Width Of Space Charge Layer Research Articles

Effects of dopant segregation on lattice-diffusional creep of nanocrystalline ceramics

The segregation of dopants whose electric charge is different from that of the parent ions and the formation of a space-charge layer induce a local electric field in the grain-boundary region. Depending upon the valence and local electrostatic potential, the charge carriers controlling diffusional creep can be either accumulated or depleted in this space-charge layer, and the creep rate is enhanced or diminished accordingly. A model was recently developed to examine the effects of a segregation-induced local electric field on the lattice-diffusional creep of nanocrystalline ceramics for spherical grains. However, in order to obtain closed-form solutions, the grain size was assumed to be much greater than the width of the space-charge layer in the existing analysis. This assumption can become inappropriate for nanocrystalline materials as the grain size is reduced; thus, a numerical method is used in the present study to resolve the existing equations without that assumption. Using yttria tetragonal zirconia as an example, the difference between the existing approximate closed-form solutions and the present numerical results is shown to be significant when the grain size is less than 50 nm.

Raman scattering from LO–phonon–plasmon coupled modes in Ag-coated GaN nanocrystals

Relationships are studied governing the Raman scattering spectra in GaN nanocrystals grown by HVPE on substrates of oxidized silicon with Ag atoms deposited in a UHV system. It is found that the intensity of an LO–phonon–plasmon coupled mode in GaN nanocrystals is increasing initially as metal atoms are deposited on their surface to a thickness of 3 nm and then decreasing. It is suggested that the intensity increase of this mode is due to increasing near-surface barrier height and the space-charge layer width.

Interface electronic structure of organic semiconductors with controlled doping levels

Abstract We investigate the properties of inorganic–organic interfaces by ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) and transport experiments. In particular, we study the interface between inorganic conductive substrates and organic layers that are intentionally p-type doped by co-evaporation of a matrix material and acceptor molecules. The photoemission spectra clearly show that the Fermi levels shift due to the doping and that the space charge layer width changes with doping (high doping – small width). The changes in the electronic structure of the interface due to doping agree well with results of transport experiments.

Some properties of Au/n-CdTe Schottky barriers as established by I–V characteristics and photocurrent spectroscopy

A study of the electric and photoelectric behaviour of electrodeposited Au/n-CdTe Schottky diodes in a wide temperature range is presented. The dependence of quantum efficiency on photon energy, temperature, and bias is used to determine the Schottky barrier height, the built-in voltage, the space charge layer width, the field strength and the bulk carrier concentration. The built-in voltage is strongly temperature dependent. At temperatures above 100 K there is no surface inversion. Several arguments indicate the existence of interface states at the Au/CdTe interface. All information is combined to establish the band diagram of the structure and to determine parameters relevant for solar-cell applications.

Detailed temperature dependence of the space charge layer width at grain boundaries in acceptor-doped SrTiO 3-ceramics

The influence of temperature and the donor-type grain boundary (GB) states on the width of the space charge layer d GB at GBs in SrTiO 3 ceramics was investigated by comparing numerical simulation results with experimental data. According to recent results for the potential barrier height at the GB, the temperature behaviour of d GB can be divided into two different regimes. For lower temperatures, d GB is independent of the temperature, for higher temperatures it decreases with rising temperature due to a change in the occupation of the electronic GB donor states. By decorating the GB with suitable dopants, the space charge layer width and its temperature dependence can be influenced. This is due to a change of the effective GB charge.

The mechanism of positive streamer acceleration and expansion in air in a strong external field

Positive streamers in atmospheric pressure air in strong uniform external fields (40, 50 and 60 kV ) are simulated within the scope of the diffusion-drift model. Photo-ionization was taken into account. A simple model which describes streamer acceleration and expansion is proposed on the basis of the simulation results. It is shown that photo-ionization is essential only during the early stage of streamer formation. In strong external field further streamer evolution is governed by collisional multiplication of seed photoelectrons in a gap. The detailed structure of the streamer head is presented. It is shown that streamer advancement occurs due to avalanche ionization in a thin ionization domain (ID) with a width of the order of the space-charge layer's width. The ID can be considered as a region where some initial level of electron density is rapidly converted to the density in the streamer channel . This allows one to assume that the shape of the streamer head coincides with the instant position of the contour line on the seed electron density surface. This assumption leads to a simple relation for the streamer velocity which is consistent with the simulation results.

Internal field emission at metal/diamond contact and performance of thin film field emitters: computer simulation

Abstract Emission characteristics of metal/diamond (insulating or donor-doped) contact field emitters have been calculated. In the calculation, Fowler-Nordheim type electron tunneling including Schottky effect and continuity of current, when needed, are taken into account. When the diamond layer is insulating or of low donor concentration, the emission performance is not satisfactorily improved in comparison with conventional metal field emitters, because the unfavorable reduction ( 1 5.7 ) in the interfacial electric field due to the dielectric nature of diamond is adverse to the favorable effect of possible decrease in barrier height (typically from 5 to 2 eV). However, when the donor density is sufficiently high, the space-charge effect in the donor-doped diamond layer can enhance the interfacial electric field leading to a high emission current. The thickness of the diamond film in this case is preferable to be close to the space-charge layer width. Too much reduction of the diamond layer thickness may lose the advantage of utilizing space-charge effect of ionized donors. The possibility of diamond metal/insulator/metal (MIM) field emitters with negative work function at the face metal is also discussed in comparison with conventional MIM vacuum emitters proposed so far.

Application of the Gartner model to elucidate parameters adversely affecting photoactivity of thin film PbO in [formula omitted] electrolyte

A photoactive α-PbO phase is synthesized by potentiodynamic anodization of lead (in alkaline medium) in the potential region of −0.75 V to 1.25 V (vs Hg HgO ). The oxide electrode formed after 18 min of anodization shows an energy conversion efficiency of 0.9% with an open circuit voltage of 800 mV and short circuit current of 2.2 mA cm −2. Mott-Schottky measurement shows that the flat band potential of this photoanode is −0.54 V (vs Hg HgO ) with a donor concentration of 1.14 × 10 16. The influence of variation in space charge layer width, diffusion length and absorption coefficient on the photoactivity of PbO is studied using the Gartner model. It is concluded that low conversion efficiency of the PbO film is due to formation of a small depletion region (3680 Å) and a large diffusion region (2443 Å) as compared to a very large 1/α value (14084 Å) at 500 nm.

Theoretical and practical investigation of the thermal generation in gate controlled diodes

The different components of thermal generation in a gate controlled diode are studied theoretically and experimentally. Expressions for the generation current in the space charge layer, the diffusion current from the quasi-neutral bulk and the surface generation current are derived for a gated-diode. The width of the generation zone within the space charge layer is calculated as a function of the energy level of the trap and the diode reverse voltage. This leads to a characteristic of the leakage current as a function of the space charge layer width. It is pointed out that the diffusion current can influence the leakage current and cannot be neglected in structures with a low dark current. In the second part the gate controlled diode is used to characterize the thermal generation in structures with a homogeneous and low dark current. A generation lifetime of 5.5 msec and a surface generation velocity at a depleted surface of 1.5 cm/sec is derived. The generation lifetime is found to be constant as a function of depth into the substrate. A considerable diffusion current is measured which is comparable to the generation current in the space charge layer.

Simulation of electrical characteristics and properties of defect clusters in irradiated semiconductors I. Approximation of electric charge density

A simulation is made of electrical characteristics of defect clusters (DC) in semiconductors. Approximations of the functional dependence of the potential barrier height φ0 on the doping level of the original semiconductor are obtained. The field of their applications depending on the energy position of the level and the defect concentrations in the core is given. The specific character of the φ0 dependence on the defect concentration in the core for various doping levels of the original semiconductor is shown. The functional dependences of the initial charge-carrier removal rate RT ∼ n, the DC size r2 ∼ n and the space-charge layer width L ∼ n on the origin semiconductor doping level are obtained. [Russian Text Ignored].

Optimum transit angles of millimeter-wave Si IMPATT diodes

Optimum transit angles for maximum output power in the millimeter-wave IMPATT diodes are calculated analytically by considering the carrier diffusion process through the space-charge region. It is found that the optimum transit angle decreases, as the space-charge layer width reduces. The theoretical results show satisfactory agreement with the previously published experimental results for Si p+-n-n+abrupt junction diodes.

Experimental verification of inhomogeneous field distribution in negatively bevelled high-voltage p-n junctions by means of photomultiplication

Laser-beam-excited photocurrent measurements of the space-charge layer width close to the surface of a negatively bevelled diffused p-n junction revealed the existence of a well defined region of carrier multiplication at high applied voltages. The region is located within the space-charge layer on the highly doped side of the junction and close to the surface. The laser-light wavelength was 6328 Å and the beam diameter was about 5 μm. The experimental results are consistent with the theoretically predicted existence of a field maximum in this region of negatively bevelled p-n junctions.

The built-in voltage and space charge layer capacitance of p− n junctions

The accuracy of the abrupt space charge edge (ASCE) approximation is studied. It is shown that the ASCE approximation is useful for exact capacitance calculations if the built-in voltage is assumed to be dependent on the space charge layer width. The classical formula C = ϵ w is not valid in this case. The new, general set of equations for capacitance calculation is derived. It can be used for an arbitrary impurity profile, and remains valid at forward bias and at any temperature. Analytical relationships for linearly graded junctions are presented. The capacitance is nearly a cube-root function of the applied voltage, as predicted by Shockley, but the value of the C −3 vs U intercept with U-axis is about 14kT 3q lower than the value obtained from the classical theory. The set of equations described in the paper can be easily solved numerically for an arbitrary impurity profile. The results of calculations for real diffused junctions are presented and compared with experiment. Some of the discrepancies between theoretical and experimental results, recently reported by Van Overstraeten and Nuyts, are explained.

A fast algorithm for the calculation of junction capacitance and its application for impurity profile determination

A fast algorithm is described which calculates the space charge layer width and junction capacitance for an arbitrary impurity profile and for plane, cylindrical and spherical junctions. The algorithm is based on the abrupt space charge edge (ASCE) approximation. A method to use the algorithm for the determination of impurity profiles for two-sided junctions is presented. An expression is derived for the built-in voltage to be used for capacitance calculations with the ASCE approximation. Experimental evidence is given that the algorithm permits very accurate capacitance calculations and also predicts the exact temperature dependence of the junction capacitance.

Zener and avalanche breakdown in silicon alloyed p- n junctions—II : Effect of temperature on the reverse characteristics and criteria for distinguishing between the two breakdown mechanisms

The effect of temperature on the reverse current and breakdown voltage of silicon p- n junctions for a wide range of breakdown voltages has been investigated and the criteria for differentiating the Zener breakdown from avalanche breakdown have been presented. Narrow p- n junctions with soft characteristics have a temperature insensitive reverse current, contributed by internal field emission and show a negative temperature coefficient β of breakdown voltage which is determined up to large extent by the available phonon density present in tunnelling transitions. Wider junctions with V B > 13 V show a highly temperature sensitive current and a positive value of β, which is in good agreement with the avalanche multiplication theory. For values of V B from 3.5 to 13 V there is a transition region from Zener to avalanche type of breakdown in which the temperature dependence of the reverse characteristics is complex and cannot be accounted by a single mechanism. Apart from the temperature dependence of breakdown voltage such properties as generation of microplasma noise, dependence of breakdown voltage on impurity concentration, space-charge layer width and maximum field strength at breakdown have been found to show distinct differences between Zener and avalanche regions and give a further support to the conclusions reached from the temperature studies.

Space-Charge Layer Width and Junction Capacitance of a Hyper-Abrupt p-n Junction

The space-charge layer width and the junction capacitance of a hyper-abrupt p - n junction were studied, especially on the characteristics of temperature-dependence. The solution of Shockley's equation in space-charge case gives a fairly good approximation of the characteristics of such a junction. The temperature dependence of the space-charge layer width can be explained mainly by the change in the built-in barrier voltage. However, there still remains a tendency that, at higher temperature, the space-charge layer width is narrower than that of the expected magnitude.

The Calculation of Space-charge Layer Widths, Maximum Field and Junction Capacitance of p-n Junctions with Arbitrary Impurity Profiles†

ABSTRACT A semi-graphical method of space-charge layer width computation is described which enables the determination of space-charge layer penetrations to both sides of the metallurgical junction, and of the maximum junction field, as a function of potential difference to be carried out for junctions of any profile. Illustrative examples comprising step and linearly graded junctions, exponential, errer function and ‘ step-error function’ profiles are given.

Space-charge layer width in diffused junctions

This paper outlines a calculation of space-charge layer width in a planar junction made by diffusing an n or p impurity (assumed to follow a Gaussian or a complementary error function distribution) into a uniformly doped crystal of opposite conductivity type. The collector junction of most drift transistors conforms closely to this model. An exponential approximation to the impurity distribution permits curves to be drawn of the space-charge layer penetration in each direction from the junction as a function of applied reverse voltage, and of the electric field distribution. The quantities involved are normalized in terms of the initial doping level N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> , the impurity diffusion length <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L = 2 \sqrt{Dt}</tex> , and the junction depth x <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</inf> . The curves should be useful in calculating depletion-layer capacitance, transistor punch-through voltage and junction breakdown voltage.