Search

A temperature-dependent RF large-signal model is constructed by modifying the Verilog-A code of the Angelov model for unique characteristics of GaN MOSHFETs. Different from the previously reported EEHEMT-based model, the present electro-thermal model can fit the temperature effects on threshold shift and transconductance degradation, the drain current in the linear region, and the gate capacitance near the cutoff region. As the result, the power, gain, efficiency, linearity, drain current, and gate current of both Class-A and Class-AB amplifiers are accurately simulated over a wide range of input powers, matching impedances, and ambient temperatures. Index Terms--Gallium compounds, MODFETs, MOSFETs, power amplifiers, pulse measurement, semiconductor device modeling, temperature measurement, thermal resistivity.

The feasibility of submicron low-threshold metal-oxide semiconductor (MOS)-double heterostructure field-effect transistor (DHFET) devices is discussed. Results suggest an effective suppression of the short-channel effect and gate leakage currents in AlGaN-InGaN-GaN MOS-DHFET.

By combining global positioning system observations of surface displacements and the locations of aftershocks, we infer that the 8 October 2005 Kashmir earthquake occurred on multiple fault planes. Mean slip of ~5.1 m occurred on a rupture between Bagh and Balakot with strike 331[degrees] and dip 29[degrees]. Additional slip occurred at depth on a NNE-dipping fault plane extending WNW from Balakot, and on an intersecting nearly flat dislocation at ~5 km depth, forming an active wedge thrust. Both the simple fault plane and the blind wedge accommodate convergence between Peshawar and Leh, Ladakh, accumulating at 7 [+ or -] 2 mm/yr, suggesting a 680 [+ or -] 150 yr recurrence interval for Kashmir 2005-like events. Keywords: Kashmir earthquake, wedge thrust, Himalayan tectonics.

The analytical approach for the designing of the field plate geometry in a general heterostructure field-effect transistor (HFET) structure is discussed. The design helps in achieving the lowest electric field either along the two-dimensional electron gas (2-DEG) channel or along the surface with minimum enhancement of the gate capacitance.

An advantage for some wide bandgap materials, that is often overlooked, is that the thermal coefficient of expansion (CTE) is better matched to the ceramics in use for packaging technology. It is shown that the optimal choice for uni-polar devices is clearly GaN. It is further shown that the future optimal choice for bipolar devices is C (diamond) owing to the large bandgap, high thermal conductivity, and large electron and hole mobilities. A new expression relating the critical electric field for breakdown in abrupt junctions to the material bandgap energy is derived and is further used to derive new expressions for specific on-resistance in power semiconductor devices. These new expressions are compared to the previous literature and the efficacy of specific power devices, such as heterojunction MOSFETs, using GaN are discussed. Index Terms--Breakdown voltage, critical electric field, diamond, GaN, power electronics, SiC, specific on-resistance, wide bandgap.

We describe the properties of novel III-N-based insulating gate heterostructure field-effect transistors (HFETs). For the gate isolation, these devices use either Si[O.sub.2] layer (in metal-oxide-semiconductor HFET (MOSHFET) structures) or [Si.sub.3][N.sub.4] layer (in metal-insulator-semiconductor HFET structures). These insulating gate HFETs have the gate-leakage currents 4-6 orders of magnitude lower than HFETs, even at elevated temperatures up to 300[degrees]C. A double-heterostructure MOSHFET with Si[O.sub.2] gate isolation exhibits current collapse-free performance with extremely low gate-leakage current. Insulating gate devices, including large periphery multigate structures, demonstrate high-power stable operation and might find applications in high-performance power amplifiers and microwave and high-power switches with operating temperatures up to 300[degrees]C or even higher. Index Terms--AlGaN, FET, GaN, heterostructure FET (HFET), microwave, metal-oxide-semiconductor HFET (MOSHFET).

A new study investigates the temperature dependencies of low-frequency noise in heterostructure field-effect transistors.

A new study investigates the high frequency noise characteristics of AlGaN/GaN HEMTs.

The late Proterozoic terrane of the Arabian Shield uniquely exposes the records of magmatic processes and deformation patterns that shaped the Earth over half a billion years ago. This Precambrian to Cambrian terrane is composed of supracrustal rocks underlain and intruded by igneous rocks. The plutonic shedding and accretion of the crest has been studied in detail. Satellite images (TM Landsat and SPOT) were processed to enhance both lithologic content and structural patterns, using spectral transformation and spatial filtering techniques. Results obtained by integrating the image interpretation with detailed ground studies are discussed. The supracrustal rocks display complex fold interference patterns, associated with the emplacement of the basic plutons (Jarshah and Mifsah) and mantled gneiss-domes (Qardath and Miktaa) which took place coeval with crustal shortening by regional deformation. The intrusion of a younger pluton (Arafadi) caused the collision of two pre-existing plutons (Jarshah and Mifsah), involved extensive sideway shedding of supracrustal rocks and formed a major shear zone (Qurayn). Finally, a subvolcanic complex (Mahala) was emplaced, which includes a 2.5-km radius ringdike of several hundred meters thickness. The geologic features of our study area can be explained by a simple model of island arc evolution in six stages: (I + II) Pre-tectonic and pre-plutonic, paleogeographic situation (0.9 to 0.66 Ga); (III) Syntectonic, basic plutonism (0.65 Ga); (IV) Syntectonic gneiss-domes (0.62 Ga); (V) Post-tectonic, acidic plutonism (0.59 to 0.54 Ga); and (VI) Post-orogenic volcanism (0.54 to 0.5 Ga). In our scenario, the relative ages of events inferred on the basis of field relationships are compatible with the geochronologic ages of rock units previously dated by radiometric methods only. The accretion rate of the Arabian Shield region is here estimated at 0.1 km(super 3) yr(super -1) or 10% of the total global accretion rate at the present-day. The average Arabian arc addition rate is 33 km(super 3) km(super -1) Ma(super -1) for the late Proterozoic, which is rather close to estimates of modem island-arc accretion rates of 30 km(super 3) km(super -1) Ma(super -1). We conclude that the process and rate of island-arc accretion onto active continental margins of Arabia in the late Precambrian eon evolved in a fashion similar to that of island-arc accretion in the Phanerozoic eon. This conclusion is important because it counters the view that, based on unrealistic assumptions, accretion rates for the late Proterozoic crest of the Arabian craton were as much as 10 times faster than the modem crustal accretion rates. Keywords: structural geology; geochronology; field geology

Low-frequency noise in n-type gallium nitride (GaN) deposited on sapphire with electron mobility of 790 square cm/V s at 300 K has been measured. Results indicate that the level of flicker noise in GaN is highly dependent on the structure of the material. The effects of band-to-band illumination on the low-frequency noise indicate that 1/f noise in GaN may result from the occupancy fluctuations of the tail states near the band edges.

Detailed mapping and geochronologic investigations from the eastern, southern, and western Nanga Parbat-Haramosh massif reveal two thrust-displacement shear zones that have a spatial and temporal link with granite plutonism from ca. 10 to 1 Ma. The shear zones define a crustal-scale antiformal pop-up structure, with dominant west-northwest-vergent and subordinate east-southeast-vergent thrusting. This is substantially different than the surrounding area where the main exposed Himalayan structures are oriented parallel to the orogenic trend and are early to middle Miocene or older. Structures mapped throughout Nanga Parbat demonstrate that its rapid and young exhumation is not due to orogen-scale structural unroofing, and that sustained high erosion rates are required. The observed west-northwest-directed shortening is proposed to be a result of differential arc-parallel motion accommodated at the syntaxial bend of the northwest Himalaya.

The persistent photoconductivity effect linked to a two-dimensional electron gas (2DEG) in an AlGaN/GaN heterojunction device has been established. Permanent photoinduced increase in 2DEG carrier mobility and density has a causal relationship with the device. Photoexcited carrier density and 2DEG carrier mobility have a positive linear correlation. Device applications using AlGaN/GaN heterojunctions are covered.

Surface polygonization is the process by which a representative polygonal mesh of a surface is constructed for rendering or analysis purposes. This work presents a new surface polygonization algorithm specifically tailored to be applied to a large class of models which are created with parametric surfaces having triangular meshes. This method has particular application in the area of building virtual environments from computer-aided-design (CAD) models. The algorithm is based on an edge reduction scheme that collapses two vertices of a given triangular polygon edge onto one new vertex. A two step approach is implemented consisting of boundary edge reduction followed by interior edge reduction. A maximum optimization is used to determine the location of the new vertex. The criterion that is used to control how well the approximate surface represents the actual surface is based on examining the angle between surface normals. The advantage of this approach is that the surface discretization is a function of two, user-controlled variables, a boundary edge angle error and a surface edge angle error. The method presented here differs from existing methods in that it takes advantage of the fact that for many models, the exact surface representation of the model is known before the polygonization is attempted. Because the precise surface definition is known, a maximum optimization procedure, that uses the surface information, can be used to locate the new vertex. The algorithm attempts to overcome the deficiencies in existing techniques while minimizing the number of triangular polygons required to represent a surface and still maintaining surface integrity in the rendered model. This paper presents the algorithm and testing results.

Large scale biostratigraphic analysis of Waziristan and Kurram in northwest Pakistan reveals that the accretionary prism and the trench strata were the first to collide with the northwest Indian passive margin during the India-Asia collision. The collision produced significant changes in Cenozoic and Cretaceous oceanography, and faunal life and climate.

A study on the chemical composition of chromium aluminum ferric iron spinels from the Chilas complex in Pakistan is presented. Results indicate that chromium aluminum ferric iron oxides undergo a variety of oxide phases. Furthermore, findings show that medium to high-grade metamorphic rocks are the most likely place for exsolved and coexisting pairs of spinel and magnetite to occur.