Resonators in Open-Loop Sigma–Delta Modulators

In this paper, we introduce the modulo resonator for use in analog-to-digital open-loop sigma-delta modulators (OLSDMs). The OLSDM presented in this paper is intended for use in high-accuracy (14-bit) high-speed analog-to-digital converters. The modulo resonator is used with a modulo notch filter to insert a zero in the noise transfer function at a nonzero frequency. The effect of finite gain in modulo integrators and resonators is described and verified through simulation. The modulo resonator and a previously published modulo integrator are used in a behavioral model of a switched-capacitor fifth-order OLSDM with more than 13-bit effective number of bits for an oversampling ratio of four. We prove for the N -order OLSDM that the number of bits in the quantizer (B) must be larger than N to ensure equivalence between open-loop sigma-delta modulation and sigma-delta modulation.     

Piecewise parabolic negative magnetoresistance of two-dimensional electron gas with triangular antidot lattice

Extraordinary piecewise parabolic behavior of the magnetoresistance has been experimentally detected in the two-dimensional electron gas with a dense triangular lattice of antidots, where commensurability magnetoresistance oscillations are suppressed. The magnetic field range of 0–0.6 T can be divided into three wide regions, in each of which the magnetoresistance is described by parabolic dependences with high accuracy (comparable to the experimental accuracy) and the transition regions between adjacent regions are much narrower than the regions themselves. In the region corresponding to the weakest magnetic fields, the parabolic behavior becomes almost linear. The observed behavior is reproducible as the electron gas density changes, which results in a change in the resistance by more than an order of magnitude. Possible physical mechanisms responsible for the observed behavior, including so-called “memory effects,” are discussed.     

Transport anomalies in electron billiards with a lack of symmetry

Magnetotransport properties of two-dimensional electron gas in low-symmetry periodic lattices of antidots have been investigated. The comparison of resistivity tensor components has shown that the magnetoresistance features of this system originate from the electron trajectories running along the lattice arrays. Deviations in the Hall resistance, connected with a lack of symmetry in the system, have been found which were absent in the square lattices with the same ratio of the period to the antidot radius.     

Magnetotransport phenomena in single AlGaAs/GaAs quantum wires grown on laterally patterned substrates

Single, sub-micrometer wide quantum wires have been fabricated using molecular beam epitaxy on mesa-etched GaAs substrates, where the GaAs wire is embedded in AlGaAs. By using a Hall bar pattern, potential probes were directly attached to the wires while grown. To avoid problems associated with anisotropic etching and regrowth, the wire structures were oriented along the 100 crystallographic directions. From this fabrication technique, described in detail by Shitara et al., Appl. Phys. Lett. 66, 2385 (1995), one can expect the formation of high quality “self-aligned” quantum wires with a confinement potential determined by the conduction band discontinuity of AlGaAs and GaAs. Here we study the four-point magnetoresistance of 50 μm long single quantum wires with widths between 250 and 700 nm from 1.3 to 21 K. A distinct weak localization peak and universal conductance fluctuations dominate the low magnetic field regime and are used to estimate the phase-coherence length of the electrons. Pronounced 1/B periodic quantum oscillations at magnetic fields above 1 T are consistent with the picture of wires with a square-well shaped confining potential.     

Improved Frey Chaotic Digital Encoder for Trellis-Coded Modulation

In this brief, nonlinear digital filters with finite precision are analyzed as recursive systematic convolutional (RSC) encoders. An infinite-impulse-response (IIR) digital filter with finite precision (wordlength of N bits) is a rate-1 RSC encoder over a Galois field GF(2N). The Frey chaotic filter is analyzed for different wordlengths N, and it is demonstrated that the trellis performances can be enhanced by proper filter design. Therefore, a modified definition for the encoding rate is provided, and a trellis design method is proposed for the Frey filter, which consists of reducing the encoding rate from 1 to 1/2. This trellis optimization partially follows Ungerboeck's rules, i.e., increasing the performances of the encoded chaotic transmission in the presence of noise. In fact, it is demonstrated that for the same spectral efficiency, the modified Frey encoder outperforms the original Frey encoder only for N = 2. To show the potential of these nonlinear encoders, it is demonstrated that a particular nonlinear digital filter over GF(4) is equivalent to a GF(2) conventional optimum RSC encoder. The symbol error rate (SER) is estimated for all the proposed schemes, and the results show the expected coding gains as compared to their equivalent nonencoded and linear versions.     

Metal-insulator transition in Sb-doped short period Si/SiGe superlattices

Low-temperature magnetotransport measurements (50 mK < T < 4.2 K) on Sb-doped short period Si/SiGe superlattices (d ≈ 40 Å) are presented. The experiments show evidence for single particle quantum interference and enhanced electron-electron interaction effects. Furthermore, for a Sb doping concentration of 4.5 × 10¹⁸ cm⁻³, a field induced metal to insulator transition (MIT) is observed for magnetic fields B parallel to the superlattice layers, whereas for the perpendicular direction of B, the MIT does not take place. The anisotropy and dimensionality of the SL-samples is discussed.     

Identical-Capacity Channel Decomposition for Design of Universal LDPC Codes

Design of low-density parity-check (LDPC) codes suitable for all channels which exhibit a given capacity C is investigated. Such codes are referred to as universal LDPC codes. First, based on numerous observations, a conjecture is put forth that a code working on N equal-capacity channels, also works on any convex combination of these N channels. As a supporting evidence, we prove that a code satisfying the stability condition on N channels, also satisfies the stability condition on the convex hull of these N channels. Then, a channel decomposition method is suggested which spans any given channel with capacity C in terms of a number of identical-capacity basis channels. We expect codes that work on the basis channels to be suitable for any convex combination of the bases, i.e., all channels with capacity C. Such codes are found over a wide range of rates. An upper bound on the achievable rate of universal LDPC codes is suggested. Through examples, it is shown that our codes achieve rates extremely close to this upper bound. In comparison with existing LDPC codes designed for a given channel, significant performance gain is reported when codes are used over various channels of equal capacity.     

Hydrodynamic hot-electron transport simulation based on the Monte Carlo method

A hydrodynamic hot electron model is used to study electron transport through a submicron N⁺ --- N --- N⁺ GaAs structure. This study is used to investigate improvements which the unique features of this model offer to analysis of devices operating under nonstationary transport conditions. The model is based upon semiclassical “hydrodynamic” conservation equations for the average carrier density, momentum and energy. The general model includes particle relaxation times, momentum relaxation times, energy relaxation times, electron temperature tensors and heat flow vectors as a function of average carrier energy for the Γ, X and L valleys of GaAs. For this study, we utilized a simplified single electron gas version of our model to clearly reveal the impact of the nonstationary terms in the model. Results from both a drift-diffusion model approach and a Monte Carlo analysis are used to show the relative accuracy and facility this new model offers for investigating practical submicron device structures operating under realistic conditions.     

The use of generalised models to explain the behaviour of ohmic contacts to n-type GaAs

The use of two generalised carrier transport models to account for the N_D⁻¹ dependence of the specific contact resistance (ρ_c) of metal-semiconductor Ohmic contacts to n-type GaAs is proposed. Both models include the effects of thermionic emission and diffusion across the high-low barrier junction a priori. Calculations of ρ_c, and comparison with experimental data, show conclusively that thermionic emission is the dominant transport mechanism across the barrier. It is stressed that these models do not rely on prior choices of either of the transport processes. These conclusions are arrived at a posteriori.     

High-performance P-n-p heterojunction bipolar transistor design

The performance of P-n-p AlGaAs/GaAs heterojunction bipolar transistors (HBTs) for analog and digital circuit applications is compared to that of N-p-n HBTs. The theoretical analysis shows that the P-n-p HBTs are comparable with N-p-n HBTs in high-speed digital operation, while the N-p-n HBTs exhibit higher cut-off frequency in microwave and millimeter-wave operation. Analytical equations and SPICE circuit simulation are used in support of the comparison between the P-n-p and N-p-n HBTs. Optimization of device doping profile design for P-n-p and N-p-n heterojunction bipolar transistors in analog and digital circuits are presented.     

Quantum corrections to the conductivity of a two-dimensional system with antidots

The quantum corrections to the conductivity of a two-dimensional electron system with antidots in the limit of a small antidot density are investigated. The corrections to the conductivity and magnetoconductivity due to the presence of antidots in a magnetic field perpendicular to the plane of the system are considered. It is assumed that the mean free path l of electrons on the impurities is far smaller than the antidot radius. Fiz. Tekh. Poluprovodn. 32, 1461–1466 (December 1998)     

Asymptotic Theorems for the Product of Certain Structured Random Matrices and Their Application to Analysis of Asynchronous CDMA

This paper consists of two parts. In the first part, asymptotic theorems about the product of certain structured random matrices are developed by means of the moment convergence theorem (MCT) and the free probability theory. This product of random matrices is a generalization of the product of a sample covariance matrix and an arbitrary Hermitian matrix. In the second part, the theoretical results obtained in the first part are applied to analyze a randomly spread asynchronous direct sequence-code-division multiple-access (DS-CDMA) system with both the number of users K and the number of chips per symbol N approaching infinity but the ratio K/N kept as a finite constant. Two levels of asynchronism are considered; one is symbol-asynchronous but chip-synchronous, and the other is chip-asynchronous. Asymptotic spectral distribution (ASD) of cross-correlation matrix and asymptotic spectral efficiency are investigated. Conditions under which CDMA systems with various synchronism levels (synchronous and two levels of asynchronism) have the same performance are also established.     

Kinetics of hillock growth in Al and Al-alloys

Hillock growth kinetics and size distribution were investigated in Al, Al:Si 1% and Al:Si1%:Cu 0.5% layers. Metallization surface was examined by optical, SEM and TEM microscopy, stylus profiling and an automatic method of hillock recognition from a microscope image. The method allowed for counting hillocks in a desired range of their diameter d. Surface density of hillocks was measured as a function of time of furnace annealing at 400°C and as a function of temperature of RTP annealing. A maximum hillock size was found to increase linearly with metallization layer thickness and with logarithm of annealing time. A total area occupied by hillocks was evaluated. Hillock density decreased versus 1/T with an activation energy of 0.28 eV for Al and 0.31 eV for Al:Si. It was found, that a normalized hillock density N may be expressed by a formula N=N₀ exp(−cd). Values for N₀ and c are given together with a short discussion.     

High stability and low driving voltage green organic light emitting diode with molybdenum oxide as buffer layer

Green organic light emitting diodes (OLEDs) with copper phthalocyanine (CuPc), 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenymine (m-MTDATA) and molybdenum oxide (MoO_x) as buffer layers have been investigated. The MoO_x based device shows superior performance with low driving voltage, high power efficiency and much longer lifetime than those with other buffer layers. At the luminance of 100 cd/m², the driving voltage is 3.8 V, which is 0.5 V and 2.2 V lower than that of the devices using CuPc (Cell-CuPc) and m-MTDATA (Cell-m-MTDATA) as buffer layer, respectively. Its power efficiency is 13.6 Lm/W, which is 38% and 30% higher than that of Cell-CuPc and Cell-m-MTDATA, respectively. The projected half-life under the initial luminance of 100 cd/m² is 42,400 h, which is more than 3.8 times longer than that of Cell-m-MTDATA and 24 times that of Cell-CuPc. The superior performance of Cell-MoO_x is attributed to its high hole injection ability and the stable interface between MoO_x and organic material. The work function of MoO_x measured by contact potential difference method and the J–V curves of “hole-only” devices indicate that a small barrier between MoO_x/N,N′-di(naphthalene-1-y1)-N,N′-dipheyl-benzidine (NPB) leads to a strong hole injection, resulting in the low driving voltage and the high stability.     

Quantized conductance in a Si/Si0.7Ge0.3 split-gate device and impurity-related magnetotransport phenomena

We have defined a quantum point-contact by the split-gate technique in a Si/SiGe heterostructure containing a two-dimensional electron gas with an elastic mean free path of about 1.3 μm. The conductance of this device shows typical steps very close to multiples of 4e² h⁻¹. Upon application of a perpendicular magnetic field the spin and valley degeneracies are lifted and magnetic depopulation of the one-dimensional subbands can be observed. The appearance of Aharonov-Bohm oscillations for B ≥ 2T and of resonant tunneling peaks close to “pinch-off” indicates the presence of impurities close to the constriction.     

Kinetic theory of negative magnetoresistance as an alternative to weak localization in semiconductors

A time-independent solution to the kinetic equation for the one-electron density matrix at an arbitrary magnetic field and linear in the electric field has been obtained within the nonequilibrium electron gas approximation. The case of scattering on deformation potential is considered. Expressions for the conductivity tensor are obtained in the form of sums over the magnetic quantization states. They are shown to coincide with classical ones in the absence of a magnetic field. Under weak magnetic fields, the magnetoresistance is positive when electron mobility is high and negative when it is low. It changes sign as the field increases when the mobility is intermediate. The magnetoresistance of the degenerate electron gas is nonzero. The conductivity tensor describes its oscillations in the quantizing magnetic field. Their amplitude and the magnetoresistance mean values increase with increasing magnetic field and mobility.     

N∗Nakagami: A Novel Stochastic Model for Cascaded Fading Channels

A generic and novel distribution, referred to as Nakagami, constructed as the product of N statistically independent, but not necessarily identically distributed, Nakagami-m random variables (RVs), is introduced and analyzed. The proposed distribution turns out to be a very convenient tool for modelling cascaded Nakagami-m fading channels and analyzing the performance of digital communications systems operating over such channels. The moments-generating, probability density, cumulative distribution, and moments functions of the N _*Nakagami distribution are developed in closed form using the Meijer's G -function. Using these formulas, generic closed-form expressions for the outage probability, amount of fading, and average error probabilities for several binary and multilevel modulation signals of digital communication systems operating over the N _*Nakagami fading and the additive white Gaussian noise channel are presented. Complementary numerical and computer simulation performance evaluation results verify the correctness of the proposed formulation. The suitability of the N _*Nakagami fading distribution to approximate the lognormal distribution is also being investigated. Using Kolmogorov--Smirnov tests, the rate of convergence of the central limit theorem as pertaining to the multiplication of Nakagami-m RVs is quantified.     

Carrier Frequency Offset Mitigation in Asynchronous Cooperative OFDM Transmissions

Carrier frequency offset (CFO) mitigation is critical for orthogonal frequency-division multiplexing (OFDM)-based cooperative transmissions because even small CFO per transmitter may lead to severe performance loss, especially when the number of cooperative transmitters is large. In this paper, we show that cyclic prefix (CP) can be exploited to mitigate or even remove completely the CFO. The mitigation performance increases along with the CP length. In particular, long CP with length proportional to NI, where N is the fast Fourier transform (FFT) block length and I is the number of cooperative transmitters, can guarantee a complete CFO removal. While this comes with a reduction in bandwidth efficiency, the long CP in the proposed scheme is exploited to enhance transmission power efficiency in a way similar to spread-spectrum systems, and thus is different from conventional CP that degrades both bandwidth and power efficiency. An efficient CFO-mitigation algorithm is developed that has complexity at most O(NI ²), or even linear in N approximately in some cases. Implemented as a preprocessing procedure independently from cooperative encoding/decoding details, this algorithm makes the CFO problem effectively transparent to and thus has general applications in OFDM-based transmissions.     

Magneto transport on antidot arrays, fabricated by an atomic force microscope

A recently introduced lithographic tool employing an atomic force microscope is improved and utilized to fabricate nanometer-scale antidot arrays. The AFM periodically creates holes in a thin photoresist, covering a conventional mesa-etched GaAs---AlGaAs heterostructure. To minimize the period of produced holes in the resist we now use carbon AFM supertips, prepared in a scanning electron microscope. To create holes or lines in the resist, a mechanical force of typically 1 μ N is exerted at ambient conditions. We thus succeed in fabricating hole arrays with a periodicity down to 15 nm and a hole diameter of only few nanometers. The resist mask is transferred onto the two-dimensional electron gas either by a shallow wet chemical etch step or by an ion beam irradiation technique, respectively. In magneto-resistance studies at T = 4.2 K on antidot-devices with periods down to 85 nm, we clearly observe commensurability oscillations of the longitudinal resistance R_xx, demonstrating the successful pattern transfer to the electron system. Resistance fluctuations measured at T = 25 mK indicate a clear dependence on the period of the antidots.