Characterisation and modelling of parasitic effects and failure mechanisms in AlGaN/GaN HEMTs

This work is a study of the degradations of AlGaN/GaN HEMTs induced by 2000 h of ageing tests. The methodology is based on cross-characterisation analysis.The life tests (HTO 150 °C, HTO 175 °C and HTRB 175 °C and Idq 90 °C) have mainly induced a decrease of the saturation drain current, occurring during the first 50 h, followed by a stabilisation. There is a shift of the pinch-off voltage in the range of 0.1–0.2 V while the Schottky contact is rather stable after ageing. The evolution of the electrical characteristics after ageing does not depend on the bias conditions but rather more on the channel temperature. It seems to be neither field nor current driven. Low frequency drain current noise demonstrates that there is no trap creation and the weak evolution of the 1/f noise confirms that there is no degradation in the channel. Moreover, pulsed I–V measurements show a weak evolution of gate lag and drain lag rates after ageing. The same degradation mode is demonstrated for all life tests with rather high activation energy of 1.6 eV. The weak evolution of electrical characteristics observed during the life tests cannot be obviously explained by a single physical mechanism and results from a combination of trap-related effects before stabilisation.     

Progesterone induces calcitonin gene expression in human endometrium within the putative window of implantation

The human endometrium acquires the ability to implant the developing embryo within a specific time window that is thought to open between days 19-24 of the secretory phase of the menstrual cycle. During this period the endometrium undergoes pronounced structural and functional changes induced by the ovarian steroids, estrogen and progesterone, that prepare it to be receptive to invasion by the embryo. The identification of reliable biochemical markers to assess this critical receptive phase in the context of the natural cycle remains one of the major challenges in the study of human reproduction. Our previous studies in a rat model system demonstrated that the expression of calcitonin, a peptide hormone involved in calcium homeostasis, is transiently induced by progesterone in the glandular epithelium at the onset of implantation. Attenuation of calcitonin synthesis in the uterus during the preimplantation phase by administration of calcitonin antisense oligodeoxynucleotides severely impairs implantation of rat embryos, suggesting that this peptide hormone plays a critical role in uterine receptivity. To investigate whether calcitonin is also expressed in the human endometrium during implantation, we monitored the spatio-temporal expression of calcitonin on various days of the menstrual cycle. Our studies employing RT-PCR showed that calcitonin messenger ribonucleic acid is expressed in human endometrium during the postovulatory midsecretory phase (days 17-25) of the menstrual cycle, with maximal expression occurring between days 19-21. Very little calcitonin expression was detected in the endometrium in either the preovulatory proliferative (days 5-14) or the late secretory (days 26-28) phase. In situ hybridization and immunocytochemical analyses localized the calcitonin expression predominantly in the glandular epithelial cells of the endometrium. Our studies further showed that calcitonin expression in the human endometrium is under progesterone regulation. Treatment of women with an antiprogestin, mifepristone (RU-486), drastically reduced calcitonin expression in the endometrium. Collectively, these findings reveal that progesterone-induced expression of calcitonin in the secretory endometrium temporally coincides with the putative window of implantation in the human.     

A divide-and-conquer/cellular-decomposition framework for million-to-billion atom simulations of chemical reactions

To enable large-scale atomistic simulations of material processes involving chemical reactions, we have designed linear-scaling molecular dynamics (MD) algorithms based on an embedded divide-and-conquer (EDC) framework: first principles-based fast reactive force-field (F-ReaxFF) MD; and quantum-mechanical MD in the framework of the density functional theory (DFT) on adaptive multigrids. To map these O(N) algorithms onto parallel computers with deep memory hierarchies, we have developed a tunable hierarchical cellular-decomposition (THCD) framework, which achieves performance tunability through a hierarchy of parameterized cell data/computation structures and adaptive load balancing through wavelet-based computational-space decomposition. Benchmark tests on 1920 Itanium2 processors of the NASA Columbia supercomputer have achieved unprecedented scales of quantum-mechanically accurate and well validated, chemically reactive atomistic simulations—0.56 billion-atom F-ReaxFF MD and 1.4 million-atom (0.12 trillion grid points) EDC–DFT MD—in addition to 18.9 billion-atom non reactive space–time multiresolution MD. The EDC and THCD frameworks expose maximal data localities, and consequently the isogranular parallel efficiency on 1920 processors is as high as 0.953. Chemically reactive MD simulations have been applied to shock-initiated detonation of energetic materials and stress-induced bond breaking in ceramics in corrosive environments.     

Serological tests as indicators of immunity against Pasteurella multocida infection in sheep

Five serological tests, i.e. single tube agglutination, doubling dilution tube agglutination, agar agglutination, passive hemagglutination and passive mouse protection tests were evaluated for their efficacy in predicting the fate of vaccinated and unvaccinated sheep on challenge with an ovine strain of Pasteurella multocida. The passive hemagglutination test predicted the fate of unvaccinated sheep while the agar agglutination test indicated the immune status of vaccinated sheep.     

Measurement of gain tilt and its contribution to composite second-order distortion in the analog-digital transmission systems with Er/Yb fiber amplifiers

Fiber-optic-based analog or digital community antenna television systems experience composite second-order (CSO) distortion caused by the interaction between the gain tilt of the doped fiber amplifier and the laser chirp due to modulation. The gain tilt for an analog-digital transmission system with a high-power erbium/ytterbium-codoped fiber amplifier has been experimentally measured and its contribution to the CSO distortion of the system is evaluated. The results are in good agreement when compared with the direct measurement of the CSO distortion of the transmission system with and without the amplifier. The dependence of the gain tilt on the modulation frequency and wavelength of the input light is also investigated.     

Design and Performance of a Heterogeneous Grid Partitioner

An important characteristic of distributed grids is that they allow geographically separated multicomputers to be tied together in a transparent virtual environment to solve large-scale computational problems. However, many of these applications require effective runtime load balancing for the resulting solutions to be viable. In this paper we present a novel latency-tolerant partitioner, called MinEX, that dynamically balances processor workloads while minimizing data movement and runtime communication for applications that are executed in a parallel distributed grid environment. We also compare the performance of MinEX with that of METIS, a popular multilevel family of partitioners, using simulated heterogeneous grid configurations. A solver for the classical N-body problem is implemented to provide a framework for the comparisons. Experimental results show that the proposed MinEX partitioner provides superior quality partitions while being competitive to METIS in terms of execution speed.     

Disaggregation-aggregation based model reduction for refinery-wide optimization

In this paper, reduced nonlinear refinery models are developed by generating and using input-output data from a process simulator. In particular, rigorous process models of continuous catalytic reformer (CCR) and naphtha splitter units are used for generating the data. To deal with complexity associated with large amounts of data, that is usually available in the refineries, a disaggregation-aggregation based approach is presented. The data is split (disaggregation) into smaller subsets and reduced artificial neural network (ANN) models are obtained for each of the subset. These ANN models are then combined (aggregation) to obtain an ANN model which represents all the data originally generated. The disaggregation step can be carried out within a parallel computing platform. Refinery optimization studies are carried out to demonstrate the applicability and the usefulness of the proposed model reduction approach.     

Long-term reliability of Ti–Pt–Au metallization system for Schottky contact and first-level metallization on SiC MESFET

The stability of metal layers on semiconductors is a key issue for the device electrical performances. Therefore, the reliability of SiC/Ti/Pt/Au system was investigated using storage steady-stress testing, AES (Auger Electron Spectrometry), and SIMS (Secondary Ions Mass Spectrometry) analysis. The study was conducted on different patterns for gate and interconnection structure to underline the different reliability problems. Auger and SIMS analysis showed important modifications in the three-metal structure without reactions with the SiC substrate. The resistance degradation was assigned to interdiffusion phenomena. It was analyzed with a diffusion-controlled model. Activation energies and mean time to failure (MTF) were calculated for a failure criterion defined as a 10% increase of the resistance. Finally, the different rules of the metallization degradation in MESFET behaviours for interconnections and gate were discussed.