External Gate Research Articles

Dynamic band alignment modulation of ultrathin WOx/ZnO stack for high on/off ratio field-effect switching applications.

A two-dimensional (2D) WOx/ZnO stack reveals a unique carrier transport behavior, which can be utilized as a novel device element to achieve a very high on/off ratio (>106) and an off current density lower than 1 nA cm-2. These unique behaviors are explained by a dynamic band alignment between WOx and ZnO, which can be actively modulated by a gate bias. The performance of FET utilizing the WOx/ZnO stack is comparable to those of other 2D heterojunction devices; however, it has a unique benefit in terms of process integration because of very low temperature process capability (T < 110 °C). The high on/off switching with extremely low off current density utilizing the dynamic band alignment modulation at the WOx/ZnO stack can be a very useful element for future device applications, especially in monolithic 3D integration or flexible electronics.

Investigation of Negative DIBL Effect and Miller Effect for Negative Capacitance Nanowire Field-Effect-Transistors

In this study, the negative DIBL (N-DIBL), negative differential resistance (NDR), and Miller effect of a negative capacitance nanowire filed-effect-transistor (negative capacitance (NC) NWFET) were analyzed by employing the custom-built SPICE model. In the simulation, the minimum subthreshold swing (SS) reduced to 40 mV/decade with negligible hysteresis, and the on-current amplified by approximately three times. The N-DIBL effect was analyzed by building a model, and the results indicated that the N-DIBL is negatively correlated with the SS. Hence, it is indispensable to make trade-offs between the N-DIBL and SS in NC NWFET applications. Moreover, the Miller effect of a NCFET-based inverter was investigated for the first time. The Miller effect of the NC NWFET-based inverter was considerably improved owing to a high on-current and negative internal gate voltage (when external gate voltage is set to 0V), which is beneficial for high-speed circuit building based on NC NWFETs. The overshoot of the NC NWFET-based inverter is ~43.1% less than that of the NWFET-based inverter, and the propagation delay of the NC NWFET-based inverter is ~73.1% less than that of the NWFET-based inverter at ferroelectric thickness $\text{T}_{\mathrm{ FE}}=3$ nm.

Subthreshold Characteristics of a Metal-Oxide–Semiconductor Field-Effect Transistor with External PVDF Gate Capacitance

An organic ferroelectric capacitor, using polyvinylidene difluoride (PVDF) as the dielectric, was fabricated. By connecting the PVDF capacitor in series to the gate of a commercially purchased metal-oxide–semiconductor field-effect transistor (MOSFET), drain current (ID)–drain voltage (VD) characteristics and drain current (ID)–gate voltage (VG) characteristics were measured. In addition, the subthreshold slopes of the MOSFET were determined from the ID–VG curves. It was found that the subthreshold slope could be effectively reduced by 23% of its original value when the PVDF capacitor was added to the gate of the MOSFET.

Electrically tunable chiral Majorana edge modes in quantum anomalous Hall insulator–topological superconductor systems

Chiral Majorana edge modes are theoretically proposed to perform braiding operations for the potential quantum computation. Here, we suggest a scheme to regulate trajectories of the chiral Majorana fermion based on a quantum anomalous Hall insulator (QAHI)-topological superconductor heterostructure. An applied external gate voltage to the QAHI region introduces a dynamical phase so that the outgoing Majorana fermions can be prominently tuned to different leads. The trajectory is mechanically analyzed and the electrical manipulation is represented by the oscillating transmission coefficients versus the gate voltage. Through the optimization of devices, the conductance is likewise detectable to be periodically oscillating, which means an experimental control of chiral Majorana edge modes. Besides, this oscillating period which is robust against disorder also provides an attainable method of observing the energy dispersion relation of the edge mode of the QAHI. Furthermore, the oscillating behavior of conductance serves as smoking-gun evidence of the existence of the chiral Majorana fermion, which could be experimentally confirmed.

A Compact Series-Connected SiC MOSFETs Module and Its Application in High Voltage Nanosecond Pulse Generator

Nanosecond pulse discharge plasma has many prospects in industrial applications, and high-voltage repetitive nanosecond pulse generators with compact design and light weight have become one of the key issues limiting its development in some applications. This paper presents a high voltage series-connected silicon carbide (SiC) metal-oxide -semiconductor field effect transistor ( MOSFET s) module which can be served as the main switch in a repetitive high-voltage nanosecond pulse generator. This kind of series-connected MOSFET s module with only single external gate driver requiring very few components is very suitable for compact assembly. By analyzing the working principle, three topologies of series-connected MOSFET s module are proposed. The switching behaviors of the three different topologies with four SiC MOSFET s series-connected are compared experimentally. The variation of switching characteristics of series-connection SiC MOSFET s module with different numbers of devices are investigated. The layout is also optimized to shorten pulse front time and improve output pulse quality. Furthermore, a 10 kV SiC MOSFET s module with a turn- on transition time ∼10 ns is developed. The double pulse test result demonstrates excellent switching performances. Finally, a compact and high-voltage pulse generator composed of three 10 kV SiC MOSFET s module is tailored, with a typical rise time ∼40 ns and peak voltage of ∼30 kV.

Static Polystyrene Gate Charge Density Modulation of Dinaphthothienothiophene with Tetrafluorotetracyanoquinodimethane Layer Doping: Evidence from Conductivity and Seebeck Coefficient Measurements and Correlations

We relate the static charging of organic field effect transistor (OFET) gate dielectric polymers, the resulting change in threshold voltage shift, and the modulation of thermoelectric properties of the transistor semiconductor. We utilize a bottom gate OFET structure with cross-linkable polystyrene and plain polystyrene (XLPS/PS) bilayer dielectrics and dinaphthothienothiophene (DNTT) organic molecular semiconductor with tetrafluorotetracyanoquinodimethane (F4TCNQ) doping layer. The conductivity and Seebeck coefficient are measured before and after charging. We find that the conductivity increases significantly, while the Seebeck coefficient decreases correspondingly in a trend of S ∝ ln(σ) after the dielectrics were negatively charged, resulting in an average of 5-fold increase in power factor at the maximum charging level. Additionally, temperature-dependent conductivity measurements show that the activation energy decreases with the increasing conductivity. We calculate the Seebeck coefficient based on the activation energy and find the result is consistent with the Mott mobility edge model. This work confirms the charging mechanism in our previous published work, including the location of static charges in the bulk of the dielectric, and demonstrates modulation and enhancement of the thermoelectric properties of organic thermoelectric materials by adjacent chargeable dielectrics without changing the molecular microstructure or applying external gate voltage during operation.

Tunable localized surface plasmon graphene metasurface for multiband superabsorption and terahertz sensing

We propose a plasmon induced tunable metasurface for multiband superabsorption and terahertz sensing. It consists of a graphene sheet that facilitates perfect absorption where the graphene pattern at the top layer creates an enhanced evanescent wave that facilitates the metasurface to work as a sensor. The modelling and numerical analysis are carried out using Finite Element Method (FEM) based software, CST microwave studio where a genetic algorithm (GA) is used to optimize the geometric parameters, and metasurface tunability is achieved via an external gate voltage on the graphene. By exploiting graphene’s tunable properties we demonstrate a multiband superabsorption spectra having a maximum absorption of 99.7% in a frequency range of 0.1–2.0 THz that also maintain unique optical performance over a wide incidence angle. Further results show how the superabsorber can be used as a sensor, where the resonance frequency shifts with the refractive index of the surrounding environment.

Efficiency and Conducted EMI Evaluation of a Single-Phase Power Factor Correction Boost Converter Using State-of-the-Art SiC Mosfet and SiC Diode

SiC-based diodes and mosfet s switch extremely quickly with low conduction losses. Thus, from the perspective of efficiency, such devices are ideal for a continuous conduction mode (CCM) boost power factor correction (PFC) converter. However, the circuit parasitic becomes alive while switching with high $dv/dt$ and $di/dt$ values, which necessitates the need for electromagnetic compatibility (EMC) compliance measurements. Employing the best available low-loss SiC mosfet and SiC diode, in this paper, a 1-kW PFC boost converter prototype was designed, developed, and evaluated with the objective of quantifying the efficiency and EMC signature. The efficiency is evaluated through two approaches, namely, a circuit simulation and a laboratory measurement. With the first approach, the switching losses are obtained using a widely accepted double-pulse test methodology, and the conduction losses are taken from the data sheet, whereas with the second approach, the current and voltage are recorded at the input and output of the PFC converter using power analyzer. The electromagnetic interference (EMI) is monitored using line impedance stabilizing network and EMC analyzer. To maximize the efficiency, a fast, clean switching of the silicon carbide (SiC) is necessary. Utilizing a low-parasitic printed circuit board design approach and switching the selected low-loss SiC devices with a 0 $\Omega$ external gate drive resistance, this PFC boost yields a peak efficiency of 97.2% at full-rated power when switched at 250 kHz. Furthermore, the EMI noise was measured at 66 and 250 kHz. It was found that the same EMI filter size satisfies the CISPR 11 Class B conducted EMI limit at both switching frequencies with a noise of approximately 10 dB higher at 250 kHz. As the main contribution of this paper, the best case efficiency and worst case EMI are evaluated in this paper.

Electron-hole superfluidity controlled by a periodic potential

We propose to control of an electron-hole superfluid in semiconductor coupled quantum wells and double layers of two-dimensional (2D) material by an external periodic field. This can either be created by the gates periodically located and attached to the quantum wells or double layers of 2D material or by the Moir\'e pattern of two twisted layers. The dependence of the electron-hole pairing order parameter on the temperature, the charge carrier density, and the gate parameters is obtained by minimization of the mean-field free energy. The second order phase transition between superfluid and electron-hole plasma, controlled by the external periodic gate field, is analyzed for different parameters.

Spin transport in proximity-induced ferromagnetic phosphorene nanoribbons

Abstract We investigate the spin-dependent transport properties in normal/ferromagnetic/normal phosphorene nanoribbon (PNR) junctions where an external gate electrode is attached to the ferromagnetic region. The exchange splitting in ferromagnetic phosphorene is induced by ferromagnetic proximity effect. We found that the current through this junction is spin polarized due to the exchange field induced in phosphorene. The spin polarization oscillates with the external gate voltage, and the spin polarization can be reversed from 100% to −100% by a slight change of the gate voltage. The controllable spin transport in phosphorene nanoribbons can have useful applications in the development of spintronic devices.

Phase inversion of THz radiation from silicon nanostructures

The experimental data of the optical and electric features of the silicon nanosandwiches obtained by silicon planar technology in the frameworks of the Hall geometry are presented. Silicon nanosandwiches represent the ultra-shallow silicon quantum wells of 2nm wide that are confined by δ-barriers heavily doped with boron, which appear to be used as the phase invertors and modulators of optical THz spectra and electric signals. The negative-U dipole boron centers formation, which appear to confine the edge channels, results in the effective mass dropping and corresponding reduction of the electron-electron interaction thereby giving rise to the macroscopic quantum phenomena at high temperatures up to room temperature. The modulation phase shift of the THz electroluminescence spectrum and phase control of the longitudinal conductance are observed by changing either the magnitude of the source-drain current or the voltage applied to the external gates of the silicon nanosandwiches within the quantum Faraday effect.

High Dynamic Range Externally Time-Gated Photon Counting Optical Time-Domain Reflectometry

Single photon detector (SPD) has a maximum count rate due to its dead time, which results in that the dynamic range of photon counting optical time-domain reflectometry (PC-OTDR) de-creases with the length of monitored fiber. To further improve the dynamic range of PC-OTDR, we propose and demonstrate an externally time-gated scheme. The externally time-gated scheme is realized by using a high-speed optical switch, i.e. a Mach-Zehnder interferometer, to modulate the back-propagation optical signal, and to allow that only a certain segment of the fiber is monitored by the SPD. The feasibility of proposed scheme is first examined with theoretical analysis and simulation; then we experimentally demonstrate it with our experimental PC-OTDR testbed operating at 800 nm wavelength band. In our studies, a dynamic range of 30.0 dB is achieved in a 70 meters long PC-OTDR system with 50 ns external gates, corresponding to an improvement of 11.0 dB in dynamic range comparing with no gating operation. Furthermore, with the improved dynamic range, a successful identification of a 0.37 dB loss event is detected with 30-seconds accumulation, which could not be identified without gating operation. Our scheme paves an avenue for developing PC-OTDR systems with high dynamic range.

Allosteric gate modulation confers K+ coupling in glutamate transporters.

Excitatory amino acid transporters (EAATs) mediate glial and neuronal glutamate uptake to terminate synaptic transmission and to ensure low resting glutamate concentrations. Effective glutamate uptake is achieved by cotransport with 3 Na+ and 1 H+, in exchange with 1 K+. The underlying principles of this complex transport stoichiometry remain poorly understood. We use molecular dynamics simulations and electrophysiological experiments to elucidate how mammalian EAATs harness K+ gradients, unlike their K+‐independent prokaryotic homologues. Glutamate transport is achieved via elevator‐like translocation of the transport domain. In EAATs, glutamate‐free re‐translocation is prevented by an external gate remaining open until K+ binding closes and locks the gate. Prokaryotic GltPh contains the same K+‐binding site, but the gate can close without K+. Our study provides a comprehensive description of K+‐dependent glutamate transport and reveals a hitherto unknown allosteric coupling mechanism that permits adaptions of the transport stoichiometry without affecting ion or substrate binding.

Gate Drive Technology Evaluation and Development to Maximize Switching Speed of SiC Discrete Devices and Power Modules in Hard Switching Applications

To understand the limitation of maximizing the switching speed of SiC low-current discrete devices and high-current power modules in hard switching applications, double pulse tests are conducted and the testing results are analyzed. For power modules, the switching speed is generally limited by the parasitics rather than the gate drive capability. For discrete SiC devices, the conventional voltage source gate drive (VSG) is not sufficient to maximize the switching speed even if the external gate resistance is minimized. The limitation of existing current source gate drives (CSG) is analyzed, and a CSG dedicated for SiC discrete devices is proposed, which can provide constant current during the switching transient, regardless of the high Miller voltage and large internal gate resistance. Compared with the conventional VSG, the proposed CSG achieves 67% faster turn on time and 50% faster turn off time, and 68% reduction in switching loss at full-load condition.

Theory of Thermionic Carrier Injection in Graphene/Organic Schottky Interface

Understanding the physics of charge transport in organic materials and charge injection across organic-based interface is critically important for the development of novel organic electronics and optoelectronics. Despite extensive efforts devoted to the study of transport and injection phenomena in organic materials and interfaces, the physics of thermionic carrier injection across graphene/organic interface remains largely incomplete thus far. Here we construct a model of thermionic carrier injection across a graphene/organic Schottky interface based on the Lengevin theory of charge recombination and the detailed balance formalism. We show that, due to the strong electrostatic doping effect in graphene under the influence of an external gate voltage, the electrical current traversing the interface differs significantly from conventional bulk-metal/organic Schottky interface and the injection current can be efficiently modulated by a gate-voltage to achieve an on-off ratio well-exceed $10^7$. The model developed here shall provide a theoretical foundation for the understanding graphene/organic Schottky interface, thus paving the way towards the development of novel nanoscale graphene-hybrid organic electronic and optoelectronic devices.

Microwave second-harmonic generation due to mobile carriers in doped graphene lacking center-of-inversion symmetry

We present a Boltzmann equation analysis of microwave second-harmonic generation (SHG) due to mobile carriers in the doped graphene layer grown epitaxially on a SiC substrate (or, alternatively, deposited on a h-BN substrate) and lost its space inversion symmetry owing to the graphene-substrate interaction. The latter also induces a finite gap between the conduction and the valence band at the corners of the graphene’s Brillouin zone. Within such a gapped band model, which includes, as a key necessary ingredient, the trigonal warping of the graphene conduction band involved, we derive an explicit analytical expression for the SHG response function of the graphene’s mobile carriers at room temperature, when the effect of electron-momentum relaxation is primarily due to acoustic phonon scattering. The results obtained show that the possibility to tune the doping level of graphene by an external gate voltage allows one to maximize the output power of the microwave SHG, which may be of practical interest for the designs of graphene-based nonlinear devices operating in the millimeter-wave range.

High‐Performance P‐Type Copper(I) Thiocyanate Thin Film Transistors Processed from Solution at Low Temperature

AbstractSemiconducting copper(I) thiocyanate (CuSCN) is actively studied for electronic and optoelectronic applications. Although various kinds of CuSCN‐based transistors are reported, these devices suffer from low charge carrier mobility of about 0.01–0.1 cm2 V−1 s−1. Here, ion gel electrolyte consisting of network polymer and ionic liquid is used as a high capacitance gate insulator to achieve high performance CuSCN‐based electrolyte‐gated transistors (CuSCN‐EGTs) with low operation voltage below 1 V. 30 nm thick CuSCN semiconductor film can be formed by a simple solution process with a low processing temperature (≈100 °C) that is directly applicable to flexible plastic substrates. By doping copper iodide to the CuSCN semiconductor, device performance including drain current and charge carrier mobility of the CuSCN EGT can be improved significantly. The measured charge carrier mobility of ≈0.3 cm2 V−1 s−1 is the highest among the reported CuSCN transistors using various gate insulators. These CuSCN‐EGTs also display good operation stability under continuous quasistatic external gate voltage sweeps. Such superior electrical performance and versatile processability of ion gel–gated CuSCN transistors make them suitable for use in complimentary circuits and large‐area flexible electronics.

Voltage-controllable multifunctional spin polarizer based on side-coupled quantum dots

Abstract Quantum dot system is considered as the prime candidate for nanospintronics devices, within which to generate spins one by one is the long term goal for quantum information and quantum computation. In this paper, with the aid of the Wilson’s numerical renormalization group method, we find a side-coupled double quantum dot molecule could be used to implement a multifunctional spin polarizer in the strongly correlated regime without switching the direction of the magnetic field. The spin polarizer reveals almost 100 % efficiencies, and the polarizing scenarios could be controlled easily by the initial charge state through external gate voltage, leading to various kinds of spin polarizers, e.g., unidirectional, bidirectional, and dual spin polarizers. We demonstrate the Zeeman effect and the inter-dot transport processes are responsible for the spin-polarized transport. Our scheme is robust for any inter-dot coupling and may also be useful for quantum dots connected with magnetic leads.

Probing topology and symmetry in topological crystalline insulators with magnetism

Topological crystalline insulators are classes of materials in which the electronic structure hosts non-trivial topology protected by some symmetry of the crystal. The paradigm of these are (Sn/Pb)Te alloys, for which the protection comes from mirror symmetry. The topology necessitates that gapless surface states are present for a bounded system even as the bulk is gapped. When magnetic dopants which are exchange-coupled to the electrons are added to the system, near the surface they may order. We demonstrate that the magnetic ordering at the surface reflects the symmetry of the bulk and leads to a rich set of possible ferromagnetic groundstates, with the particular form of this chosen out by the electron surface density, which in principle may be controlled by an external gate. The effective spin stiffness at the surface has an unusual “emergent” long-range form which follows from the Dirac nature of the electronic dispersion at the surface, with interesting consequences for the domain walls. Possible experimental signatures of this physics are discussed.

Effects of the electrostatic environment on superlattice Majorana nanowires

Finding ways of creating, measuring and manipulating Majorana bound states (MBSs) in superconducting-semiconducting nanowires is a highly pursued goal in condensed matter physics. It was recently proposed that a periodic covering of the semiconducting nanowire with superconductor fingers would allow both gating and tuning the system into a topological phase while leaving room for a local detection of the MBS wavefunction. We perform a detailed, self-consistent numerical study of a three-dimensional (3D) model for a finite-length nanowire with a superconductor superlattice including the effect of the surrounding electrostatic environment, and taking into account the surface charge created at the semiconductor surface. We consider different experimental scenarios where the superlattice is on top or at the bottom of the nanowire with respect to a back gate. The analysis of the 3D electrostatic profile, the charge density, the low energy spectrum and the formation of MBSs reveals a rich phenomenology that depends on the nanowire parameters as well as on the superlattice dimensions and the external back gate potential. The 3D environment turns out to be essential to correctly capture and understand the phase diagram of the system and the parameter regions where topological superconductivity is established.