Semiconductor devices “from inside”

A recent study of secondary electron (SE) spectra in an Auger spectrometer demonstrated unique features indicative of the chemical nature of the tested material. The scanning electron microscope (SEM) naturally generates SEs; therefore, in this paper, we combine the concept of using differential voltage contrast (DVC) with SE spectroscopy to identify the chemical nature of a material. It is demonstrated that this method reveals the uniqueness of electron energy distribution in the conduction band of a solid or, what is the same, the uniqueness of a build-up of the outer electron shell system, and avoids errors due to the changes in the angular distribution or yield of the SE in the SEM. A theory of this new microanalytical method is developed. The experimental limitations of the SEM for this type of study are examined as well.

The thermal step method is a nondestructive technique for determining electric charge distribution across solid insulating structures. It consists in measuring and analyzing a transient capacitive current due to the redistribution of influence charges when the sample is crossed by a thermal wave. This work concerns the application of the technique to inhomogeneous insulating structures. A general equation of the thermal step current appearing in such a sample is established. It is shown that this expression is close to the one corresponding to a homogeneous sample and allows using similar techniques for calculating electric field and charge distribution.

“Giant” undulations on the equatorward boundary of the evening diffuse aurora, occasionally observed during a substorm, are numerically simulated. A “giant” undulation is defined as a waveform having the crest‐to‐trough amplitude comparable to the wavelength. The two‐dimensional particle code is used for studying the motion of magnetospheric plasma perpendicular to the geomagnetic field. Based on the observational facts, it is assumed that in the auroral magnetosphere there exist azimuthally aligned sheets of enhanced density, which are called arc sheets. (Arc sheets inside the inverted‐V structure can actually be responsible for discrete arcs.) The simulation model for giant undulations assumes the presence of an arc sheet equatorward of the proton diffuse aurora. Anomalous cross‐field diffusion of the arc sheet protons leads to polarization of the arc sheet. The polarized arc sheet is such that a negative charge sheet is sandwiched between two positive charge sheets. The polarized arc sheet is subject to a Kelvin‐Helmholtz (K‐H) instability due to a velocity shear generated in the arc sheet. The K‐H waves associated with the arc sheet can spatially modulate the equatorward boundary of the proton diffuse aurora. In the actual simulation, the temporal evolution of the diffuse auroral pattern, as displayed in photographs from the satellites, can be visualized by following the cluster of the energetic protons. The outstanding indentations characteristic of observed giant undulations are successfully reproduced. There have been a few examples of the satellite measurements of electric fields, field‐aligned currents, and fluxes of precipitating particles around the undulating diffuse auroral boundaries. The distributions of electric fields and space charges obtained from the simulation are found to be consistent with these observations. Therefore, giant undulations on the equatorward diffuse auroral boundary can be identified with spatial modulation of the energetic protons by the K‐H waves arising from the polarized arc sheet.

In this work, a high-resolution microscopic gate-modulation imaging (μ-GMI) technique is successfully developed to visualize inhomogeneous charge and electric field distributions in operating organic thin-film transistors (TFTs). We conduct highly sensitive and diffraction-limit gate-modulation sensing for acquiring difference images of semiconducting channels between at gate-on and gate-off states that are biased at an alternate frequency of 15 Hz. As a result, we observe unexpectedly inhomogeneous distribution of positive and negative local gate-modulation (GM) signals at a probe photon energy of 1.85 eV in polycrystalline pentacene TFTs. Spectroscopic analyses based on a series of μ-GMI at various photon energies reveal that two distinct effects appear, simultaneously, within the polycrystalline pentacene channel layers: Negative GM signals at 1.85 eV originate from the second-derivative-like GM spectrum which is caused by the effect of charge accumulation, whereas positive GM signals originate from the first-derivative-like GM spectrum caused by the effect of leaked gate fields. Comparisons with polycrystalline morphologies indicate that grain centers are predominated by areas with high leaked gate fields due to the low charge density, whereas grain edges are predominantly high-charge-density areas with a certain spatial extension as associated with the concentrated carrier traps. Consequently, it is reasonably understood that larger grains lead to higher device mobility, but with greater inhomogeneity in charge distribution. These findings provide a clue to understand and improve device characteristics of polycrystalline TFTs.

High resolution secondary electron (SE) images can be obtained in a scanning transmission electron microscope (STEM) by collecting the type I secondary electrons generated directly by the incident electron beam. The high resolution SE signal is, in general, localized to about 1 nm in the generation process. The escape depth of the secondary electrons, however, varies from about 1 nm for most metals to about 10 nm for some insulating materials. Surface steps can be imaged by collecting high resolution secondary electrons Among other possible contrast mechanisms for SE imaging of surface steps, topographic contrast plays an important role. The topographic contrast of surface steps depends on: (1) the incident electron probe size; (2) step height to SE escape depth ratio; (3) localization of the generated SE signals and (4) the incident beam angle relative to the specimen surface.In our UHV STEM (MIDAS) instrument, both the entrance and exit surfaces of the sample can be imaged by collecting secondary electrons. For thin samples, the resolution is < lnm for SE images of entrance and exit surfaces but for thicker samples the resolution of the SE image obtained from the exit surface will be degraded by the beam broadening effects. For studying steps on thick or bulk crystal surfaces, secondary electrons emitted from the entrance surfaces are, in general, collected to form images with high contrast and high resolution. In this report, all the SE images are obtained, in MIDAS, by collecting secondary electrons emitted from the entrance surfaces of the samples. Figure 1 is a SE image of a MgO smoke crystal revealing the surface growth steps with high contrast. The parallel straight steps, along <100> directions, constitute the edges of unfinished atomic planes on the MgO {100} crystal surfaces. This image indicates that the {100} surfaces of MgO smoke crystals are flat surfaces. Figure 2 shows a SE image of a MoO3 crystal with surface growth steps spiraling from an end-on screw dislocation (the black dots in the SE image represent electron beam induced reduction products). The dark lines (indicated by D) represent up-steps and the bright lines (indicated by B) represent down-steps. It is interesting to note that the two-dimensional crystal growth around the dislocation is anisotropic, forming terraces with long edges parallel to [010] direction.

On the basis of the principle that an applied electric field will cause a change in spatial distribution of the dielectric constant produced by a small applied temperature difference due to the interaction of the gradient of the dielectric constant with the field, the electric field distributions along the interface between a dielectric fluid and a dielectric solid can be measured using a Schlieren technique. Some experimental results for the interfaces between n-hexane and a glass plate surface, and between n-hexane and a semiconducting layer surface obtained with this technique show that the space charge is the dominant factor controlling the field distributions. The other factors, such as the dielectric constants and resistivities of the two materials forming the interface, would play an important role in determining the field distributions if and only if the space charge effect is negligibly small or can be ignored. This technique can be used to study electric field and space charge distributions along fluid-solid dielectric interfaces for practical insulation systems under both steady-state and transient conditions.

SummaryTurn-off semiconductor devices have vastly promoted modern power electronics during the last decade. Included are bipolar power transistors. Field Effect Transistors (MOSFETs), Gate Tun-off Thyristors (GTOs) and Insulated Gate Bipolar Transistors (IGBTs). Additional elements are Static Induction Transistors and Thyristors (SITs and SIThs) as well as MOS Controlled Thyristors (MCTs) for higher frequency applications.The prensent status of semiconductor power devices is discussed including switching behavior, losses, gate driver, snubber, overcurrent and protection. The standard control is nowadays pulse width modulation, Resonant circuits are growing in importance because they allow for higher frequency, lower storage components and transformers and reduction of switching losses in semiconductor devices, Main field of application are discussed and future trends given.

Detailed knowledge of the electric charge or field distribution in the bulk of dielectrics is necessary for the understanding of charge-storage and charge-transport phenomena. The most successful techniques for the experimental determination of spatial charge or field distributions in dielectrics, are based on the transit of a compression wave through the sample [1-6]. In the present study, the basic equations for pressure-pulse and pressure-step experiments are derived from first principles. The pressure dependence of the relative permittivity e is investigated, and the influence of this dependence on the signal amplitude is calculated for nonpolar and for polar dielectrics. The response equations for pressure-step, pressure-pulse and arbitrary pressure-profile experiments are calculated.

The generation of double resonance in a nanostructure, thus permitting the modulation of optical field at two frequencies simultaneously, offers new application opportunities for surface enhanced Raman scattering (SERS) and surface enhanced fluorescence (SEF). Here, we present a simple composite nanostructure of silver nanodecahedron (Ag ND)/silica spacer/gold film/glass substrate for achieving double resonance under the normal incidence of polarized light. The optical responses of the composite structure have been theoretically studied by varying the thickness of silica spacer layer from 5 nm to 35 nm for mediating the interaction between Ag ND and gold film. Results indicate that the extinction spectrum of the composite system is strongly dependent on the separation between Ag ND and gold film. The electric field and charge distribution during resonance have been investigated in order to obtain a detailed understanding on the coupling between these two objects. More importantly, due to the anisotropic geometry of Ag ND, double resonance with two plasmonic modes (dipole and gap modes) whose responses can be adjusted through varying the size of Ag ND and mediating its coupling with the gold film respectively, has been achieved in the composite structure under the excitation with polarization parallel to the Ag ND edge adjacent to the spacer surface. The knowledge gained through this work will benefit the development of applications based on local field enhancement.

We present a self-consistent analytical model describing transport processes in quantum well infrared photodetectors (QWIPs) under dark conditions. The model takes into account electron thermionic emission from the quantum wells, thermionic injection from the emitter contact, and features of transport and capture in the self-consistent electric field in the QWIP active region. Using an assumption that the rates of the electron escape from and capture into a QW are functions of the electric fields only in the barriers sandwiching this QW, we calculate the electric field and charge distributions as well as dark current–voltage characteristics. We clarify the origin of steep dark current characteristics. It is confirmed that the effect of the emitter contact substantially weakens with increasing number of QWs in the QWIP structure.

The dielectric barrier discharge (DBD) in the glow regime in neon has been investigated by experiment and two-dimensional (2D) fluid modeling. The discharge was carried out in a planar DBD system with segmented-electrodes driven by square-wave voltage. The results show that the glow DBD originates in the center of the electrode and expands outward to the electrode edge during each half cycle of the voltage, forming a radial structure. The discharge decays firstly in the inner area but sustains longer in the edge area, showing a reversed discharge area. The discharge cannot completely cover the entire electrode surface, but remains a border of non- or weak discharge. The fluid modeling shows a similar result in agreement with the experiments. The simulations indicate that the electric field in the edge area is distorted due to the boundary effect so that the electric field and charge distribution are different from that in the inner part. The distorted field reduces the longitudinal component near the edge and causes the local field to be lower than that in the center, and hence makes the discharge behindhand. It also induces a transverse field that makes the discharge extend radially outward to the edge. The boundary effect plays an important role in the glow DBD structure.

Room temperature contactless electroreflectance technique was used to investigate the built‐in electric field in AlGaN/GaN heterostructure grown by metaloorganic chemical vapour deposition on [0001] 4H‐SiC substrate. Two heterostructures have been studied: one grown with a misorientation angle (8 degrees toward [11‐20] crystallographic direction of the substrate) and second without misorientation angle. For both heterostructures strong AlGaN‐related resonance with Franz‐Keldysh oscillation (FKO) has been observed. The built‐in electric field in the AlGaN layer was extracted from the FKO period [0.26 MV/cm and 0.47 MV/cm for samples without and with misorientation, respectively]. It has been concluded that the substrate orientation angle has a great impact on the built‐in electric field and charge distribution across the AlGaN/GaN structure. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal equilibration of soot charge distributions by coagulation

Interdigital transducers are systems of conducting strips on a piezoelectric halfspace; in some systems, the groups of strips repeat periodically over a transducer counting hundreds of strips that can be considered infinite. An infinite planar periodic system of groups of strips with adequate period, sufficiently large if necessary, is considered to model an electric field in the plane of strips. There are electric charge distributions on strips either resulting from an external voltage source applied to the strips or induced by a propagating surface acoustic wave in the piezoelectric substrate. The latter case is equivalent, in the electrostatic approximation applied in this paper, to strips embedded in an already existing, spatially variable electric field. A method is presented for evaluation of the spatial spectrum of charge distribution that is responsible for either the surface wave generation or the scattering by strips in weak piezoelectrics. It overcomes certain numerical difficulties arising when Fourier transform is computed from the already evaluated numerical representation of the square-root singular charge distribution at the strip edges.

Based on relativistic field equations of superconducting state, the structure and forming mechanism of interface layer of superconductor are studied for the condition of weak magnetic field. It is shown that electric field, potential difference, Hall effect and sandwich of charge density exist in this layer,which originate from the measurable effect of potential field limited by relativistic covariance. In order to verify the result of this paper, a design of experiment is given.