Comparitive Study of Coaxial Bragg Resonators

A coaxial Bragg resonator can be performed in four kinds of structures: it is constructed by inserting a regular waveguide between two selective Bragg reflectors (being called the upstream reflector and downstream reflector, respectively), or it consists of only two Bragg reflectors without regular waveguide section between them, where connection of these two reflectors is smooth with phase difference Δ??=?0 or is step-changed with corrugation phase difference Δ? = +π or ?π, respectively. The present paper is devoted to a comparative study for the response of reflectivity and quality factor on frequency in these four kinds of coaxial Bragg resonators, including the influences of the location and corrugation-phase difference of the step-changed connection of the two reflectors. Results show that a coaxial Bragg resonator possesses most attractive properties when the corrugation-phase difference is Δ? = +π at the point of connection and the upstream reflector is longer than the downstream reflector.     

Transistor noise theory in the Middlebrook limit of high injection

Transistor noise in p-n-p transistors at high injection is calculated for a one-dimensional model for arbitrary values of p₀(w_B)/p₀(0), where p₀(0) is the hole concentration on the emitter side of the base region and p₀(w_B) the hole concentration on the collector side of the base region, and it is found that additional terms occur in the noise expressions. The results are then applied to the low-frequency case. The equivalent circuit of the transistor is calculated for the Middlebrook limit, where p₀(w_B)/p₀(0) attains a limiting value, and it is found that this circuit is greatly simplified in comparison with the case p₀(w_B)/p₀(0) < 1. Finally the correlation impedance Z_cor of the transistor noise is calculated at relatively low frequencies and it is found that the theory cannot explain the large values of Z_cor observed by Tong and van der Ziel.     

On completeness and multi-channel sampling

A bandlimited input x is applied as a common input to m linear time-invariant filters (channels) which are ‘independent’ in a certain sense. We investigate, from a unified point of view using the concept of completeness, the problem of uniquely reconstructing the input x for all time values from samples of the m outputs, each output being sampled at the uniform rate of σ/gqm samples/second, where σ is the positive frequency bandwidth of the input signal.For the lowpass input case and independent channels, perfect reconstruction is always possible; similarly for the case of a bandpass input and an even number of channels, recovery of the input can also be accomplished at a rate of σ/mπ samples/second. However, for an odd number m of channels and a bandpass input, it is shown that the rate σ/mπ samples/second at the outputs no longer suffices to determine the input uniquely unless 2ω₀m/σ is an odd integer, a constraint on the relation between the center frequency of the band, ω₀, and the bandwidth σ. To obtain a sampling theorem when the number of channels is odd and in the absence of such a positioning constraint, a higher sampling rate per channel must be employed.In those cases which permit a unique determination of the input from samples of the output channels, it is shown that a linear scheme involving m linear time-invariant post-filters can be used to effect the input reconstruction.     

Simulation of optical properties of silicon solar cells textured with penetrating <Emphasis Type="Italic">V</Emphasis>-shaped grooves

The coefficients of reflection (R), transmission (T), and absorption (A) of light for two wavelengths λ = 1000 and 1100 nm for silicon wafers that have thicknesses t = 50, 100, and 200 μm and are textured with penetrating V-shaped grooves with various geometries have been calculated; the half-width of groove’s base w (10, 20, and 30 μm) and the depth of the groove d (0 ≤ d ≤ t) have been varied. In the case of an increase in the aspect ratio d/w (in the case of λ = 1100 nm), the absorption curve A(d/w) monotonically ascends from 6.6 to 67.6%, whereas, for λ = 1000 nm, a nontrivial dependence A(d/w) is observed: the absorption coefficient first increases to 54%, attains then a maximum of 97% at d/w = 3, and then decreases at d > t/2 for all values of w. This effect of a decrease in absorption with an increase in d/w distinguishes texturing with penetrating grooves from conventional surface texturing. Distributions of angles of deviations of photons in the plane of bottoms of grooves are obtained; these distributions are represented by a set of δ-type functions.     

Effects of soft segment mixtures with different molecular weight on the properties and reliability of UV curable adhesives for electrodes protection of plasma display panel (PDP)

A series of urethane acrylate oligomers has been synthesized from isophorone diisocyanate (IPDI), mixture of soft segments with different M_w, i.e., poly(ethylene-co-1,2-butylene)diol (PEBD, M_w = 2400) and 2-butyl-2-ethyl-1,3-propanediol (BEPD, M_w = 160) and 2-hydroxyethyl methacrylate (HEMA) using dibutyl tin dilaurate (DBTL) as a catalyst. Subsequently, the UV curable adhesives were fabricated by mixing the prepared oligomers, reactive diluents (Isobonyl acrylate and 4-hydroxybutyl acrylate) and photo initiator (1-hydroxy-cyclohexyl-phenyl-ketone). The physical and mechanical properties of UV cured adhesives such as coefficient of thermal expansion (CTE), elastic modulus (E′), glass transition temperature (T_g), adhesion strength and water vapor transmission rate (WVTR) were measured. And electrochemical migration of Ag electrodes, which brings on the degradation of surface insulation resistance, was also investigated by monitoring insulation resistance during 100 h of 50 °C, 90%RH storage test.     

The profile of temperature and voltage dependent series resistance and the interface states in (Ni/Au)/Al0.3Ga0.7N/AlN/GaN heterostructures

The temperature dependence of capacitance-voltage (C-V) and the conductance-voltage (G/w-V) characteristics of (Ni/Au)/Al_0.3Ga_0.7N/AlN/GaN heterostructures were investigated by considering the effect of series resistance (R_s) and interface states N_ss in a wide temperature range (79-395 K). Our experimental results show that both R_s and N_ss were found to be strongly functional with temperature and bias voltage. Therefore, they affect the (C-V) and (G/w-V) characteristics. The values of capacitance give two peaks at high temperatures, and a crossing at a certain bias voltage point (∼3.5 V). The first capacitance peaks are located in the forward bias region (∼0.1 V) at a low temperature. However, from 295 K the second capacitance peaks appear and then shift towards the reverse bias region that is located at ∼−4.5 V with increasing temperature. Such behavior, as demonstrated by these anomalous peaks, can be attributed to the thermal restructuring and reordering of the interface states. The capacitance (C_m) and conductance (G/w-V) values that were measured under both reverse and forward bias were corrected for the effect of series resistance in order to obtain the real diode capacitance and conductance. The density of N_ss, depending on the temperature, was determined from the (C-V) and (G/w-V) data using the Hill-Coleman Method.     

Suppression of Residual Side-lobes in a Coaxial Bragg Reflector

This note demonstrates the existence and possible suppression of the residual side-lobes in a coaxial Bragg reflector by employing Blackman window distribution, no matter if the phase difference between the outer and inner corrugations is 0, or π/2, or π. Physical explanation is qualitatively given to the effect of the window distribution.     

Influence of spatial arrangements of π-spacer and acceptor of phenothiazine based dyes on the performance of dye-sensitized solar cells

Three phenothiazine based organic dyes PTA, PDTA and PTDA with D–π–A, π–D–π–A and A–π–D–π–A frameworks were designed and synthesized for the dye sensitized solar cells (DSSCs). Phenothiazine with octyloxyphenyl moiety acts as donor while thiophene and cyanoacetic acid units act as a π-spacer and an acceptor, respectively. The effects of the molecular structures of the dyes on the performance of the DSSCs were investigated systematically along with their photophysical and photoelectrochemical properties. The dye PTDA with A–π–D–π–A framework exhibited a better light harvesting capacity and an effective electron extraction pathway from the electron donor to the TiO₂ surface, leading to an efficiency of 6.82% under 100 mW cm⁻² light illumination, while the dyes PTA and PDTA with D–π–A and π–D–π–A frameworks delivered efficiencies of 6.34% and 5.12%, respectively.     

Two-electron centers with negative correlation energy in Pb1−x SnxSe solid solutions

In Pb_1?x?y Sn_xNa_ySe solid solutions, the chemical potential F lies lower than the valence-band top E _v , and the dependences F(T) are linear, with the extrapolation to T = 0 K yielding a value E _V ? F ₀ = (210 ± 10) meV for solid solutions containing 0.3 at % of sodium and (250 ± 10) meV for those with 0.5 at % of sodium (F ₀ is the position of the chemical potential at T = 0 K). The dependences of F ₀ on the content of sodium are also linear, with the extrapolation to zero content yielding a value F ₀ = 162 ± 10 meV for solid solutions containing 2 at % of tin. The correlation energy U of two-electron tin centers in PbSe is independent of the content of tin and sodium and equal to U = (?65 ± 10) meV.     

Approximating linearizations for nonlinear systems

Given a nonlinear control system $$\dot x(t) = f(x(t)) + \sum\limits_{i = 1}^m {u_i (t)g_i (x(t))}$$ on ? ⁿ and a pointx ₀ in ? ⁿ , we want to approximate the system nearx ₀ by a linear system. Of course, one approach is to use the usual Taylor series linearization. However, the controllability properties of both the nonlinear and linear systems depend on certain Lie brackets of the vector field under consideration. This suggests that we should construct a linear approximation based on Lie bracket matching atx ₀. In general, the linearizations based on the Taylor method and the Lie bracket approach are different. However, under certain mild assumptions, we show that there is a coordinate system for ? ⁿ nearx ₀ in which these two types of linearizations agree. We indicate the importance of this agreement by examining the time responses of the nonlinear system and its linear approximation and comparing the lower-order kernels in Volterra expansions of each.     

Maximum energy window with constrained spectral values

The optimum energy criterion continuous and discrete prolate spheroidal wave function windows had a profound effect on the basic understanding and practical optimization of many problems in communication theory, signal processing, and antenna theory. However, in many situations, there is interest in not only packing maximum window energy in some frequency interval, but also interest in imposing precise spectral window values and spectral nulls over some specified frequencies. We have formulated and solved such an optimization problem in an analytical sense as well as in an efficient computational sense. The problem is first expressed as a constrained maximization of a normalized quadratic form w′Aw/w′w, where A is a positive-definite matrix specified by the window energy concentration interval, w is the window weighting vector, and the constraining subspace is specified by the spectral window values and locations. This problem is then transformed to a nonconstrained maximization of a normalized quadratic form w′ PAPw/w′w, where P is a projection operator onto the orthogonal complement of the original constraining subspace. Persymmetric properties of A and PAP are used to reduce the computational complexity of the solution of the optimum window. Numerical maximization can be readily performed using the iterative power method. Specific examples are presented.     

An Effective Approach to Finding Differentiator Window Functions Based on Sinc Sum Function

An approach to finding digital differentiator window functions is studied. The frequency response of the truncated ideal differentiator is expressed by two parts. One is the ideal frequency response and the other is the deviation on the interval from ω=0 to ω=π. The deviation expression is the sum of weighted functions, where the general expression of these functions is equal to the half-sum of a pair of sinc sum functions plus π, and each weight is a window constant. Using the properties of the sinc sum function eight properties of the general expression and six properties of the deviation expression are deduced. By these properties both the relative errors of the passband and the change of their ripples can be small if each weight is proper and the truncated ideal differentiator is ideal at ω=0. From the expression of the deviation a matrix equation with window constants as unknowns can be written. Examples are given about how to write the matrix equations and how to find the optimized window constants. Four new differentiator windows as a family are obtained. These windows belong to the fixed window. Different from existing windows, the new windows are optimized in terms of reducing the relative errors of the passband. Comparisons show that new windows are better or much better than the Hanning, Hamming, Blackman, Kaiser, Chebyshev and polynomial windows in terms of differentiator performances.     

On the profile of temperature dependent series resistance in Al/Si3N4/p-Si (MIS) Schottky diodes

The temperature dependence of capacitance-voltage (C-V) and conductance-voltage (G/w-V) characteristics of metal-insulator-semiconductor (Al/Si₃N₄/p-Si) Schottky barrier diodes (SBDs) was investigated by considering series resistance effect in the temperature range of 80-300 K. It is found that in the presence of series resistance, the forward bias C-V plots exhibit a peak, and experimentally show that the peak positions with a maximum at 260 K shift toward lower voltages with increasing temperature. The C-V and (G/w-V) characteristics confirm that the interface state density (N_ss) and series resistance (R_s) of the diode are important parameters that strongly influence the electric parameters of MIS structures. The crossing of the G/w-V curves appears as an abnormality compared to the conventional behavior of ideal Schottky diode. It is thought that the presence of series resistance keeps this intersection hidden and unobservable in homogeneous Schottky diodes, but it appears in the case of inhomogeneous Schottky diode. In addition, the high frequency (C_m) and conductance (G_m/w) values measured under both reverse and forward bias were corrected for the effect of series resistance to obtain the real diode capacitance.     

Some extremal properties of Daubechies filters and other orthonormal filters

New extremal properties of Daubechies 4-tap orthonormal filters are given: they maximize a certain functional, have the largest gain in (0,π/2), and allow maximum energy compaction in [0,π/2]. These properties do not carry over to Daubechies filters of arbitrary length. They complement what is known about Daubechies filters and highlight the specific role of the 4-tap filter. Moreover, we demonstrate that these properties cannot be fulfilled by any other orthonormal lowpass filter, regardless of its length.     

Low-LUMO 56π-electron fullerene acceptors bearing electron-withdrawing cyano groups for small-molecule organic solar cells

56π-Electron fullerene derivatives with electron-withdrawing cyano groups were synthesized by the reaction of 58π-electron fullerenes with NaCN, followed by in situ treatment with TsCN or MeOTf. The fullerene derivatives have low-lying LUMO levels, which are comparable with or lower than that of pristine C₆₀. They were used as solution-processable electron acceptors in small-molecule organic solar cells that showed a maximum power conversion efficiency of 2.0% (J_SC = 7.05 mA/cm², V_OC = 0.62 V and FF = 0.45) using chloroindium phthalocyanine as an electron donor.     

A test for lateral nonuniformities in MOS devices using only capacitance curves

Capacitance vs voltage (C-V) curves of metal-oxide-semiconductor (MOS) devices are affected both by interface states and by lateral nonuniformities (such as barrier-height or fixed-charge variations across planes parallel to the interface plane). If the doping profile of the device is known, then it is shown that from C-V curves alone one can distinguish between nonuniformity effects and interface state effects. Two ways to make this distinction are described and tried experimentally. One method uses the quasi-static C-V curve to determine the interface-potential, ψ, and the high-frequency C-V curve to determine the depletion width, w. Then this experimental w-ψ relation is compared with the w-ψ relation expected for the known doping profile. Any discrepancy can be due only to nonuniformities. The second (preferred) approach is to use the quasi-static and high-frequency C-V curve to determine an apparent doping profile corrected for interface-states. This apparent profile is then compared with the known profile. Again, only nonuniformities can cause the two profiles to differ.Our method of detecting nonuniformities is also useful in assessing whether it is the interface state density or the interface uniformity which is altered during a given fabrication or aging process performed upon an MOS device. If a quasi-static and a high-frequency C-V curve are measured both before and after the process under study, then “before” and “after” apparent doping profiles (or w-ψ relations) can be compared. Any difference between the “before” and “after” profiles can be interpreted in terms of an effect of the process upon device uniformity.Experimental measurements on devices known to be nonuniform show the method to be viable at least for gross nonuniformities.     

Modular realization of multi-dimensional filters

A method is proposed for the exact decomposition of a general multi-dimensional (m-d) rational transfer function in terms of order one, each one of which is a function of only one of the m variables. This method is used for the realization of general, linear m-d filters, with great modularity, and high parallelism. By using known nonsingular matrices, the coefficients of the decomposed filter can be expressed in terms of the coefficients of the given transfer function and the elements of these matrices. An algorithm for the determination of these coefficients is given and a class of matrices is proposed which leads to simple realizations.     

On the temperature dependence of series resistance and interface states in Al/SiO2/p-Si (MIS) Schottky diodes

The capacitance-voltage-temperature (C-V-T) and conductance-voltage-temperature (G/w-V-T) characteristics of metal-semiconductor (Al/p-Si) Schottky diodes with thermal growth interfacial layer were investigated by considering series resistance effect in the wide temperature range (80-400 K). It is found that in the presence of series resistance, the forward bias C-V plots exhibit a peak, and experimentally shows that the peak positions shift towards higher positive voltages with increasing temperature, and the peak value of the capacitance has a maximum at 80 K. The C-V and (G/w-V) characteristics confirm that the N_ss and R_s of the diode are important parameters that strongly influence the electric parameters in (Al/SiO₂/p-Si) MIS Schottky diodes. The crossing of the G/w-V curves appears as an abnormality when seen with respect to the conventional behaviour of the ideal MS or MIS Schottky diode. It is thought that the presence of a series resistance keeps this intersection hidden and unobservable in homogeneous Schottky diodes, but it appears in the case of inhomogeneous Schottky diode. In addition, the high frequency (C_m) and conductance (G_m/w) values measured under both reverse and forward bias were corrected for the effect of series resistance to obtain the real diode capacitance.     

A general factorization method for multivariable polynomials

The problem of factorizing a multivariable or multidimensional (m-D) polynomialf (z ₁,z ₂, ...,z _m ), with real or complex coefficients and independent variables, into a number of m-D polynomial factors that may involve any independent variable or combination of them is considered. The only restriction imposed is that all factors should be linear in one and the same variable (sayz ₁). This type of factorization is very near to the most general type: $$\begin{gathered} f(z_1 ,z_2 , \ldots ,z_m ) = \prod\limits_{i = 1}^{N_1 } {\lbrack {\mathop \sum \limits_{\epsilon _1 = 0}^{\epsilon _{i,1} } \cdots \mathop \sum \limits_{\epsilon _m = 0}^{\epsilon _{i,m} } a_{i;\epsilon _1 ,\epsilon _2 , \ldots ,\epsilon _m } z_1^{\epsilon _1 } \cdots z_m^{\epsilon _m } + c_i } \rbrack} \hfill \\ (\epsilon _1 , \ldots ,\epsilon _m ) \neq (0, \ldots ,0) \hfill \\ \end{gathered}$$ and appears to be the most general type available. The method is first briefly sketched for the convenience of the reader, and then is presented in detailed form through a number of theorems. These theorems provide a clear algorithmic way for the factorization, which may be automated via a suitable computer code. The factorization of m-D polynomials simplifies the stability analysis and the realization of m-D systems, as well as the solution of distributed parameters systems.     

A new closed form method for design of variable bandwidth linear phase FIR filter using different polynomials

In this paper, a new method for the design of variable bandwidth linear-phase finite impulse response (FIR) filters using different polynomials such as shifted Chebyshev polynomials, Bernstein polynomials and shifted Legendre polynomials is proposed. For this purpose, the transfer function of a variable bandwidth filter, which is a linear combination of fixed-coefficient linear-phase filters and the above polynomials are separately exploited as tuning parameters to control bandwidth of the filter. In order to determine the filter coefficients, mean squared difference between the desired variable bandwidth filter and the practical filter is minimized by differentiating it with respect to its coefficients leading to a system of linear equations. The matrix elements can be expressed in form of Toeplitz-plus-Hankel matrix, which reduces the computational complexity. Several examples are included to demonstrate effectiveness of the proposed method in terms of passband error (e_p), stopband error (e_s) and stopband attenuation (A_s).