Search

Low-temperature MOS-gated Hall measurements and gated diode capacitance-voltage (C-V) measurements are used for characterizing both trap density and Hall mobility on 4H-silicon carbide MOSFETs with gate oxides grown by sodium enhanced oxidation (SEO) and thermally grown in [N.sub.2]O. The temperature-dependent Hall mobility measurements have revealed that inversion layer mobility in both the SEO and baseline samples are limited by Coulomb scattering from interface trapped charge and surface roughness.

The main scattering mechanisms limiting mobility in SiC MOSFETs were examined with the help of Hall-effect mechanisms of n-channel MOS devices. Findings reveal that surface-roughness scattering and Coulomb scattering were the main scattering mechanisms limiting electron mobility found in SiC MOSFETs at room temperature.

Research involving high-field effects in GaN MODFETs is described.

Intrinsic noise sources and their correlation in gallium-nitride high electron-mobility transistors are extracted and studied. Microwave noise measurements have been performed over the frequency range of 0.8-5.8 GHz. Using measured noise and scattering parameter data, the gate and drain noise sources and their correlation are determined using an equivalent-circuit representation. This model correctly predicts the frequency-dependent noise for two devices having different gate length. Three noise mechanisms are identified in these devices, namely, those due to velocity fluctuation, gate leakage, and traps. Index Terms--Correlation, leakage currents, semiconductor device noise, shot noise, white noise.

Research involving s low transients observed in AlGaN/GaN HFETs that are subjected to high bias stress is described and discussed.

Dynamic loadline analysis illustrating the microwave performance limits of state-of-the-art AlGaN/GaN high electron-mobility transistors (HEMTs) under pulsed RF biasing and drive conditions are presented. Calculation of dynamic loadlines concurrent with load-pull measurements show the increase of the device RF knee voltage with increasing drain voltage as the cause of reduced output power-added efficiency (PAE) at high drain biases. In this study, an 8-GHz saturated output power of 14.1 W (9.4 W/mm) is achieved on a 1500 x 0.25 x /[micro][m.sup.2] AlGaN/GaN HEMT at a pulsed drain bias of [V.sub.D] = 40 V. The pulsed-bias conditions considered here preclude device self-heating as a mechanism responsible for the lower than expected output power and decrease in PAE with increasing drain bias. These data suggest electron trapping associated with the surface of the device between the gate and drain as the mechanism that limit the ultimate power, PAE, and linearity of AlGaN/GaN HEMTs. Index Terms--Dynamic loadline, gallium nitride, high electron-mobility transistor (HEMT), silicon carbide.

Broad-band high-power cascode AlGaN/GaN high electron-mobility transistor monolithic-microwave integrated-circuit (MMIC) amplifiers with high gain and power-added efficiency (PAE) have been fabricated on high-thermal conductivity SiC substrates. A cascode gain cell exhibiting 5 W of power at 8 GHz with a small-signal gain of 19 dB was realized. A nonuniform distributed amplifier (NDA) based on this process was designed, fabricated, and tested, yielding a saturated output power of 3-6 W over a dc-8-GHz bandwidth with an associated PAE of 13%-31%. A broad-band amplifier MMIC using cascode cells in conjunction with a lossy-match input matching network showed a useful operating range of dc-8 GHz with an output power of 5-7.5 W and a PAE of 20%-33% over this range. The third-order intermodulation products of the amplifiers under two-tone excitation were studied and third-order-intercept values of 42 and 43 dBm (computed using two-tone carrier power) for the lossy match and NDA amplifiers were obtained. Index Terms--Broad-band amplifier, cascode, distributed amplifier, GaN, high electron-mobility transistor, silicon carbide.

The fabrication of high electron mobility transistors using undoped AlGaN/GaN structures and the induction of a two-dimensional electron gas are described.

The interaction of cesium (Cs) atom/ion with the oxidant and carbon cluster defects at the SiC/Si[O.sub.2] interface is examined by using atomistic scale computational methods and characterization methods. The results have shown that Cs has not reduced the penetration barrier of the impinging oxidant.

Raman spectroscopy is of significant importance in industrial gas analysis due to its unique capability of quantitative multigas measurement, especially diatomics (N2 and H2), with a single laser. This paper presents the development of a gas analyzer system based on high pressure Raman scattering in a multipass Raman cell and demonstrates its feasibility for real-time natural gas analysis. A 64-pass Raman cell operated at elevated pressure (5 bar) is used to create multiplicative enhancement (proportional to number of passes times pressure) of the natural gas Raman signal. A relatively low power 532-nm continuous wave laser beam (200 mW) is used as the source and the signals are measured through a cooled charge-coupled device grating spectrometer (30-s exposure). A hybrid algorithm based on background-correction and least-squares error minimization is used to estimate gas concentrations. Individual gas component concentration repeatability of the order of 0.1% is demonstrated. Further, the applicability of the technique for natural gas analysis is demonstrated through measurements on calibrated gas mixtures. Experimental details, analyzer characterization, and key measurements are presented to demonstrate the performance of the technique. [ABSTRACT FROM AUTHOR]

This paper presents the first diode laser-based measurements of trace hydrogen sulfide at the 2646.35 nm absorption line and evaluates the feasibility of this line for natural gas sensing applications. The balanced detection technique is used to achieve a detection limit of 25 parts per million-meter H2S in 150 Torr (T) balance N2. Spectral interference effects of major natural gas constituents (methane, ethane, and propane) are also presented. It is shown that methane interference can be avoided by lowering the sample pressure, while propane has no interference. However, strong interfering lines of ethane are observed around the 2646.35 nm region. To our knowledge, this is the first report where individual ethane lines in this spectral region are observed. These observations make the 2646.35 nm H2S line unfavorable for natural gas applications. This line, however, still remains attractive for H2S sensing applications with low ethane content. [ABSTRACT FROM AUTHOR]

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-Ω 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. [ABSTRACT FROM AUTHOR]

Very slow drain current and surface potential transients have been observed in AlGaN/GaN heterostructure field effect transistors that are subjected to high bias stress. Simultaneous measurements of drain current and surface potential indicate that large change in surface potential after stress is responsible for the reduction in drain current in these devices. Measurements of surface potential profile from the gate edge toward the drain as a function of time indicate that surface potential changes occur mostly near the gate. It is proposed that the surface potential changes are caused by electrons which tunnel from the gate under high bias stress and get trapped at the surface states near the gate. Passivation of the surface with SiN[sub ℵ] reduces the transient magnitudes to a large extent. This correlates with a large improvement in microwave power performance in these devices after passivation. UV illumination of these devices totally eliminates the drain current and surface potential transients. [ABSTRACT FROM AUTHOR]

Gallium nitride cantilevers with metal-insulator-semiconductor capacitors were fabricated and characterized as a function of strain to determine the effective piezoelectric constant. These cantilevers were tested at room temperature through a combination of cantilever bending experiments, strain gauge measurements, and finite element modeling. By measuring the shift in flatband voltage to applied strain the magnitude of the piezoelectric charge induced is determined. The effective piezoelectric constant e31′ was determined from these measurements to be -0.57±0.03 C/m2. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The properties of the SiO2/GaN interface were characterized using metal-oxide-semiconductor capacitors on polar c-plane (0001) and nonpolar m-plane (0 1 -1 0) GaN crystal faces. GaN m-plane samples show the absence of pyroelectric polarization effects. Additionally, capacitance-voltage hysteresis is less on m-plane compared to c-plane GaN surfaces, suggesting a lower interface-state density at the m-plane GaN/SiO2 interface. [ABSTRACT FROM AUTHOR]