Effects of SiN passivation and high-electric field on AlGaN-GaN HFET degradation

The authors report on the effects of silicon nitride (SiN) surface passivation and high-electric field stress (hot electron stress) on the degradation of undoped AlGaN-GaN power HFETs. Stressed devices demonstrated a decrease in the drain current and maximum transconductance and an increase in the parasitic drain series resistance, gate leakage, and subthreshold current. The unpassivated devices showed more significant degradation than SiN passivated devices. Gate lag phenomenon was observed from unpassivated devices and removed by SiN passivation. However, SiN passivated devices also showed gate lag phenomena after high-electric field stress, which suggests possible changes in surface trap profiles occurred during high-electric field stress test.     

Recognizing, assessing, and intervening with problems of professional competence.

Working collaboratively, psychologist educators and trainers at the doctoral, internship, and postdoctoral levels; credentialers; practitioners; and students offer 8 proposals for psychologists to consider in recognizing, assessing, and intervening with problems of professional competence in students and practicing professionals. In the proposals, the authors address the following topics: definitions and categories; preparing the system; self-assessment; remediation; diversity; communication across various levels of the system; confidentiality; and ethical, regulatory, and legal underpinnings. They also propose future directions for the assessment of problems in professional competence in both students and practicing psychologists. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

High-breakdown-voltage AlInAs/GaInAs junction-modulated HEMT's(JHEMT's) with regrown ohmic contacts by MOCVD

A technology for increasing both the two-terminal gate-drain breakdown and subsequently the three-terminal-off-state breakdown of AlInAs/GaInAs high-electron-mobility transistors (HEMTs) to record values without substantial impact on other parameters is presented. The breakdown in these structures is dependent on the multiplication of electrons injected from the source (channel current) and the gate (gate leakage) into the channel. In addition, holes are generated by high fields at the drain and are injected back into the gate and source electrodes. These phenomena can be suppressed by increasing the gate barrier height and alleviating the fields at the drain. Both have been achieved by incorporating a p⁺-2DEG junction as the gate that modulates the 2DEG gas and by utilizing selective regrowth of the source and drain regions by MOCVD. The 1-μm-gate-length devices fabricated have two-terminal gate-drain and three-terminal-off-state breakdown voltages of 31 V and 28 V, respectively     

Development and testing of a holographic projection system

We describe the development and testing of a holographic projection system that is used to produce micro-optical devices. The projector uses a two-dimensional two-phase-level diffraction grating to produce multiple coherent beams and an interferometric optical system behind the grating to recombine the beams to produce interference patterns that have been recorded within a photosensitive substrate. The two different substrates that we used are a diazo imaging material and a bisbenzocyclobutene (BCB) polymeric resin for fabrication of surface relief microstructures. After the exposed photosensitive substrate is developed, the recorded interference pattern forms a micro-optical device. The analysis and testing of these micro-optical devices show promise that this technique can form patterns uniformly over a region of several centimeters in diameter on flat or curved substrates. The experimental testing results of these micro-optical devices demonstrate that this method is a simple and energy efficient system to produce microstructures compared with other methods. These devices may be used as a new generation of directional light filters or monolithic microlenslet arrays that may have applications in communications, display, and components technologies.     

Shared book reading: When and how questions affect young children's word learning.

Shared book reading, and the conversation that accompanies it, can facilitate young children's vocabulary growth. To identify the features of extratextual questions that help 3-year-olds learn unfamiliar words during shared book reading, two experiments explored the impact of cognitive demand level, placement, and an approximation to scaffolding. Asking questions about target words improved children's comprehension and production of word–referent associations, and children with larger vocabularies learned more than children with smaller vocabularies. Neither the demand level nor placement of questions differentially affected word learning. However, an approximation to scaffolding, in which adults asked low demand questions when words first appeared and high demand questions later, did facilitate children's deeper understanding of word meanings as assessed with a definition task. These results are unique in experimentally demonstrating the value for word learning of shifting from less to more challenging input over time. Discussion focuses on why a scaffolding-like procedure improves children's acquisition of elaborated word meanings. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Influence of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> passivation on surface trapping in SiC metal-semiconductor field-effect transistors

The SiC metal-semiconductor field-effect transistors (MESFETs) have been reported to have current instability and strong dispersion caused by trapping phenomena at the surface and in the substrate, which degrade direct-current (DC) and radio-frequency (RF) performance. This paper illustrates the change in electrical characteristics of SiC MESFETs after Si₃N₄ passivation. Because of a reduction of surface trapping effects, Si₃N₄ passivation can diminish current collapse under pulsed DC conditions, increasing the RF power performance. The reduction of surface trapping effects is verified by the change in the ratio of the drain current to the gate current under pinch-off conditions.     

Effect of cladding layer thickness on the performance ofGaAs-AlGaAs graded index separate confinement heterostructure singlequantum-well lasers

The thinning of cladding layers of GaAs-AlGaAs graded index separate confinement heterostructure single quantum-well (GRINSCH-SQW) lasers offers several advantages. These advantages include easier fabrication of surface grating-based lasers and ridge lasers, the reduction of growth time and source-material use, and the more effective removal of heat due to lower thermal resistance. Experimental results from GRINSCH-SQW lasers showing that typical cladding thicknesses of 1.5 to 2 μm are much thicker than necessary are presented. Lasers with cladding layers as thin as 4500 Å have not shown any increase in threshold current. Theoretical analysis shows good agreement with the experimental results on the minimum cladding thickness necessary to prevent an increase in the threshold current. The differential quantum efficiency is theoretically considered and is found to be more sensitive to cladding-layer thickness     

Chlorination and dechlorination of nuclear reactor cooling water

Effects from chlorination and dechlorination of Savannah River water were studied during the development of biofouling countermeasures for a proposed cooling tower system required for thermal mitigation of nuclear reactor cooling water effluent. Testing was conducted to assess chlorine demand and dissipation rates as well as the environmental acceptability of using sodium sulfite as a dechlorinating agent. Chlorine demand varied significantly, but in an unpredictable manner during seven seasonal sampling dates. A chlorine dosage of 3–5 mg/l was generally adequate to provide a free chlorine residual of 1 mg/l. Static 48-h bioassays with bluegill showed no acute toxicity for chlorinated/dechlorinated cooling water containing up to 64 times the calculated stoichiometric concentration of sodium sulfite required for dechlorination. Experiments measuring the depletion of dissolved oxygen and flow-through (96-h) bioassays with bluegill and largemouth bass further substantiated the environmental acceptability of using sodium sulfite as a dechlorinating agent.