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We consider the stationary characteristics and high-frequency conductance of dual-gate field-effect transistors with a substantially inhomogeneous capacitive connection between gates and a current-conducting channel for an electron enhancement control mode. In most cases, such transistors are quasihomogeneous because their stationary JV-characteristics are not different from characteristics of perfectly homogeneous devices, which can be described by a certain effective channel length. But a nonstationary small-signal conductance of such transistors with the same stationary characteristics can be noticeably different since it is dependent on detailed space distributions of the capacitive inhomogeneity. In particular, a high-frequency gate current can depend on the stationary current direction in channels with symmetrical stationary characteristics. We discuss in detail the cases when a specific channel-gate capacitance increases or decreases monotonically along the channel current direction. Then high-frequency gate currents depend substantially on the stationary channel current direction (in spite of the same stationary JV-characteristics for both directions). [ABSTRACT FROM AUTHOR]

A very short-channel field-effect transistor (FET), which is controlled by a nonplanar prismoidal gate, is proposed and its stationary JV characteristics are derived. The prismoidal gate effectively depletes a thin electron channel in a very narrow strip around the gate apex. Interaction of the channel electrons with a controlling gate potential is additionally focused by a selected two-layer dielectric separating the channel and gate. It is shown that in the considered FETs there exist both “standard” regimes characterized by quasisaturation of the drain current jD, with an increase in the drain-source voltage uDS, and nonstandard regimes, with drooping jDuDS characteristics and a “self-blocking” current jD. We consider the dependence of the channel conductance on the gate curvature near the apex. [ABSTRACT FROM AUTHOR]

The first subject considered in the article is a donor center embedded in a thin heterostructural barrier separating a semiconductor medium into two halves. As a result of the small thickness of this barrier, the wave function of an electron bound by the donor center shifts almost completely into both halves of the surrounding semiconductor medium. The ground and first excited electron states of such a donor center are separated from each other by a narrow energy gap determined by the symmetric-antisymmetric tunnel splitting. Such structures can be implemented in both GaAs/AlXGa1-XAs and Si/GeXSi1-X material systems. The second considered subject is an exciton formed in analogous heterostructures when the staggered band alignment takes place between the heterobarrier and semiconductor medium. As a result of such band alignment, the hole participating in the exciton creation is located in the formed quantum well and the electron, which is the hole’s opponent, is separated into halves (on different sides of the quantum well) as before. Unlike the donor center, the exciton can be shifted and localized in arbitrary positions along the staggered “barrier-well” boundary by inhomogeneous electric fields of external controlling gates. [ABSTRACT FROM AUTHOR]

Electron wave tunneling through a rectangular heterostructural emitter barrier is considered in the case of a homogeneous high-frequency (hf) alternating electric field directed normal to the barrier interfaces. This hf field leads not only to the well-known increase in a stationary tunnel current through the emitter barrier, which is proportional to EB2 (where EB is the electric-field amplitude) but also to a linear (∼EB) increase in an alternating current (ac) through this barrier with the same frequency ω as the electric-field frequency. The ac is a sharp function of ω, which grows significantly with an increase in ω (typically in the terahertz range). In a certain intermediate current and frequency region, the above-mentioned increase in the ac is the dominating effect of the alternating field. Such an effect can be used to optimize tunnel barrier emitters for ballistic transit-time terahertz-range oscillators. [ABSTRACT FROM AUTHOR]

A very thin positively charged metallic filament separated from a surface of a semiconductor (S) by a thin nontunneling potential barrier (B) induces a quantum wire (QWr) in the semiconductor at the B/S interface. Single-electron quantum states of this QWr are controlled by a potential (and a charge) of the metallic filament. Two close parallel metallic filaments placed over such a B/S interface form a double-quantum wire with the ground and the first excited electron states, which appear as a result of a symmetric–antisymmetric splitting of the ground electron state in the single QWr. Two crossed metallic filaments, which are parallel to the B/S interface, form a quantum dot with completely localized electron states under the crossing point of the metallic filaments. The analogous crossing of a metallic filament by a pair of close metallic filaments forms a double-quantum dot (DQD). The latter can serve as a two-level qubit cell. Such qubits can be controlled by potentials of three independent metallic filaments inducing the above-mentioned DQD. Besides this “outside” metallic wire control, the DQDs can be connected to each other across the “inside” quantum wires, which have formed these DQDs by crossing. [ABSTRACT FROM AUTHOR]

A plane electron wave incident on a tunnel-transparent potential barrier formed by the potential V(x,t)=V0(x)+V1(x)cos ωt generates, in addition to the usual stationary transmitted and reflected stationary waves, also “transmitted” and “reflected” electron waves oscillating with the same frequency ω. The transmitted oscillating wave can serve as the basis for transit-time microwave generators oscillating in the terahertz range. (Such oscillators are ballistic analogs of the tunnel-emission transit-time diode oscillators suggested almost half a century ago.) In the special case of a rectangular potential barrier, we describe the dependence of a small transmitted oscillating wave amplitude on the frequency co and the value of V1(x). We consider two forms of V1(x): (1) homogeneous oscillation of the height of the rectangular barrier and (2) V1(x)-aδ(x-x1) [where δ(x) is the Dirac delta function and 0

We develop an analytic theory based on an earlier model of the admittance of a ballistic transit time diode terahertz oscillator with tunnel emission of electrons into a transit space. The focus of this work is on the actual case when electrons are injected with high enough energy to move from the start with maximal (saturated) ballistic velocity (∼1×10[sup 8] to 2×10[sup 8] cm/s). On the one hand, such diodes have maximal oscillation frequencies and, on the other hand, a simple analytic theory describes them and allows us to avoid a cumbersome numerical procedure, which characterizes the general case. Such a description is analogous to the description of oscillatory diodes with diffusive transport and saturated drift velocity. We have also considered a special case when a small part of the ballistic electrons crossing the transit space scatter into a diffusive subsystem with a small drift velocity. The appearance of such slow-drifting electrons substantially increases space charge in the transit space and influences the static JV-characteristic but the high-frequency admittance is almost invariable. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have considered interactions between ballistic (or quasiballistic) electrons accelerated by a dc electric field in an undoped transit space (T space) and a small ultrahigh frequency ac electric field and have calculated the linear admittance of the T space. Electrons in the T space have a conventional, nonparabolic dispersion relation. After consideration of the simplest specific case when the current is limited by the space charge of the emitted electrons, we turned to an actual case when the current is limited by a heterostructural tunnel barrier (B barrier) separating the heavily doped cathode contact and the T space. We assumed that the B barrier is much thinner than the T space and both dc and ac voltages drop mainly across the T space. The emission tunnel current through the B barrier is determined by the electric field E(0) in the T space at the boundary B barrier/T space. The more substantial is, the tunnel current limitation the higher the electric field E(0) becomes. We have shown that for a space-charge limited current the change from parabolic dispersion to the nonparabolic branch induces narrowing and closing of the frequency windows of transit-time negative conductance starting with the lowest-frequency windows. These narrowing and closing frequency windows become effective only for very high voltages U across the T space: U>mV[sub S][sup 2]/2e, where m is the effective mass for the parabolic branch and V[sub S] is the saturated velocity for the nonparabolic branch. For moderate voltages U, the effects of nonparabolicity are not very substantial. The tunnel current limitation decreases the space-charge effects in the T space and diminishes the role of the detailed electron dispersion relation. As a result, restoration of the frequency windows of transit-time negative conductance and an increase in the value of this negative conductance occur. The implementation of the considered tunnel injection transit time oscillator diode promises to lead to... [ABSTRACT FROM AUTHOR]

A dispersion relation for an electron in a two-layer (and also multilayer) quantum well (QW) is formed as a result of a certain combination of initial dispersion relations for each of the forming layers. Such a combination can be used to engineer new dispersion relations with desirable properties. The same relates to a two-dimensional electron gas (2DEG) induced in a multilayer medium. In this study, we consider first such a 2DEG in a specific two-layer structure where a superlattice (SL) plays the role of the second half-infinite layer, and electrons with large wave numbers along the SL vector spread from the first ordinary QW layer to this SL. As a result of such a quantum (dynamic) real-space transfer, electrons become heavier, and the dispersion relation achieves an additional negative effective mass (NEM) section. Such NEM dispersion relations were studied for several different material systems, including the two most interesting three-material systems: (1) an isomorphic Al[sub 0.15]Ga[sub 0.85]As//GaAs/Al[sub 0.5]Ga[sub 0.5]As structure and (2) a strained In[sub 0.5]Ga[sub 0.47]As//In[sub x]Ga[sub 1-x]As/In[sub y]Al[sub 1-y] As structure (x> 0.53, y < 0.52) with a strain-balanced In[sub x]Ga[sub 1-x]As/In[sub y]Al[sub 1-y]As SL. Most of the results were verified using a simplified 1D model, but some of them were verified by more complicated 2D-model calculations. [ABSTRACT FROM AUTHOR]

Absolute absorption and photoionization cross sections of methane have been measured with an accuracy of about 2% or 3% over most of the wavelength range from 950 to 110 Å. Also, dissociative photoionization cross sections were measured for the production of CH+4 , CH+3 , CH+2 , CH+, and C+ from their respective thresholds to 159 Å, and for H+ and H+2 measurements were made down to 240 Å. Fragmentation was observed at all excited ionic states of CH4. [ABSTRACT FROM AUTHOR]

The total absorption, photoionization, and dissociative photoionization cross sections of ammonia have been measured from 80 to 1120 Å. All possible fragment ions have been observed including doubly ionized ammonia. The absolute ionization efficiencies have also been measured in this spectral range. The appearance potentials of the fragment ions have been measured and are compared with the calculated appearance potentials derived from published heats of formation and ionization potentials of the fragments. [ABSTRACT FROM AUTHOR]

Presents a study which examined the effects of biaxial strain on the intervalence-band absorption spectra of indium gallium arsenide/indium phosohide systems. Calculation of the valence-band structure and the band structures of the layers; Discussion of momentum matrix elements; Calculaion of intervalence-band absorption coefficients.

Discusses a study on the Wigner function simulations of structures of resonant tunneling diodes with high peak-to-valley ratios. Methods and equations; Device structure and direct current simulation results; Small-signal analysis and estimated power generation.

Discusses a study which presented a boundary condition method to simulate the interaction with ideal particle reservoirs at the device boundaries. Factor that impeded the numerical solution of the time-dependent Schrödinger equation for resonant-tunneling diodes; Way in which the transient and small-signal behavior of resonant-tunneling diodes were calculated; Requirement of the open nature of the semiconductor device problem.

Presents a study which examined the typical power generating capabilities of silicon carbide impact avalanche transit time (IMPATT) diodes. Properties of silicon carbide; Operating characteristics of double-drift IMPATT devices; Device design and simulation.

Total absorption and photoionization cross sections of water vapor have been measured between 100 and 1000 Å with an accuracy of ±3%. Recent calculations are shown to be in reasonable agreement with the present data. [ABSTRACT FROM AUTHOR]

Presents a study which investigated the dependence of the performance characteristics of In[sub0.52]Al[sub0.48]As/In[sub0.53]Ga[sub0.47]As resonant tunneling diodes upon molecular-beam-epitaxial growth parameters. Parameters that affect the negative differential resistance characteristics; Illustration of the resonant tunneling diodes lattice matched to indium phosphide grown by molecular-beam epitaxy; Way of realizing the improvement of the peak-to-valley ratio.

Reports the use of a transfer matrix method to establish a formalism for determining the dispersion relations, electrostatic potentials, and Frohlich interaction Hamiltonians of the interface optical phonons. Multiple-interface heterostructure systems within the framework of the macroscopic dielectric continuum model; Facilitation of calculations for complex structures.

Presents a study which examined the effects of built-in biaxial strain on Γ-X transport in GaAs/InxAl[sub1-x]As/GaAs pseudomophic heterojunction barier structures. Experimental details; Theoretical methods; Conclusion.

Presents a numerical study of gallium-arsenic (GaAs)/aluminum-arsenic (AlAs)/gallium-arsenic (GaAs) quantum well structures. Equations used in the study; Features of a single barrier GaAs/AlAs/GaAs.

Presents a theoretical model for quantum well varactors. Comparison of quantum well varactors with Schottky barrier varactors; Characters for varactors of various structures; Advice for an accurate evaluation of the quantum mechanical current density and the dependence of the capacitance on applied voltage of a quantum device structure.

Presents a study which considered the properties of graded Al[subx]Ga[sub1-x]As heterojunction barriers using a self-consistent ensemble Monte Carlo method. Structure and transport model of heterojunction barriers; Method used in describing the structure; Equation of the graded barrier region.

Presents a study which reported a magnetohydrodynamic model of a plasma torch that describes the complete torch system from the gas injection. Theoretical studies of plasma torches; Description of the plasma torch model used; Diagram of the plasma torch; Characteristics of voltage-current and voltage-flow; Details on temperature in the plume of the plasma torch.

Deals with a study which discussed the development of an improved boundary condition treatment for the time-dependent Schrödinger equation applied to resonant-tunneling diode simulation. Features of the treatment; Model that requires the formulation of open-system boundary conditions; Problems avoided by typically time-dependent Schrödinger calculations.

Presents a study which investigated nonalloyed and alloyed low-resistance ohmic contacts with good morphology for gallium arsenide using a graded InGaAs cap layer. Ways of decreasing the contact resistance between a semiconductor device and the contact; Reason for the attractiveness of indium arsenide for good ohmic contacts to III-V compounds; Results of the transmission-line model measurements on the nonalloyed sample.

Presents information on a study which calculated the switching speeds for GaAs-AlGaAs resonant-tunneling diode structures with different barrier widths from the time-dependent Schrödinger equation. Factors that complicate the application of resonant-tunneling diode switching times in determining device switching speeds; Calculated transients of the structure.

A self-consistent quantum mechanical simulation is used to study the effect of spacer layer thickness on such resonant tunneling diode properties as the peak current and peak-to-valley current ratio. It is found that with a low cathode doping the peak current is insensitive to the commonly used spacer layer thickness. However, for higher cathode doping the peak current decreases with increasing spacer layer thickness. This phenomenon is explained on the basis of the junction potential between the heavily doped cathode contact region and the undoped double-barrier region. Thus, for device applications where a high current density is desired the cathode spacer layer should be designed as thin as possible. [ABSTRACT FROM AUTHOR]

The effect of incorporating a spatially variable effective mass in the Schrödinger equation method of resonant tunneling device modeling is investigated. It is shown that inclusion of this effect can produce an order of magnitude difference in the calculated peak current density of the static current voltage (I-V) curve for the resonant tunneling diode. Results for a particular In0.53Ga0.47As-AlAs structure show that much better agreement between theory and experiment is obtained by including this effect. Also, comparison of transient results for an In0.53Ga0.47As-In0.52Al0.48As structure shows a significant change in the diode switching transients. [ABSTRACT FROM AUTHOR]

Intrinsic bistability has been observed experimentally and attributed to the effect on the potential profile from stored charge in the quantum well through Poisson’s equation. This effect leads to two possible current states corresponding to a single voltage within the negative resistance region. In this letter a simulation method is presented which clearly shows bistability in the current-voltage curve of a resonant tunneling diode. This method self-consistently combines a Thomas–Fermi equilibrium model for the electron concentrations outside the double-barrier structure with a quantum calculation for the concentration inside the structure. [ABSTRACT FROM AUTHOR]

Measurements of normalized radial emission coefficients in a free-burning arc in argon are presented as a function of position in the arc. The variation of these emission coefficients with pressure indicates departures from local thermodynamic equilibrium at 1-atm pressure. [ABSTRACT FROM AUTHOR]