Study on GaAs/GaAlAs MQW structure for photovoltaic applications

GaAs/GaAlAs MQW structures with varying well widths having narrowest wells on the top layers and gradually wider wells for the inner layers were studied and confirmed by Auger spectroscopy and photoluminescence measurements. Multiplication of quantum wells gives stronger photoluminescence due to higher density of quantized states in the structure. This MQW structure was experimented in the photoconductivity and photocurrent measurements at room temperature. It is found that at low multiplication of quantum wells in MQW structure, the photoconductivity effect was mainly controlled by bulk material as shown in the spectral response. Photocurrent at low bias voltage shows relatively better spectral sensitivity at shorter wavelength. The photoconductivity and photocurrent measurements indicate that appropriate MQW structures acting as broader absorbers due to graded characters of quantized energy states are needed for photovoltaic application. Integration of MQW structure to solar cell structure was also investigated.     

AlGaAs/GaAs/InGaAs composite MQW structures for photovoltaic applications

AlGaAs/GaAs/InGaAs composite MQW structures were theoretically studied and simulated. The computer simulation indicated that an appropriate composite MQW, both with symmetrical and non-symmetrical structures, could keep |ψ|² of quantized carriers at proper locations in electric-field-tilted quantum wells, so the efficient transition by photon absorption would be possible and applicable for photovoltaic cells which have the composite MQW as the active region. The AlGaAs/GaAs MQW and GaAs/InGaAs (4x) SQW structures were separately prepared by MBE and were evaluated for their spectral responses. The AlGaAs/GaAs MQW has a high response at a short wavelengths (peak at 685 nm) due to the quantized states in GaAs wells, while the GaAs/InGaAs SQW has a broader spectral response covering longer wavelengths (600–850 nm) because of the strong absorption in the GaAs barrier and substrate. However, (4x) photoluminescence peaks at 900–1100 nm that werefound from GaAs/InGaAs strained quantum wells at room temperature are promising evidence for the longer wavelength spectral response. The AlGaAs/GaAs/InGaAs composite SQW and MQW samples were experimentally prepared by MBE techniques and tested for their optical properties. The broader photoluminescence peak was observed and reflected the nature of the composite structure. The study on the photospectral response of composite MQW structures has been conducted which provides the basic information for high performance solar cell design.     

Prevalence of coronary heart disease and major cardiovascular risk factors in Thailand

BACKGROUND: Coronary heart disease (CHD) is expected to become one of the major health problems in developing countries such as Thailand where prevalence data are scarce. This study reports the prevalence of CHD, as indicated by electrocardiogram (ECG) Minnesota coding, and its risk factors in Thailand. METHODS: In 1991 we conducted a cross-sectional ECG survey in a multistage random sample of the Thai population, aged > or =30. All major cardiovascular risk factors were measured. Standard supine 12-lead ECG data were collected; amplitudes and intervals were measured manually and entered into a computer. Abnormal tracings were verified by five cardiologists, and agreement among at least three of them was accepted as final. RESULTS: The total sample included 3822 men and 4967 women aged > or =30 years. The age-standardized prevalence rate of CHD was 9.9/1000 (men 9.2/1000, women 10.7/ 1000). The age-standardized level of major cardiovascular risk factors among men and women respectively were: total cholesterol 4.8 mmol/l (187.3 mg/dl), 5.1 mmol/l (197.7 mg/dl); hypercholesterolaemia (> or =6.2 mmol/l) 12.2%, 16.9%; systolic blood pressure (mmHg) 117.8, 117.7; diastolic blood pressure (mmHg) 76.9, 75.8; body mass index (kg/m2) 21.7, 22.8; fasting blood sugar 4.8 mmol/l (87.9 mg/dl), 5.0 mmol/l (90.3 mg/dl); hypertension (> or =160/95 +/- on antihypertensive drugs) 6.3%, 8.1%; smoking 65.1%, 8.5%; diabetes mellitus (> or =7.8 mmol/l) 2.4%, 3.7%; obesity (>25 kg/m2) 15.2%, 27.2%. CONCLUSIONS: Most of the age-adjusted mean values and proportion of major cardiovascular disease risk factors as well as the prevalence of total CHD in the Thai population were much lower than the median of those values found in developing countries.     

A Class of Frobenius Norm-Based Algorithms Using Penalty Term and Natural Gradient for Blind Signal Separation

We consider the blind signal separation (BSS) problem of instantaneous mixtures using penalty term and natural gradient. A class of Frobenius norm-based algorithms consisting of the offline/block processing (BP), online processing (OP) algorithms, and their normalized versions is proposed for separating nonstationary and nonwhite signals. The BP and OP algorithms, respectively, suitable for blind separation with offline and online data, are derived by using the nonstationarity and nonwhiteness of signals and the natural gradient method in conjunction with an appropriate penalty term. Associated with almost all algorithms employing a gradient method is a gradient noise problem. We thus develop, from BP and OP, their normalized versions in which the update of an unknown demixing matrix is based on the minimal disturbance principle. We show that the resulting updates are in the same direction as those of the original algorithms but with a scaling factor whose upper bound is unity. Algorithms using the nonstationarity and nonwhiteness properties have been proposed before but, due to the use of logarithms in their derivation, they are not capable of separating signals that are not persistently active and require regularization parameters to mitigate the problem. In this paper, the superior performance of the proposed algorithms to the previously proposed logarithm-based algorithms with and without regularization when separating nonpersistently active source signals is presented through some illustrative numerical experiments.