, changing the electron's energy by an amount proportional to $|{\ensuremath{\Psi}(x)|}^{2}$. By recording the change in energy as the SPM tip is moved along the length of the dot, the density profile of the electronic wave function can be found along the length of the quantum dot.

Bistable Charge Configuration of Donor Systems near the GaAs(110) Surfaces

In gated semiconductor devices, the space charge layer that is located under the gate electrode acts as the functional element. With increasing gate voltage, the microscopic process forming this space charge layer involves the subsequent ionization or electron capture of individual dopants within the semiconductor. In this Letter, a scanning tunneling microscope tip is used as a movable gate above the (110) surface of n-doped GaAs. We study the build-up process of the space charge region considering donors and visualize the charge states of individual and multi donor systems. The charge configuration of single donors is determined by the position of the tip and the applied gate voltage. In contrast, a two donor system with interdonor distances smaller than 10 nm shows a more complex behavior. The electrostatic interaction between the donors in combination with the modification of their electronic properties close to the surface results in ionization gaps and bistable charge switching behavior.

The relevance of electrostatics for scanning-gate microscopy

Scanning-probe techniques have been developed to extract local information from a given physical system. In particular, conductance maps obtained by means of scanning-gate microscopy (SGM), where a conducting tip of an atomic-force microscope is used as a local and movable gate, seem to present an intuitive picture of the underlying physical processes. Here, we argue that the interpretation of such images is complex and not very intuitive under certain circumstances: scanning a graphene quantum dot (QD) in the Coulomb-blockaded regime, we observe an apparent shift of features in scanning-gate images as a function of gate voltages, which cannot be a real shift of the physical system. Furthermore, we demonstrate the appearance of more than one set of Coulomb rings arising from the graphene QD. We attribute these effects to screening between the metallic tip and the gates. Our results are relevant for SGM on any kind of nanostructure, but are of particular importance for nanostructures that are not covered with a dielectric, e.g. graphene or carbon nanotube structures.

Scanning gate imaging of quantum dots in 1D ultra-thin InAs/InP nanowires

We use a scanning gate microscope (SGM) to characterize one-dimensional ultra-thin (diameter≈30 nm) InAs/InP heterostructure nanowires containing a nominally 300 nm long InAs quantumdot defined by two InP tunnel barriers. Measurements of Coulomb blockade conductanceversus backgate voltage with no tip present are difficult to decipher. Using the SGM tip asa charged movable gate, we are able to identify three quantum dots along the nanowire: thegrown-in quantum dot and an additional quantum dot near each metal lead. The SGMconductance images are used to disentangle information about individual quantum dotsand then to characterize each quantum dot using spatially resolved energy-levelspectroscopy.

Design of New Logic Architectures Utilizing Optimized Suspended-Gate Single-Electron Transistors

The operation and performances of the suspended-gate single-electron transistor (SET) are investigated through simulation. The movable gate is 3-D optimized, so that low actuation voltage (0.4 V), fast switching (1 ns), and ultralow pull-in energy (0.015 fJ) are simulated. A two-state capacitor model based on the 3-D results is then embedded with a SET analytical model in a SPICE environment to investigate the operation of the device. Through the control of the Coulomb oscillation characteristics, the position of the movable gate enables a background charge insensitive coding of the information. New circuit architectures with applications in cellular nonlinear network and pattern matching are also proposed and simulated.

Pull-In and Release Voltage Design for Nanoelectromechanical Field-Effect Transistors

The Euler-Bernoulli beam equation is solved simultaneously with the Poisson equation in order to accurately model the switching behavior of nanoelectromechanical field-effect transistors (NEMFETs). Using this approach, the shape of the movable gate electrode and semiconductor potential across the width of the channel are derived for the various regimes of transistor operation (before gate pull-in, after gate pull-in, and at the point of gate release). The impact of various transistor design parameters such as the body doping concentration, gate work function, gate stiffness, and as-fabricated actuation gap thickness, as well as source-to-body bias voltage and surface forces, on the gate pull-in and gate release voltages are examined. A unified pull-in/release voltage model is developed to facilitate NEMFET design for digital- and analog-circuit applications.

Nonlinear Internal Wave Run-Up on Impermeable Steep Slopes

Laboratory experiments were conducted to investigate the run-up of internal solitary waves (ISWs) on steep uniform slopes in a two-layered fluid system with a free surface. A 12 m long wave flume, which incorporated a movable vertical gate for generating ISWs, was used in the experiments. A steep uniform slope was modeled at one end of the flume. In the present study we looked at internal waves with small and large amplitudes using a steep uniform slope (from one of θ=30, 50, 60, 90, 120, and 130 deg) much longer than those previously published in the literature. Results collected from a wide range of experimental runs show that the run-up height depends on the planar slope, while the breaking depth is only antecedent to the amplitude of an ISW. The overall quantitative agreement with the linear feature aspects of the incident wave amplitude is encouraging.

Scanning gate microscopy measurements on a superconducting single-electron transistor

We present measurements on a superconducting single-electron transistor (SET) in which the metallic tip of a low-temperature scanning force microscope is used as a movable gate. We characterize the SET through charge stability diagram measurements and compare them to scanning gate measurements taken in the normal conducting and the superconducting states. The tip-induced potential is found to have a rather complex shape. It consists of a gate voltage-dependent part and a part which is independent of gate voltage. Further scanning gate measurements reveal a dependence of the charging energy and the superconducting gap on the tip position and the voltage applied to it. We observe an unexpected correlation between the magnitude of the superconducting gap and the charging energy. The change in EC can be understood to be due to screening, however the origin of the observed variation in Δ remains to be understood. Simulations of the electrostatic problem are in reasonable agreement with the measured capacitances.

Visualization of local gate control in a ZnO inter-nanowire junction device

Electronic transport behavior of a ZnO inter-nanowire junction transistor was studied with atomic force microscopy (AFM) and scanning gate microscopy. Source–drain currents exhibit strong dependences on gate voltage, applied by a movable local gate. Noticeable local gating was observed when the tip is above the junction area of the inter-nanowire device, demonstrating that active region in the device is confined. Transport mechanism in the device was understood in terms of band bending by electronic doping. Our device structure is analogous to that of two ZnO grains separated by an insulating layer in polycrystalline varistor devices.

Gateless Gating Model vs. Gated Pore Model: Phase Pinning of Guanidinium Toxins in Sodium Channels

Unlike the gated pore model, which assumes the existence of a movable gate within the molecule, the gateless gating model (H. R. Leuchtag, Voltage-Sensitive Ion Channels, Springer 2008) (VSIC) explains ion-channel gating as a phase transition. Under this model, the closed channel conformation imposed by the resting electric field is a compact, ordered phase with behavior, such as critical temperature, hysteresis, fractional dispersion exponent (constant phase angle) and Curie-Weiss law, similar to that of phases seen in ferroelectric liquid crystals (VSIC, pp. 355-383). In the absence of a toxin molecule, threshold membrane depolarization indirectly brings about a stochastic phase transition to a less ordered phase in which S4 segments expand by the mutual repulsion of their positively charged residues. The resulting wider pitch of permeation pathway α helices elastically linked to the S4s permits ion replacement in the interloop H bonds and the subsequent percolation of permeant ions through the channel (VSIC, 477f, 506f). With an externally applied tetrodotoxin (TTX) or analog molecule complexed in the channel, however, the ordered phase is pinned by the toxin, inhibiting the transition to the ion-conducting phase (VSIC, 76, 382f). Phase pinning by impurities is an established effect in ferroelectric liquid crystals. In the toxin, a guanidinium group, H2N+=C(NH2)2, a highly resonant, planar, positive ion, is active in pinning the closed phase. The fact that guanidinium is also found in ferroelectric crystals such as guanidinium aluminum sulfate hexahydrate suggests that TTX enhances the spontaneous polarization of the resting phase. This explanation by the gateless gating model of specific toxin action is based on physical principles; in contrast, the phrase “TTX blocks the pore” offered by the gated pore model is vague and at the macroscopic rather than molecular scale.

A Laterally Movable Gate Field Effect Transistor

A laterally movable gate field effect transistor (LMGFET) device that directly couples lateral mechanical gate motion to drain current of a FET is presented in this paper. Lateral motion of the FET gate results in a change in channel width, keeping the channel length and the gap between the gate and the oxide layer constant. This results in a change in channel current that, in principle, is linearly proportional to mechanical motion. The operating principle of an LMGFET, along with details of the fabrication process for a depletion-type LMGFET device, is described. Fabricated LMGFET shows an average drain current sensitivity to gate motion of -5.8 ¿A/¿m at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> = 20 V and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> = 0 V for 60-¿m gate motion. A model for the fabricated LMGFET is developed based on electrical measurements. The device shows promise both as a sensor and as an actuator in MEMS and other related applications.

Synchronization effects in a strongly coupled nanomechanical single-electron transistor

The tunneling of electrons in a single-electron transistor (SET) that has a movable gate can be predictable when the energy associated with the motion of the resonating gate is nearly the same as the energy gained by the tunneling electron. This occurs as the tunneling events and the motion of the resonating gate become synchronized. Ideas from stochastic resonance can explain this phenomenon.

Imaging a one-electron InAs quantum dot in an InAs/InP nanowire

Nanowire heterostructures define high-quality few-electron quantum dots for nanoelectronics, spintronics, and quantum information processing. We use a cooled scanning probe microscope (SPM) to image and control an InAs quantum dot in an InAs/InP nanowire using the tip as a movable gate. Images of dot conductance vs tip position at $T=4.2\text{ }\text{K}$ show concentric rings as electrons are added, starting with the first electron. The SPM can locate a dot along a nanowire and individually tune its charge, abilities that will be very useful for the control of coupled nanowire dots.

Micro FET pressure sensor manufactured using CMOS-MEMS technique

The fabrication of a micro field effect transistor (FET) pressure sensor using the commercial 0.35μm complementary metal oxide semiconductor (CMOS) process and a post-process has been investigated. The pressure sensor is composed of 16 sensing cells in parallel, and each sensing cell includes a suspended membrane and an NMOS. The suspended membrane is the movable gate of the NMOS. The pressure sensor needs a post-process to obtain the suspended membrane after the CMOS process. The post-process employs etchants to etch the sacrificial layers to release the suspended membrane, and then a low-pressure chemical vapor deposition (LPCVD) parylene is used to seal the etching holes in the pressure sensor. The pressure sensor produces a change in current when applying a pressure to the sensing cells. Experimental results show that the pressure sensor has a sensitivity of 0.022μA/kPa in the pressure range of 0–500kPa.

Numerical modelling of flow through moving water-control gates by vortex method. Part II – calculation result

Computational results are presented for uniform approach flow past emergency gate model, placed in a 2–D channel with free water surface, for two cases: (1) fixed gate and (2) gate moving perpendicular to the water flow. The calculations were made in the dimensionless system, for the assumed Reynolds number Re = 106. The velocity field and vorticity field were determined by vortex method. On this basis the pressure distribution profile and hydrodynamic force were determined by means of boundary elements method. The vortex structures and the evolution of vortex shedding are illustrated by velocity field of vortex blobs for both cases – fixed and movable gate. We also presented the distribution of the pressure coefficient alongside the gate edge as well as the derivative lift and drag coefficients. The average force coefficients agree very well with the experimental values. The movement of downward of the gate substantially increases the calculated value of the lift coefficient.

Development of MEMS Capacitive Sensor Using a MOSFET Structure

The concept of a capacitive MOSFET sensor using a SOI wafer for detecting vertical force applied to its floating gate was already reported by the authors. A MOSFET is fabricated on a SOI wafer, and the box oxide under the gate is removed to release the gate structure. This sensor detects the displacement of the movable gate electrode from changes in drain current, and this current can be amplified electrically by adding voltage to the gate, i.e., the MOSFET itself serves as a mechanical sensor structure. Following this, the present paper reports the fabrication of a practical test device and its preliminary characterization. The present paper also proposes a circuitry, which converts the drain current change to the voltage change while compensating the temperature change. The performance of this circuitry is confirmed by SPICE simulation. In accelerometer application, a comparatively heavy proof mass and thin supporting beams are necessary for increasing the sensitivity. For this purpose, a fabrication process of depositing a thick mass structure using electroplating is newly proposed.