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.     

Synchrotron Radiation for Detecting Movement of a Volatile Soil Liquid

Heat and volatile liquid transport in soils are physically coupled processes. Soil water movement driven by thermal gradients has received considerable attention both experimentally and via models. This is appropriate since the thermal regime of underground electric power cables, pipelines, agricultural soils, and nuclear-waste repositories influences their ability to function effectively. However, available experimental liquid concentration data are of relatively low resolution, generally about one measurement per cm. By using synchrotron x rays we were able to nondestructively obtain high (50 μm) spatial resolution, time-lapsed data for the thermally driven movement of dibromomethane (DBM) in soil. Under repeated temperature cycling during a 22 h period, DBM movement within a sealed, 25-mm-long quartz sand (D50 = 0.25?mm) soil column was highly repeatable. About four complete scans of the column were completed each hour. Average DBM vol?% within the column as a whole was 4.0 and the porosity was 0.31. Measurements of DBM concentration at the same position and equivalent times during different cycles usually differed by less than 0.2 vol?%. Experimental data of this precision will permit better comparison with detailed model predictions.     

Optimum tapered buffer

Driver stages in MOS circuitry have been extensively investigated during the last decade. recently a tapering rule for CMOS buffers was derived showing that the tapering factor (β) is determined by the ratio of output to input capacitance. The derivation fails to account for the correlation between the short-circuit current and β. As a result, the derived formula consistently overpredicts the value of optimum β, especially for large input/output capacitance ratios. The authors present a modified formula and a method to account for the effect of the short-circuit current that is viable for buffer stages over a wide range of output/input capacitance ratios; this newly derived formula accurately predicts the optimum tapering factors for BiCMOS as well as CMOS buffer chains     

Coupling efficiency in grating tuned carbon dioxide waveguide lasers

The coupling efficiency in typical CO₂ waveguide lasers when the feedback element is a diffraction grating is investigated theoretically. A scalar diffraction integral approach is adopted, and the laser is assumed to operate on its lowest loss waveguide mode.     

Design and implementation of a full profile sub-cm ruby laser based Thomson scattering system for MAST

A major upgrade to the ruby Thomson scattering (TS) system has been designed and implemented on the Mega-ampere spherical tokamak (MAST). MAST is equipped with two TS systems, a Nd:YAG laser system and a ruby laser system. Apart from common collection optics each system provides independent measurements of the electron temperature and density profile. This paper focuses on the recent upgrades to the ruby TS system. The upgraded ruby TS system measures 512 points across the major radius of the MAST vessel. The ruby laser can deliver one 10 J 40 ns pulse at 1 Hz or two 5 J pulses separated by 100-800 μs. The Thomson scattered light is collected at F/15 over 1.4 m. This system can resolve small (7 mm) structures at 200 points in both the electron temperature and density channels at high optical contrast; ～50% modulated transfer function. The system is fully automated for each MAST discharge and requires little adjustment. The estimated measurement error for a 7 mm radial point is <4% of T(e) and <3% of n(e) in the range of 40 eV to 2 keV, for a density of n(e)=2×10(19) m(-3). The photon statistics at lower density can be increased by binning in the radial direction as desired. A new intensified CCD camera design allows the ruby TS system to take two snapshots separated with a minimum time of 230 μs. This is exploited to measure two density and temperature profiles or to measure the plasma background light.     

High-power monolithic AlGaN/GaN HEMT oscillator

A monolithic X-band oscillator based on an AlGaN/GaN high electron mobility transistor (HEMT) has been designed, fabricated, and characterized. A common-gate HEMT with 1.5 mm of gate width in conjunction with inductive feedback is used to generate negative resistance. A high Q resonator is implemented with a short-circuit low-loss coplanar waveguide transmission line. The oscillator delivers 1.7 W at 9.556 GHz into 50-/spl Omega/ load when biased at V/sub ds/=30 V and V/sub gs/=-5 V, with dc-to-RF efficiency of 16%. Phase noise was estimated to be -87 dBc/Hz at 100-kHz offset. Low-frequency noise, pushing and pulling figures, and time-domain characterization have been performed. Experimental results show great promise for AlGaN/GaN HEMT MMIC technology to be used in future high-power microwave source applications.     

Detecting non-maxwellian electron velocity distributions at JET by high resolution Thomson scattering

The present work is motivated by a long standing discrepancy between the electron temperature measurements of Thomson scattering (TS) and electron cyclotron emission (ECE) diagnostics for plasmas with strong auxiliary heating observed at both JET and TFTR above 6–7 keV, where in some cases the ECE electron temperature measurements can be 15%–20% higher than the TS measurements. Recent analysis based on ECE results at JET has shown evidence of distortions to the Maxwellian electron velocity distribution and a correlation with the TS and ECE discrepancies has been suggested. In this paper, a technique to determine the presence of non-Maxwellian behavior using TS diagnostics is outlined. The difficulties and limitations of modern TS system designs to determine the electron velocity distribution are also discussed. It is demonstrated that small deviations such as those suggested by previous ECE analysis could be potentially detected, depending on the spectral layout of the TS polychromators. The spectral layout of the JET high resolution Thomson scattering system is such that it could be used to determine these deviations between 1 and 6 keV, and the results presented here indicate that no evidence of non-Maxwellian behavior is observed in this range. In this paper, a modification to the current polychromator design is proposed, allowing non-Maxwellian distortions to be detected up to at least 10 keV.     

Energy Equation for Volatile Liquid Transport in Porous Media

Two energy balance equations widely used to describe simultaneous transfer of heat and mass in porous media are inconsistent with control volume energy conservation. Potential energy, enthalpy, and internal energy terms are involved in the discrepancies. Energy within a volume is properly counted as the sum of internal, potential, and kinetic energy. However, one equation uses enthalpy where internal energy should have been used. In the other, potential energy and shifts in internal energy associated with heat of wetting are not included. Energy conservation for a control volume dictates summing convective fluxes of internal, potential, and kinetic energy at the control volume surface along with conducted heat and work crossing the boundary. The pressure–volume (pv) work at the volume surface may be combined with internal energy convection so that flow of enthalpy is used in the flux term. Examples of energy change versus work input in adiabatic processes illustrate the error introduced when enthalpy rather than internal energy is used to compute control volume energy content. For porous media flows kinetic energy can be dropped. A consistent equation based on the control volume approach is presented. It includes effects due to internal energy, potential energy, heat of wetting, conducted heat, non-pv work, enthalpy, and mass flow. Substantial temperature changes due to heat of wetting have been found experimentally in a separate work. A comparison is needed of the experiments and a numerical simulation based on the new equation.     

Signal generation, control, and frequency conversion AlGaN/GaN HEMT MMICs

We review the design and experimental results of three new AlGaN/GaN high electron-mobility transistor monolithic microwave integrated circuits: a voltage-controlled oscillator (VCO), a single-pole-double-throw switch (SPDT), and a resistive field-effect transistor mixer. The VCO exhibits frequency range between 8.5-9.5 GHz with maximum output power of 35 dBm (at V/sub ds/=30 V) across a 50-/spl Omega/ load. The L/S band SPDT switch at 0.9, 1.8, and 2.1 GHz was measured to have 0.87-, 0.96-, 1-dB insertion loss and 46-, 42-, and 41-dB isolation, respectively. The switch also shows linear performance for the power levels up to 1 W in the insertion mode. A singly ended X-band resistive mixer has exhibited very low intermodulation, less than -60 dBc for the second and third harmonics of the IF at the RF power level of 10 dBm, and high power handling, P/sub 1 dB/ is estimated to be at least 1 W, with the conversion loss of 17 dB.     

Fertilizer 15N Presence in Lysimeter Drainage Water

Agricultural and natural N cycling systems have been intensively studied via the stable isotope 15N. Tracing N translocation over time by applying 15N-tagged fertilizer has been used extensively. Our objective was to quantify fertilizer 15N translocation for an extended time in irrigated lysimeters containing sandy loam soil and cropped with corn and potato. Tagged fertilizer was applied to two lysimeters in 1989 and one in 1990. One lysimeter remained untreated. Breakthrough of 15N, above the background level of 0.366 atom %, was detected in drain effluent from 2.3-m-deep drains on treated lysimeters one year after initial preplant 15N application. Drainage water 15N concentration then increased rapidly to about 0.48 atom %, followed by a gradual decrease. However, 9 years after application there was still elevated 15N, 0.39 atom %, in the drainage water and 26–31% of the applied excess 15N remained in the soil profile. Cumulative 15N removed from lysimeters via drainage water during the first 7 years was between 3.0 and 3.7%. Estimated gaseous loss was 16%.