Avalanche multiplication characteristics ofAl0.8Ga0.2As diodes

The avalanche multiplication characteristics of Al_0.8Ga _0.2As have been investigated in a series of p-i-n and n-i-p diodes with i-region widths, w, varying from 1 μm to 0.025 μm. The electron ionization coefficient, α, is found to be consistently higher than the hole ionization coefficient, β, over the entire range of electric fields investigated. By contrast with Al_xGa _1-xAs (x⩽0.6) a significant difference between the electron and hole initiated multiplication characteristics of very thin Al_0.8Ga_0.2As diodes (w=0.025 μm) was observed. Dead space effects in the diodes with w⩽0.1 μm were found to reduce the multiplication at low bias below the values predicted from bulk ionization coefficients. Effective α and β that are independent of w have been deduced from measurements and are able to reproduce accurately the multiplication characteristics of diodes with w⩾0.1 μm and breakdown voltages of all diodes with good accuracy. By performing a simple correction for the dead space, the multiplication characteristics of even thinner diodes were also predicted with reasonable accuracy     

DC and microwave noise transient behavior of InP/InGaAs doubleheterojunction bipolar transistor (DHBT) with polyimide passivation

DC and microwave noise transient behavior of InP/InGaAs double heterojunction bipolar transistor (DHBT) with polyimide passivation is reported in this paper for the first time. The base transient current is believed to be due to the change of surface potential near the base-emitter junction perimeter at the polyimide/emitter interface resulting from a decrease in the amount of trapped electrons in the polyimide. We also find that the surface potential on the sidewall of collector-emitter affected by the charge trapping and detrapping in polyimide may induce a parasitic polyimide field effect transistor along the surface of the base-collector junction which results in an excess collector transient current. These base and collector current transients result in associated transient of broadband shot noise. The time dependence of microwave noise figures due to the excess transients is also investigated. The better understanding of the mechanisms of the noise transient behavior of the InP HBT device is very useful to improve the device and circuit reliability     

Dual-window dual-bandwidth spectroscopic optical coherence tomography metric for qualitative scatterer size differentiation in tissues

This study investigates the autocorrelation bandwidths of dual-window (DW) optical coherence tomography (OCT) k-space scattering profile of different-sized microspheres and their correlation to scatterer size. A dual-bandwidth spectroscopic metric defined as the ratio of the 10% to 90% autocorrelation bandwidths is found to change monotonically with microsphere size and gives the best contrast enhancement for scatterer size differentiation in the resulting spectroscopic image. A simulation model supports the experimental results and revealed a tradeoff between the smallest detectable scatterer size and the maximum scatterer size in the linear range of the dual-window dual-bandwidth (DWDB) metric, which depends on the choice of the light source optical bandwidth. Spectroscopic OCT (SOCT) images of microspheres and tonsil tissue samples based on the proposed DWDB metric showed clear differentiation between different-sized scatterers as compared to those derived from conventional short-time Fourier transform metrics. The DWDB metric significantly improves the contrast in SOCT imaging and can aid the visualization and identification of dissimilar scatterer size in a sample. Potential applications include the early detection of cell nuclear changes in tissue carcinogenesis, the monitoring of healing tendons, and cell proliferation in tissue scaffolds.     

Nonparametric distance measures of speaker verification

A nonparametric technique for speaker recognition and verification is proposed. The proposed distance measures differ from existing distance measures by their symmetry. These measures have been evaluated on a text-dependent database, achieving a 99.6% verification rate for 200 French speakers. In addition, it is shown that the covariance carries more speaker information than the sample mean     

磁控共溅射GaAs/SiO2细小纳米颗粒镶嵌材料的结构和非线性光学性质

应用磁控共溅射技术和后退火方法制备了GaAs/SiO2纳米颗粒镶嵌薄膜,并分别应用原子力显微镜、X射线衍射和卢瑟福背散射实验来观测薄膜的形貌、相结构和化学组分.结果表明GaAs纳米颗粒的平均直径很小(约为1.5～3.2nm),且均匀地分布于SiO2之中,薄膜中的GaAs和SiO2组分都符合化学计量关系.应用脉冲激光高斯光束对薄膜的光学非线性进行了Z扫描测试和分析.结果表明,薄膜的三阶光学非线性折射率系数和非线性吸收系数都由于量子限制效应而大大地增强,在非共振条件下,它们分别约为4×10-12m2/W和2×10-5m/W,在准共振的条件下,它们分别约为2×10-11m2/W和-1×10-4m/W.     

Performance of DS SS system under on-off wideband jamming

A performance analysis of a direct sequence spread spectrum (DS SS) system under a periodic on-off wideband jammer is presented. Closed-form results of system bit error rate (BER) are derived for all possible cases of jammer duty cycle     

Effect of dead space on avalanche speed [APDs]

The effects of dead space (the minimum distance travelled by a carrier before acquiring enough energy to impact ionize) on the current impulse response and bandwidth of an avalanche multiplication process are obtained from a numerical model that maintains a constant carrier velocity but allows for a random distribution of impact ionization path lengths. The results show that the main mechanism responsible for the increase in response time with dead space is the increase in the number of carrier groups, which qualitatively describes the length of multiplication chains. When the dead space is negligible, the bandwidth follows the behavior predicted by Emmons but decreases as dead space increases     

Bit allocation for joint source/channel coding of scalable video

We propose an efficient bit allocation algorithm for a joint source/channel video codec over noisy channels. The approach is to distribute the available source and channel coding bits among the subbands in such a way that the expected distortion is minimized. The constructed distortion curves bound the performance degradation should the channel be estimated incorrectly. The algorithm can be used in other similar distortion minimization problems with two constraints, such as power or complexity.     

Location Estimation via Support Vector Regression

Location estimation using the global system for mobile communication (GSM) is an emerging application that infers the location of the mobile receiver from multiple signals measurements. While geometrical and signal propagation models have been deployed to tackle this estimation problem, the terrain factors and power fluctuations have confined the accuracy of such estimation. Using support vector regression, we investigate the missing value location estimation problem by providing theoretical and empirical analysis on existing and novel kernels. A novel synthetic experiment is designed to compare the performances of different location estimation approaches. The proposed support vector regression approach shows promising performances, especially in terrains with local variations in environmental factors