Photoluminescence-Based Electron and Lattice Temperature Measurements in GaN-Based HEMTs

Nitride-based semiconductors are gaining importance not only for high-power applications but also for high-temperature electronic devices. Using photoluminescence (PL) techniques, it is now possible to simultaneously determine the temperatures of the lattice and hot electrons in these devices. Therefore, it is possible to use PL mapping measurements to derive temperature profiles for electrons and the lattice in the active region of an operating device with a single set of measurements. This work presents an experimental process to construct such spatially resolved temperature maps for a planar semiconductor device under bias and applies this approach to a specific example using the conductive channels of a biased AlGaN/GaN high-electron-mobility transistor. Studying the temperature distribution inside the conductive channels will help understand how electrons flowing in the device interact with the lattice as well as the process of heat generation within the device.     

A Study on the Effect of Electrodeposition Parameters on the Morphology of Porous Nickel Electrodeposits

Metallurgical and Materials Transactions A - In this study, the electrodeposition of nickel foam by dynamic hydrogen bubble-template method is optimized, and the effects of key deposition...     

Impact of barrier thickness on gate capacitance—Modeling and Comparative analysis of GaN based MOSHEMTs

A mathematical model is developed predicting the behavior of gate capacitance with the nanoscale variation of barrier thickness in AlN/GaN MOSHEMT and its effect on gate capacitances of AlInN/GaN and AlGaN/GaN MOSHEMTs through TCAD simulations is compared analytically. Al N/GaN and AlInN/GaN MOSHEMTs have an advantage of a significant decrease in gate capacitance up to 108 fF/ m2 with an increase in barrier thickness up to 10 nm as compared to conventional AlGaN/GaN MOSHEMT. This decrease in gate capacitance leads to improved RF performance and hence reduced propagation delay.     

Highly sensitive detection of exocytotic dopamine release using a gold-nanoparticle-network microelectrode

Here we report a new type of microelectrode sensor for single-cell exocytotic dopamine release. The new microsensor is built by forming a gold-nanoparticle (AuNP) network on a carbon fiber microelectrode. First a gold surface is obtained on a carbon fiber microdisk electrode by partially etching away the carbon followed by electrochemical deposition of gold into the pore. The gold surface is chemically functionalized with a sol-gel silicate network derived from (3-mercaptopropyl)trimethoxysilane (MPTS). A AuNP network is formed by immobilizing Au nanoparticles onto the thiol groups in the sol-gel silicate network. The AuNP-network microelectrode has been characterized by scanning electron microscopy (SEM) and steady-state voltammetry. The AuNP-network microelectrode has been used for amperometric detection of exocytotic dopamine secretion from individual pheochromocytoma (PC12) cells. The results show significant differences in the kinetic peak parameters including shorter rise time, decay time, and half-width as compared to a bare carbon fiber electrode equivalent. These results indicate AuNP-network microelectrodes possess an excellent sensing activity for single-cell exocytotic catecholamine release, specifically dopamine. Moreover, key advantageous properties inherent to bare carbon fiber microelectrodes (i.e., rigidity, flexibility, and small size) are maintained in addition to an observed prolonged shelf life stability and resistance to cellular debris fouling and dopamine polymerization.     

Environmentally motivated modeling of hygro-thermally induced stresses in the layered limestone masonry structures: Physical motivation and numerical modeling

In the present paper, environmentally motivated numerical modeling for the limestone layered masonry structures has been investigated involving the continuous hygric state field variable, i.e. relative humidity ??. By taking advantage of the Lin et?al. assumption pertaining to the relative humidity (independency of the relative humidity and temperature) (Lin MW et?al. Build. Environ. 41(5):646?C656, (2006); Khoshbakht M et?al. Finite Elem. Anal. Des. 42(5):414?C429, (2006); Khoshbakht M, Lin MW Meas. Sci. Technol 2989?C2996, (2006); Khoshbakht M Finite Elem. Anal. Des., 45(8?C9):511?C518, (2009)), we have provided a mathematical model involving hygro-thermo-mechanical aspects as well as the water vapor transfer across the porous limestone masonry walls. The numerical study substantiates the impact of hygric effects as the major key point in the thermo-hygro-mechanical degradation and effect of geometry in the real brick-line of mortar assembly. Furthermore, we have obtained the moisture entrapment at the intersection of the lines of mortar through the layered masonry wall by means of the multidisciplinary nonlinear finite element method (NFEM) for variably saturated porous media. The new outlooks and fresh departure in durability and aging have been briefly discussed.     

Heat‐Transport Mechanisms in Superlattices

The heat transport mechanisms in superlattices are identified from the cross‐plane thermal conductivity Λ of (AlN)_x–(GaN)_y superlattices measured by time‐domain thermoreflectance. For (AlN)_{4.1 nm}–(GaN)_{55 nm} superlattices grown under different conditions, Λ varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest Λ, Λ of (AlN)_{4 nm}–(GaN)_y superlattices decreases monotonically as y decreases, Λ = 6.35 W m⁻¹ K⁻¹ at y = 2.2 nm, 35 times smaller than Λ of bulk GaN. For long‐period superlattices (y > 40 nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G ≈ 620 MW m⁻² K⁻¹. For y < 40 nm, the apparent value of G increases with decreasing y, reaching G ≈ 2 GW m⁻² K⁻¹ at y < 3 nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN)_{4.1 nm}–(GaN)_{4.9 nm} superlattice irradiated by 2.3 MeV Ar ions to a dose of 2 × 10¹⁴ ions cm⁻² is reduced by <35%, suggesting that heat transport in these short‐period superlattices is dominated by long‐wavelength acoustic phonons. Calculations using a Debye‐Callaway model and the assumption of a boundary scattering rate that varies with phonon‐wavelength successfully capture the temperature, period, and ion‐dose dependence of Λ.     

The role of alloying elements in the design of nickel-base superalloys

The constituents of nickel-base superalloys have been classified into solid solution formers, precipitate formers, carbide formers and surface stabilizers. The characteristics of solutes which would make them most suitable in each category have been specified and appropriate alloying elements have been identified. Nickel-base superalloys are hardened primarily by the precipitation of Ni₃X type compounds. The occurrence and crystallography of precipitation of various kinds of Ni₃X type precipitates have been considered. The role of substitution by alloying elements on mismatch and stability of phases has been discussed. The free electron model and the Engel-Brewer model have been applied for evaluating the stabilities of precipitates, and the role of the alloying elements in determining the stabilities of external and internal surfaces such as grain boundaries have been briefly outlined.