Gate Voltage Signal Research Articles

Gate-Tunable Lifshitz Transition of Fermi Arcs and Its Transport Signatures

One hallmark of Weyl semimetals is the emergence of Fermi arcs (FAs) in surface Brillouin zones, where FAs connect the projected Weyl nodes of opposite chiralities. Unclosed FAs can give rise to various exotic effects that have attracted tremendous research interest. Configurations of FAs are usually thought to be determined fully by the band topology of the bulk states, which seems impossible to manipulate. Here, we show that FAs can be simply modified by a surface gate voltage. Because the penetration length of the surface states depends on the in-plane momentum, a surface gate voltage induces an effective energy dispersion. As a result, a continuous deformation of the surface band can be implemented by tuning the surface gate voltage. In particular, as the saddle point of the surface band meets the Fermi energy, the topological Lifshitz transition takes place for the FAs, during which the Weyl nodes switch their partners connected by the FAs. Accordingly, the magnetic Weyl orbits composed of the FAs on opposite surfaces and chiral Landau bands inside the bulk change their configurations. We show that such an effect can be probed by the transport measurements in a magnetic field, in which the switch-on and switch-off conductances by the surface gate voltage signal the Lifshitz transition. Our work opens a new route for manipulating the FAs by surface gates and exploring novel transport phenomena associated with the topological Lifshitz transition.

Numerical investigation of field-effect control on hybrid electrokinetics for continuous and position-tunable nanoparticle concentration in microfluidics.

We introduce herein an effective way for continuous delivery and position-switchable trapping of nanoparticles via field-effect control on hybrid electrokinetics (HEK). Flow field-effect transistor exploiting HEK delicately combines horizontal linear electroosmosis and transversal nonlinear electroosmosis of a shiftable flow stagnation line (FSL) on gate terminals under DC-biased AC forcing. The microfluidic nanoparticle concentrator proposed herein makes use of a simple device geometry, in which an individual or a series of planar metal strips serving as gate electrode (GE) are subjected to a hybrid gate voltage signal and arranged in parallel between a pair of 3D driving electrodes. On the application of a DC-biased AC electric field across channel length direction, all the GE are electrochemically polarized, and the action of imposed hybrid electric field on the multiple-frequency bipolar counterions within the composite-induced double layer generates two counter-rotating induced-charge electroosmotic (ICEO) micro-vortices on top of each GE. Symmetry breaking in ICEO flow profile occurs once the gate voltage deviates from natural floating potential of corresponding GE. The gate voltage offset not only results in an additional pump motion of working fluid for enhanced electroosmotic transport but also directly changes the location of FSL where nanoparticles are preferentially collected by field-effect HEK. Our results of field-effect control on HEK are supposed to guide an elaborate design of flexible electrokinetic frameworks embedding coplanar metal strips for a high degree of freedom analyte manipulation in modern micro-total-analytical systems.

Modeling of Power Module for 48 V High Power Inverter

The paper presents simulation and measurement results of high current three-phase inverter power modules used in 48V motor application. Firstly, FEM (Finite Element Method) is used to extract circuit parasitics. With the inclusion of MOSFET and passive component SPICE models, highly accurate simulation model is created. Switching characteristics of power module are simulated at 300 A load current and 48 V battery voltage. Thermal simulations estimate maximum transistor temperatures at given power losses. Electrical simulations are compared to actual measurements under identical test conditions. Comparison shows good matching between simulations and measurements. The phase voltage rise and fall times are the same in simulations and measurements. The overshoot voltages are also the same in both cases, around 28 V. The mismatch can be found in the currents of secondary loops. The gate voltage signal is similar with small mismatch of Miller plateau voltage, due to transistor model parameters mismatch.

On the Physical Meaning of Single-Value Activation Energies for BTI in Si and SiC MOSFET Devices

Bias temperature instability (BTI) has been shown to be the collective response of an ensemble of defects to a gate voltage signal. Each defect can be described using first-order reactions for either charge trapping or defect creation with a broad distribution of time constants. Consequently, the temperature dependence of BTI has to be determined by a distribution of activation energies reflecting the variety of available defects. Commonly employed single-value apparent activation energy-based modeling of BTI can therefore be at best a very rough approximation and its apparent activation energy is consequently a model parameter without physical justification. This study investigates the impact of measurement parameters, such as the extraction point in the drift curve and the recovery time, on these apparent activation energies that can be extracted either by threshold voltage shift increase (vertical method) or by stress time acceleration (horizontal method). The framework of activation energy maps enables an analytical mathematical approach to calculate the dependence of extracted apparent activation energies on the measurement parameters. A comparison between Si and SiC MOSFET devices is employed, and nonphysical negative apparent activation energies are explained. Since apparent activation energies have been repeatedly employed to justify physics-based descriptions of BTI, their physical (in)significance is discussed.

Gate-controlled generation of optical pulse trains using individual carbon nanotubes.

In single-walled carbon nanotubes, electron–hole pairs form tightly bound excitons because of limited screening. These excitons display a variety of interactions and processes that could be exploited for applications in nanoscale photonics and optoelectronics. Here we report on optical pulse-train generation from individual air-suspended carbon nanotubes under an application of square-wave gate voltages. Electrostatically induced carrier accumulation quenches photoluminescence, while a voltage sign reversal purges those carriers, resetting the nanotubes to become luminescent temporarily. Frequency-domain measurements reveal photoluminescence recovery with characteristic frequencies that increase with excitation laser power, showing that photoexcited carriers provide a self-limiting mechanism for pulsed emission. Time-resolved measurements directly confirm the presence of an optical pulse train synchronized to the gate voltage signal, and flexible control over pulse timing and duration is also demonstrated. These results identify an unconventional route for optical pulse generation and electrical-to-optical signal conversion, opening up new prospects for controlling light at the nanoscale.

A Three-Terminal Silicon-PMOSFET-Like Light-Emitting Device (LED) for Optical Intensity Modulation

A Si- p-channel metal-oxide-semiconductor field-effect transistor (PMOSFET)-like LED has been developed for light emission modulation. In contrast to a two-terminal Si-diode LED modulated by current signal, a major advantage of this three-terminal Si-PMOSFET LED is that the optical intensity modulation can be controlled by gate voltage signal, a standard CMOSFET operation to ease both logic circuit implementation and light modulation. The gate applied voltage induces carrier concentration modulation at both channel and source/drain region under the gate, thus modulating electric-field distribution and its light emission. Fabricated in a standard CMOS process technology, this Si-PMOSFET LED ensures its potential on realizing silicon optoelectronic integration.

A new oxide-trap based on charge-pumping (OTCP) extraction method for irradiated MOSFET devices: part II (low frequencies)

In this paper, we propose a new extraction method of radiation-induced oxide-trap density (/spl Delta/N/sub ot/), called Oxide-Trap based on Charge-Pumping (OTCP). This part presents the HF-OTCP method, which relies on high-frequency (HF) standard charge-pumping measurement. By applying an HF gate voltage signal, we avoid the border-trap effect in CP current (I/sub cp/) measurements. Hence, I/sub cp/ will be only due to the interface-trap contribution. We establish, using the HF-OTCP method, that /spl Delta/N/sub ot/ is only dependent on /spl Delta/V/sub th/ (threshold voltage shift) and /spl Delta/I/sub cpm/ (increase of maximum CP current), where /spl Delta/I/sub cpm/ is caused by radiation-induced interface-trap increase (/spl Delta/N/sub it/). We also clearly show that /spl Delta/V/sub th/ and /spl Delta/I/sub cpm/ can be obtained from lateral and vertical shifts of Elliot's charge-pumping curves, respectively. Thus, this new procedure allows the determination of /spl Delta/N/sub ot/ without needing any additional techniques. Finally, this procedure can be used in rapid hardness assurance test to evaluate both radiation-induced oxide and interface traps.

An alternative method for measuring flatband voltage in MOS devices

The present paper describes an experimental method that can be used to measure the flatband voltage value in MOS devices. The method is based on the detection of the non-steady-state/steady-state transition of the surface potential at the Si–SiO 2 interface when it is pulsed by a voltage signal. This detection is evidenced as follows. A set of current versus signal frequency measurements, for different voltage amplitudes, are performed. The frequency values corresponding to the maximum measured current (optimal frequencies) are read. Various plots of gate voltage signal ( V G) versus critical frequencies ( f m– V G) are performed for voltage values comprised in the range 0.2 to 2 V stepped by a 0.1 V increment. The f m– V G curves are found to undergo an abrupt change of slope at a specific gate voltage value. The value of the flatband voltage is extracted from the former curves. Experiments have been carried out on a variety of devices. The values of flatband voltage obtained are compared to that obtained by the conventional C-V method and that specified by device manufacturers. The results are found to be in quite good agreement.

Modeling Soft Breakdown of Ultra-Thin Gate Oxide Layers

ABSTRACTThe time-dependent dielectric breakdown of MOS capacitors with ultra-thin gate oxide layers is investigated. After the occurrence of soft breakdown, the gate current increases by 3 to 4 orders of magnitudes and behaves like a power law of the applied gate voltage. It is shown that this behavior can be explained by assuming that a percolation path is formed between the electron traps generated in the gate oxide layer during electrical stress of the capacitors. The time dependence of the gate voltage signal after soft breakdown is next analysed. It is shown that the fluctuations in the gate voltage are non-gaussian as well as that long-range correlations exist in the system after soft breakdown. These results can be explained by a dynamic percolation model, taking into account the trapping-detrapping of charges within the percolation cluster formed at soft breakdown.

Charge-extraction technique for studying the surface states in MOS devices

A method called the charge-extraction technique is proposed for studying the surface states in MOS devices. This new technique utilizes the substrate current arising from the non-steady-state emission of carries from the surface states instead of that arising from their steady-state recombination, as utilized in the charge-pumping method. This is achieved by confining the amplitude of the gate-voltage signal to such a magnitude that the surface region of the device does not cross the depletion limits. A new theoretical model is developed for the present case which predicts the occurrence of a maximum value of the substrate current at a certain optimum frequency of the applied gate-voltage signal. Experimental measurements have been found quite in conformity with the theoretical model. It is found that the maximum substrate current and the corresponding optimum frequency, obtained in this way, can serve as more handy and precise parameters for the determination of the surface states.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Generation of dc substrate current in metal-oxide-semiconductor structures under an oscillating gate voltage

The present letter reports the results of the experimental measurements of the dc substrate current in metal-oxide-semiconductor (MOS) structures operated under a gate-voltage signal oscillating under depletion limits. A new component has been detected in this current whose direction of flow is independent of the polarity of the applied gate voltage as well as the nature of the MOS structure. Its magnitude shows strong dependence on the frequency of the applied gate voltage, giving rise in general to three prominent peaks in the current-frequency curve. Arguments are given in support of the view that this current has its origin from the charging and discharging of the surface-states existing on the Si-SiO2 interface under non-steady-state emission.

Investigation of the charge pumping current in metal-oxide-semiconductor structures

A study of the charge pumping phenomena occurring in a metal-oxide-semiconductor structure is presented. This study, which is based on numerical simulations of charge pumping in the time domain, enables a good overall description of the charge pumping characteristics to be obtained. In particular, the influence of various charge pumping configurations is analyzed relative to gate voltage signal characteristics such as rise/fall times, low/high gate biases, pulse duration, and signal waveform, and to device parameters such as interface-state density, flat-band voltage, and threshold voltage. The numerical simulations are also compared to typical experimental charge pumping characteristics. The adequacy of analytical or simplified models generally used to exploit the charge pumping data is also discussed.

A time domain analysis of the charge pumping current

A general time domain analysis of the recombination currents involved in the charge pumping phenomenon occurring in a metal-oxide-semiconductor structure is presented. This analysis not only allows a better understanding of the physical origin of the charge pumping current, but also furnishes quantitative information about the charge pumping current intensity versus the frequency, shape, and amplitude of the applied gate voltage signal. A comparison between our numerical simulations and analytical models available in the literature is also conducted. In particular, the range of applicability of the Groeseneken formula is studied both for trapezoidal and triangular time dependence of the gate voltage.