Lag Measurements Research Articles

Groundwater recharge and time lag measurement through Vertosols using impulse response functions

Summary Throughout the world there are many stressed aquifers used to support irrigated agriculture. The Condamine River catchment (southern Queensland, Australia) is one example of a globally significant agricultural region where groundwater use has exceeded recharge over the last 50 years. There is a high dependence on groundwater in this catchment, because yearly rainfall is highly variable, and actual evapotranspiration often exceeds rainfall. To better manage the aquifer there is a need to correctly conceptualise the primary inputs and outputs of the system, and characterise the lags in system response to all forcings. In catchment models it is particularly important to correctly proportion diffuse (areal) rainfall recharge and to account for the lag between rainfall and recharge at the water table. Throughout large portions of the Condamine Catchment, groundwater levels are now 20 or more metres below the ground surface. This study aimed to better quantify the lag between rainfall and recharge at the water table using the predefined impulse response function in continuous time method (PIRFICT; von Asmuth et al., 2002; von Asmuth, 2012). The PIRFICT method was applied to 255 multi-decadal groundwater level data sets throughout the catchment. Inputs into the modelling include rainfall, irrigation deep drainage, stream water level, evapotranspiration, and groundwater extractions. As an independent check the PIRFICT model derived diffuse recharge estimates are compared to point lysimeter and geochemical recharge estimates in the Vertosol soils within this catchment. It is estimated using the PIRFICT method that in the Condamine Catchment between 1990 and 2012, the mean rain-derived groundwater recharge is 4.4 mm/year. Mean groundwater response from rainfall was determined to be 5.3 years: range 188 days to 48 years. The recharge estimates are consistent with both geochemical and lysimeter point measurements of recharge. It is concluded that where extensive groundwater level and climatic data sets are available the PIRFICT method provides an independent assessment of recharge. Importantly, the PIRFICT method highlights the need to consider the lag between rainfall and recharge at the water table. This lag is often overlooked when calibrating spatial distributed water balance models used to guide groundwater allocations.

Correlated X-ray/ultraviolet/optical variability in NGC 6814

We present results of a 3-month combined X-ray/UV/optical monitoring campaign of the Seyfert 1 galaxy NGC 6814. The object was monitored by Swift from June through August 2012 in the X-ray and UV bands and by the Liverpool Telescope from May through July 2012 in B and V. The light curves are variable and significantly correlated between wavebands. Using cross-correlation analysis, we compute the time lag between the X-ray and lower energy bands. These lags are thought to be associated with the light travel time between the central X-ray emitting region and areas further out on the accretion disc. The computed lags support a thermal reprocessing scenario in which X-ray photons heat the disc and are reprocessed into lower energy photons. Additionally, we fit the lightcurves using CREAM, a Markov Chain Monte Carlo code for a standard disc. The best-fitting standard disc model yields unreasonably high super-Eddington accretion rates. Assuming more reasonable accretion rates would result in significantly under-predicted lags. If the majority of the reprocessing originates in the disc, then this implies the UV/optical emitting regions of the accretion disc are farther out than predicted by the standard thin disc model. Accounting for contributions from broad emission lines reduces the lags in B and V by approximately 25% (less than the uncertainty in the lag measurements), though additional contamination from the Balmer continuum may also contribute to the larger than expected lags. This discrepancy between the predicted and measured interband delays is now becoming common in AGN where wavelength-dependent lags are measured.

Identification of trap locations in AlGaN/GaN high electron mobility transistors by varying photon flux during sub-bandgap optical pumping

By varying the photon flux during sub-bandgap optical pumping, the locations of traps associated with sub-bandgap laser pumping in AlGaN/GaN high electron mobility transistors (HEMTs) were identified. Fixed photon flux of sub-bandgap optical pumping was previously employed to determine the activation energies of traps in AlGaN/GaN HEMT; traps with activation energies of 0.73 eV, both 0.73 and 1.91 eV or all three trap states detected for the 671, 532, or 477 nm laser illumination, respectively. However, by illuminating the HEMTs with lower photon fluxes from 532 or 447 nm lasers, the traps with 0.73 eV were absent, and only the traps associated with higher activation energies were activated by the laser light. This indicated that the traps related to the activation energies of 1.91 and 2.35 eV are likely located at the surface of the HEMTs. Photon flux dependent gate-lag measurements were also conducted. Laser illumination of 671 nm did not affect the drain current during the gate lag measurement, implyin...

Simulations of the OzDES AGN reverberation mapping project

As part of the Australian spectroscopic dark energy survey (OzDES) we are carrying out a large-scale reverberation mapping study of ≥500 quasars over five years in the 30 deg2 area of the Dark Energy Survey (DES) supernova fields. These quasars have redshifts ranging up to 4 and have apparent AB magnitudes between 16.8 mag < r < 22.5 mag. The aim of the survey is to measure time lags between fluctuations in the quasar continuum and broad emission-line fluxes of individual objects in order to measure black hole masses for a broad range of active galactic nuclei (AGN) and constrain the radius–luminosity (R–L) relationship. Here we investigate the expected efficiency of the OzDES reverberation mapping campaign and its possible extensions. We expect to recover lags for ∼35–45 per cent of the quasars. AGN with shorter lags and greater variability are more likely to yield a lag measurement, and objects with lags ≲6 months or ∼1 yr are expected to be recovered the most accurately. The baseline OzDES reverberation mapping campaign is predicted to produce an unbiased measurement of the R–L relationship parameters for H β, Mg ii λ2798, and C iv λ1549. Extending the baseline survey by either increasing the spectroscopic cadence, extending the survey season, or improving the emission-line flux measurement accuracy will significantly improve the R–L parameter constraints for all broad emission lines.

Study of the Effects of GaN Buffer Layer Quality on the dc Characteristics of AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN high electron mobility transistors (HEMTs) are promising candidates for high power and high current applications with its high mobility and high breakdown field. Epitaxial films for device substrate are typically grown on sapphire or silicon carbide, SiC, with dislocation densities in the order of 108 - 1010cm-2 due to their thermal coefficient and lattice mismatches. The thinner GaN buffer layers on SiC have been reported to have higher threading dislocation density in the GaN, but lead to higher off-state breakdown voltages. However, thick GaN buffer layers on sapphire and their impact on dc performance have not been widely studied. In this study, the impact of GaN buffer layer quality on dc and gate-lag pulse performance of AlGaN/GaN HEMTs was studied. Transmission electron microscopy (TEM) was used to measure the dislocation density in the GaN buffer layer of the HEMT samples. Dc characteristics, including drain I-Vs, transfer characteristics, and gate pulsed drain I-V characteristics of the HEMTs, were measured to establish a correlation between the AlGaN/GaN HEMTs’ dc and gate-pulsed performance and GaN buffer layer quality. AlGaN/GaN HEMT structures grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrates were acquired from two different vendors. Both types of wafers had 24% Al concentration in the AlGaN barrier layer. One type of wafer had a 5 µm GaN buffer and the other had a 2 µm GaN buffer layer. HEMT fabrication was started with mesa patterning by conventional optical lithography. The mesa-etching was achieved using a unaxis shuttle-lock reactive ion etcher with inductively coupled plasma module (ICP) for device isolation with a Cl2/Ar plasma. Ohmic metallization of electron-beam evaporated Ti/Al/Ni/Au (25 nm/125 nm/45 nm/100 nm) was patterned by lift-off and subsequently rapid thermally annealed at 850⁰C in a flowing N2 ambient for 45 seconds. Schottky gates of e-beam evaporated Ni/Au (20 nm/80 nm) with 100 μm width and 1 μm length were obtained by standard lift-off patterning. DC current–voltage (I-V) characteristics were measured using an HP4156 parameter analyzer, and gate-lag pulse measurements were measured using an Agilent 8114A pulse generator. Figure 1 shows the bright field cross-sectional transmission electron microscopy (BF-XTEM) pictures of AlGaN/GaN surface for the top layers of (a) HEMT structure with 2 µm GaN buffer layer, (b) HEMT structure with 5 μm GaN buffer layer as well as the GaN/sapphire interface for the (c) HEMT structure with 2 μm GaN buffer layer, and (d) HEMT structure with 5 μm GaN buffer layer. Dislocations are seen to originate near the GaN/sapphire interface and propagate up through the AlGaN/GaN interface to the surface of both HEMT wafers. However, it is clear that the HEMT structure with 2 μm GaN buffer layer exhibited a much higher defect density. The calculated defect density for 2 μm GaN buffer layer substrate had an order of magnitude higher defect density. However, 5μm GaN buffer layer wafer showed that it was under high strain with larger height difference across the 2 inch with lower radius of curvature. There was no major difference in HEMTs’ dc performance was observed. In the other hand, Figure 2 (a) and (b) shows that gate lag measurement on fabricated HEMTs on 5 μm GaN buffer layer wafer showed no degradation and on 2 μm GaN buffer layer wafer showed the otherwise. The effects of defect density on HEMT dc and radio frequency performance were studied. HEMTs were fabricated on 2 and 5 μm of GaN buffer layers with the same composition of active layers. Despite there was no significant difference was seen in dc characteristics, significant reduction in pulse measurements was observed on thinner GaN layer HEMTs. These results suggest that the defects created in GaN buffer layer do not largely influence device dc performance but are closely related to high frequency performance. Figure 1

Technical Note: Intrafractional changes in time lag relationship between anterior-posterior external and superior-inferior internal motion signals in abdominal tumor sites.

To investigate constancy, within a treatment session, of the time lag relationship between implanted markers in abdominal tumors and an external motion surrogate. Six gastroesophageal junction and three pancreatic cancer patients (IRB-approved protocol) received two cone-beam CTs (CBCT), one before and one after treatment. Time between scans was less than 30 min. Each patient had at least one implanted fiducial marker near the tumor. In all scans, abdominal displacement (Varian RPM) was recorded as the external motion signal. Purpose-built software tracked fiducials, representing internal signal, in CBCT projection images. Time lag between superior-inferior (SI) internal and anterior-posterior external signals was found by maximizing the correlation coefficient in each breathing cycle and averaging over all cycles. Time-lag-induced discrepancy between internal SI position and that predicted from the external signal (external prediction error) was also calculated. Mean ± standard deviation time lag, over all scans and patients, was 0.10 ± 0.07 s (range 0.01-0.36 s). External signal lagged the internal in 17/18 scans. Change in time lag between pre- and post-treatment CBCT was 0.06 ± 0.07 s (range 0.01-0.22 s), corresponding to 3.1% ± 3.7% (range 0.6%-10.8%) of gate width (range 1.6-3.1 s). In only one patient, change in time lag exceeded 10% of the gate width. External prediction error over all scans of all patients varied from 0.1 ± 0.1 to 1.6 ± 0.4 mm. Time lag between internal motion along SI and external signals is small compared to the treatment gate width of abdominal patients examined in this study. Change in time lag within a treatment session, inferred from pre- to post-treatment measurements is also small, suggesting that a single measurement of time lag at the session start is adequate. These findings require confirmation in a larger number of patients.

ALMA and VLA measurements of frequency-dependent time lags in Sagittarius A*: evidence for a relativistic outflow

Radio and mm-wavelength observations of Sagittarius A* (Sgr A*), the radio source associated with the supermassive black hole at the center of our Galaxy, show that it behaves as a partially self-absorbed synchrotron-emitting source. The measured size of Sgr A* shows that the mm-wavelength emission comes from a small region and consists of the inner accretion flow and a possible collimated outflow. Existing observations of Sgr A* have revealed a time lag between light curves at 43 GHz and 22 GHz, which is consistent with a rapidly expanding plasma flow and supports the presence of a collimated outflow from the environment of an accreting black hole. Here we wish to measure simultaneous frequency-dependent time lags in the light curves of Sgr A* across a broad frequency range to constrain direction and speed of the radio-emitting plasma in the vicinity of the black hole. Light curves of Sgr A* were taken in May 2012 using ALMA at 100 GHz using the VLA at 48, 39, 37, 27, 25.5, and 19 GHz. As a result of elevation limits and the longitude difference between the stations, the usable overlap in the light curves is approximately four hours. Although Sgr A* was in a relatively quiet phase, the high sensitivity of ALMA and the VLA allowed us to detect and fit maxima of an observed minor flare where flux density varied by ~10%. The fitted times of flux density maxima at frequencies from 100 GHz to 19 GHz, as well as a cross-correlation analysis, reveal a simple frequency-dependent time lag relation where maxima at higher frequencies lead those at lower frequencies. Taking the observed size-frequency relation of Sgr A* into account, these time lags suggest a moderately relativistic (lower estimates: 0.5c for two-sided, 0.77c for one-sided) collimated outflow.

Pseudo flow-injection ESR technique combining spin-trapping and application to the evaluation of superoxide scavenging capacity of phenolic compound

Evaluation methods of superoxide scavenging capacity have been studied by many researchers. One of the methods is a spintrapping electron spin resonance (ESR) technique, in which 5,5-diemthyl-1-pyrroline 1-oxide (DMPO) was usually used as a spintrapping reagent. In previous reports, the evaluation method based on a competition reaction and spin-trapping ESR technique were proposed. However, a big key issue and unavoidable problem remained in this evaluation. This is the rapid decrease of DMPO-O2 adduct by the following chemical reactions. Therefore, the initial signal intensity of DMPO-O2 has to be estimated with an approximation from a decreasing curve. This approximation, however, includes at least a thirty-second time lag before starting an ESR measurement. So, the measurement of a shorter time lag is required to obtain a more accurate initial intensity of DMPO-O2 - adduct. In this study, the pseudo flow-injection ESR method using a peristaltic pump was adopted to inject the test solution directly into a quartz cell in ESR cavity. This method achieved a 10 times shorter time lag (ca. 3 seconds) compared with the previous method (ca. 30 seconds) using a hematocrit capillary. As a consequence, the initial signal intensity within 10 % error was obtained without any estimation. The second-order rate constants of the reaction between various phenolic compounds and superoxide were obtained as one of the applications of this method.

Iron K and Compton hump reverberation in SWIFT J2127.4+5654 and NGC 1365 revealed by NuSTAR and XMM–Newton

In the past five years, a flurry of X-ray reverberation lag measurements of accreting supermassive black holes have been made using the XMM-Newton telescope in the 0.3-10 keV energy range. In this work, we use the NuSTAR telescope to extend the lag analysis up to higher energies for two Seyfert galaxies, SWIFT J2127.4+5654 and NGC 1365. X-ray reverberation lags are due to the light travel time delays between the direct continuum emission and the reprocessed emission from the inner radii of an ionised accretion disc. XMM-Newton has been particularly adept at measuring the lag associated with the broad Fe K emission line, where the gravitationally redshifted wing of the line is observed to respond before the line centroid at 6.4 keV, produced at larger radii. Now we use NuSTAR to probe the lag at higher energies, where the spectrum shows clear evidence for Compton reflection, known as the Compton 'hump'. The XMM-Newton data show Fe K lags in both SWIFT J2127.4+5654 and NGC 1365. The NuSTAR data provide independent confirmation of these Fe K lags, and also show evidence for the corresponding Compton hump lags, especially in SWIFT J2127.4+5654. These broadband lag measurements confirm that the Compton hump and Fe K lag are produced at small radii. At low-frequencies in NGC 1365, where the spectrum shows evidence for eclipsing clouds in the line of sight, we find a clear negative (not positive) lag from 2--10 keV, which can be understood as the decrease in column density from a neutral eclipsing cloud moving out of our line of sight during the observation.

Modelling reverberation mapping data – I. Improved geometric and dynamical models and comparison with cross-correlation results

We present an improved and expanded simply parameterized phenomenological model of the broad line region (BLR) in active galactic nuclei (AGN) for modeling reverberation mapping data. By modeling reverberation mapping data directly, we can constrain the geometry and dynamics of the BLR and measure the black hole mass without relying on the normalization factor needed in the traditional analysis. For realistic simulated reverberation mapping datasets of high-quality, we can recover the black hole mass to $0.05-0.25$ dex uncertainty and distinguish between dynamics dominated by elliptical orbits and inflowing gas. While direct modeling of the integrated emission line light curve allows for measurement of the mean time lag, other details of the geometry of the BLR are better constrained by the full spectroscopic dataset of emission line profiles. We use this improved model of the BLR to explore possible sources of uncertainty in measurements of the time lag using cross-correlation function (CCF) analysis and in measurements of the black hole mass using the virial product. Sampling the range of geometries and dynamics in our model of the BLR suggests that the theoretical uncertainty in black hole masses measured using the virial product is on the order of 0.25 dex. These results support the use of the CCF to measure time lags and the virial product to measure black hole masses when direct modeling techniques cannot be applied, provided the uncertainties associated with the interpretation of the results are taken into account.

Suitability of magnetic single- and multi-core nanoparticles to detect protein binding with dynamic magnetic measurement techniques

We investigated the binding of biotinylated proteins to various streptavidin functionalized magnetic nanoparticles with different dynamic magnetic measurement techniques to examine their potential for homogeneous bioassays. As particle systems, single-core nanoparticles with a nominal core diameter of 30 nm as well as multi-core nanoparticles with hydrodynamic sizes varying between nominally 60 nm and 100 nm were chosen. As experimental techniques, fluxgate magnetorelaxometry (MRX), complex ac susceptibility (ACS) and measurements of the phase lag between rotating field and sample magnetization are applied. MRX measurements are only suited for the detection of small analytes if the multivalency of functionalized nanoparticles and analytes causes cross-linking, thus forming larger aggregates. ACS measurements showed for all nanoparticle systems a shift of the imaginary part's maximum towards small frequencies. In rotating field measurements only the single-core nanoparticle systems with dominating Brownian mechanism exhibit an increase of the phase lag upon binding in the investigated frequency range. The coexistence of Brownian and Neel relaxation processes can cause a more complex phase lag change behavior, as demonstrated for multi-core nanoparticle systems.

Caught in the act: measuring the changes in the corona that cause the extreme variability of 1H 0707−495

The X-ray spectra of the narrow line Seyfert 1 galaxy, 1H 0707-495, obtained with XMM-Newton, from time periods of varying X-ray luminosity are analysed in the context of understanding the changes to the X-ray emitting corona that lead to the extreme variability seen in the X-ray emission from active galactic nuclei (AGN). The emissivity profile of the accretion disc, illuminated by the X-ray emitting corona, along with previous measurements of reverberation time lags are used to infer the spatial extent of the X-ray source. By fitting a twice-broken power law emissivity profile to the relativistically-broadened iron K fluorescence line, it is inferred that the X-ray emitting corona expands radially, over the plane of the accretion disc, by 25 to 30 per cent as the luminosity increases, contracting again as the luminosity decreases, while increases in the measured reverberation lag as the luminosity increases would require also variation in the vertical extent of the source above the disc. The spectrum of the X-ray continuum is found to soften as the total X-ray luminosity increases and we explore the variation in reflected flux as a function of directly-observed continuum flux. These three observations combined with simple, first-principles models constructed from ray tracing simulations of extended coron self-consistently portray an expanding corona whose average energy density decreases, but with a greater number of scattering particles as the luminosity of this extreme object increases.

High breakdown voltage in AlN/GaN metal–insulator–semiconductor high-electron-mobility transistors

The breakdown characteristics of AlGaN/GaN based metal–insulator–semiconductor high-electron-mobility transistors (MISHEMTs) using a 10 nm thick AlN gate insulator and passivation layer deposited plasma enhanced atomic layer deposition. The AlN was effective in significantly reducing gate leakage current relative to Schottky gate devices and showed only small decreases in drain current during gate lag measurements. The devices exhibited a strong dependence of gate breakdown voltage on source–drain distance, reaching a value of 2000 V for a source–drain distance of 40 μm limited by the measurement instrument. The specific on-state resistance was 1.3 and 10.9 mΩ cm2 for the devices with the gate–drain distance of 7.5 and 37.5 μm, respectively. The saturation drain current was inversely dependent on source–drain distance and the on–off ratios were in excess of 108 due to the low gate leakage current in the MISHEMTs.

Time series analysis of active galactic nuclei: The case of Arp 102B, 3C 390.3, NGC 5548 and NGC 4051

We used the Z-transformed Discrete Correlation Function (ZDCF) and the Stochastic Process Estimation for AGN Reverberation (SPEAR) methods for the time series analysis of the continuum and the Hα and Hβ line fluxes of a sample of well known type 1 active galactic nuclei (AGNs): Arp 102B, 3C 390.3, NGC 5548, and NGC 4051, where the first two objects are showing double-peaked emission line profiles. The aim of this work is to compare the time lag measurements from these two methods, and check if there is a connection with other emission line properties. We found that the obtained time lags from Hβ are larger than those derived from the Hα analysis for Arp 102B, 3C 390.3 and NGC 5548. This may indicate that the Hβ line originates at larger radii in these objects. Moreover, we found that the ZDCF and SPEAR time lags are highly correlated (r∼0.87), and that the error ranges of both ZDCF and SPEAR time lags are correlated with the FWHM of used emission lines (r∼0.7). This increases the uncertainty of the black hole mass estimates using the virial theorem for AGNs with broader lines.

SU‐E‐J‐126: Respiratory Gating Quality Assurance: A Simple Method to Achieve Millisecond Temporal Resolution

Purpose:Low temporal latency between a gating on/off signal and a linac beam on/off during respiratory gating is critical for patient safety. Although, a measurement of temporal lag is recommended by AAPM Task Group 142 for commissioning and annual quality assurance, there currently exists no published method. Here we describe a simple, inexpensive, and reliable method to precisely measure gating lag at millisecond resolutions.Methods:A Varian Real‐time Position Management™ (RPM) gating simulator with rotating disk was modified with a resistive flex sensor (Spectra Symbol) attached to the gating box platform. A photon diode was placed at machine isocenter. Output signals of the flex sensor and diode were monitored with a multichannel oscilloscope (Tektronix™ DPO3014). Qualitative inspection of the gating window/beam on synchronicity were made by setting the linac to beam on/off at end‐expiration, and the oscilloscope's temporal window to 100 ms to visually examine if the on/off timing was within the recommended 100‐ms tolerance. Quantitative measurements were made by saving the signal traces and analyzing in MatLab™. The on and off of the beam signal were located and compared to the expected gating window (e.g. 40% to 60%). Four gating cycles were measured and compared.Results:On a Varian TrueBeam™ STx linac with RPM gating software, the average difference in synchronicity at beam on and off for four cycles was 14 ms (3 to 30 ms) and 11 ms (2 to 32 ms), respectively. For a Varian Clinac™ 21EX the average difference at beam on and off was 127 ms (122 to 133 ms) and 46 ms (42 to 49 ms), respectively. The uncertainty in the synchrony difference was estimated at ±6 ms.Conclusion:This new gating QA method is easy to implement and allows for fast qualitative inspection and quantitative measurements for commissioning and TG‐142 annual QA measurements.

Effect of Gamma Irradiation on DC Performance of Circular-Shaped AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN High Electron Mobility Transistors (HEMTs) show great promise for applications such as military radar and satellite-based communications systems. Due to the applications in extreme radiation environments, it is important to characterize the performance after irradiation. There are many studies characterizing the performance under proton, neutron, and electron irradiations for AlGaN/GaN HEMTs. However, the studies for gamma irradiation are limited. Detailed studies are needed to characterize the performance of HEMTs after gamma irradiation. In this study, we characterize the DC performance after irradiation and extract the change of Vth, carrier concentration, mobility and resistance. Also, we use gate lag measurement and drain pulse measurement to characterize the defects generated after irradiation. AlGaN/GaN HEMT heterostructures were grown on c-plane sapphire by molecular beam epitaxy (MBE). The layer structure included a thin AlGaN nucleation layer followed with a 2 µm thick undoped GaN buffer topped by a 25 nm thick unintentionally doped Al0.28GaN layer. Mesa isolation was achieved by using an inductively coupled plasma system with Ar/Cl2-based discharges. The Ohmic contacts were formed by lifting-off e-beam evaporated Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (800 Å). 1-µm gates were defined by lift-off of e-beam deposited Ni/Au metallization. The contacts were annealed at 850°C for 45 s under a flowing nitrogen ambient in Heatpulse 610T system. The final step was deposition of e-beam evaporated Ti/Au (300 Å/1200 Å) interconnection contacts. Devices were exposed to 60Co γ-rays with different doses of 50, 300, 450, or 700 Gy. Irradiations were performed at temperatures <50°C. During device irradiation, the samples were held in nitrogen ambient. The device DC performance was characterized using HP 4156 parameter analyzer.As shown in Figure 1, IDSS after irradiation was increased from 293mA/mm to 339mA/mm after irradiation. The increase of IDSSincreases with dose before 450 Gy and saturates after that. From the transmission line method (TLM), there is no significant change in sheet resistance after irradiation. However, the device total resistance, the reciprocal of the current current voltage (I-V) curve slope in the lower drain bias voltage, decreased significantly, as illustrated in Figure 1. The total resistance consisted of the channel resistance and the source resistance as well as the drain resistance. Both The source resistance and the drain resistance determined from the transmission line method change less than 2%. While the channel resistance decreased around 5% as shown in Figure 2. The reduction of the channel resistance was mainly due to the increase of mobility and carrier concentration, which were extracted from the drain IV curve in the low field region and TLM data. Kurakin et. al 1 found a similar effect that mobility increases after irradiation due to the relaxation of strain. Berthet et. al 2 attributed the drain current increases after irradiation to the introduction of donor type defects through the implantation. From gate lag measurement, the dispersion of the drain current measured at 100kHz increased as a function of the gamma irradiation dose indicating additional traps generated between gate and drain electrode and enhanced the current collapse. There was no obvious current dispersion during the drain pulse measurement showing the defects created by the gamma irradiation were less than the original defects through the material growth in the buffer layer.In conclusion, low dose gamma irradiation improved the AlGaN/GaN HEMT dc performance. The effect may be due to the strain relaxation effect and due to the donor type defects generated during the gamma irradiation.

Effect of Proton Irradiation on DC Performance and Reliability of Circular-Shaped AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN High Electron Mobility Transistors (HEMTs) show great promise for applications such as military radar and satellite-based communications systems. Circular devices, with smaller junction areas and higher effective gate width/gate length, are better suited for high power, high frequency application. Since the circular devices have been shown to improve the device performance, the effect of radiation on their reliability is an important topic. There are many studies characterizing the performance under proton, neutron, and electron irradiations for rectangular AlGaN/GaN HEMTs. However, the studies for circular-shaped devices are limited. In this work, we report the effect of proton irradiation on circular-based AlGaN/GaN HEMT. DC performance, breakdown voltage, and gate lag measurement were done to characterize the devices.AlGaN/GaN HEMT heterostructures were grown on c-plane sapphire by molecular beam epitaxy (MBE). The layer structure included a thin AlGaN nucleation layer followed with a 2 µm thick undoped GaN buffer topped by a 25 nm thick unintentionally doped Al0.28GaN layer. Mesa isolation was achieved by using an inductively coupled plasma system with Ar/Cl2-based discharges. The Ohmic contacts were formed by lifting-off e-beam evaporated Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (800 Å). Gates with the dimension of 1.5 µm × 314 µm were defined by lift-off of e-beam deposited Ni/Au metallization. The contacts were annealed at 850°C for 45 s under a flowing N2 ambient in Heatpulse 610T system. The final step was deposition of e-beam evaporated Ti/Au (300 Å/1200 Å) interconnection contacts. Devices were exposed to protons with 5 MeV and different doses of 2×1014 proton/cm2. The device DC performance was characterized using an HP 4156 parameter analyzer. The breakdown voltage was conducted with a Tektronix 370A curve tracer. FLOODS TCAD finite-element solver was used to simulate the electrical field around the gate edge to confirm the experimental result.As shown in Figure 1, drain current IVs, IDSS, decreased from 454 to 398 mA/mm after irradiation with a proton dose of 2×1014 cm-2. Moreover, resistance between source and drain increased and extrinsic transconductance decreased after irradiation. However, the drain breakdown voltage increased from 150 to 230 V. The improvement of the breakdown voltage was due to the reduction of electric field at the edge of the gate, caused by the defects generated by proton irradiation at AlGaN/GaN interface, which was confirmed with simulation, as shown in Figure 2.Both gate and drain pulse measurements were also performed to verify the introductions of implanted defects into the HMET structure, which will be discussed in the talk.

Effect of low dose γ-irradiation on DC performance of circular AlGaN/GaN high electron mobility transistors

The changes in direct current performance of circular-shaped AlGaN/GaN high electron mobility transistors (HEMTs) after 60Co γ-irradiation doses of 50, 300, 450, or 700 Gy were measured. The main effects on the HEMTs after irradiation were increases of both drain current and electron mobility. Compton electrons induced from the absorption of the γ-rays appear to generate donor type defects. Drain current dispersions of ∼5% were observed during gate lag measurements due to the formation of a virtual gate between the gate and drain resulting from the defects generated during γ-irradiation.

Effect of Proton Irradiation on DC Performance and Reliability of Circular-Shaped AlGaN/GaN High Electron Mobility Transistors

The effect of 5 MeV proton irradiation on DC performance and reliability of circular-shaped AlGaN/GaN high electron mobility transistors were investigated. The drain saturation current (IDSS) reduced 12.5% and the sheet resistance increased 9.2% after irradiation with a dose of 2×1014 1/cm2. Threshold voltage shifted more positively with a dose of 2×1014 1/cm2 compared to 2×1013 /cm2. Besides, the breakdown voltage improved 39.8% after irradiation with dose of 2×1014 1/cm2. Simulation was done and the peak electric field at the drain side of the gate edge was found to be reduced after irradiation. The defects generated after irradiation at the AlGaN/GaN interface create a virtual gate and thus reduce the electric field. Gate lag measurement was performed to characterize the defects. The current dispersion of DC to high frequency indicates the defects had been generated after irradiation, which is in agreement with the simulation data.

Effect of Gamma Irradiation on DC Performance of Circular-Shaped AlGaN/GaN High Electron Mobility Transistors

The effect of low dose gamma irradiation on DC performance of circular-shaped AlGaN/GaN high electron mobility transistors weas investigated. The drain saturation current (IDS) increased 11.44% after irradiation with a dose of 700 Gy. Sheet resistance (Rs) was measured from transfer line method and it decreased 3.6% after irradiation. By extracting the resistance between source and drain in the drain I-V curve and combining with TLM data, mobility was found to increase 34.53% after irradiation. The mobility increase may come from the donor-type defects or the strain relaxation effect. Gate lag measurement was also performed and 5% current dispersion was found after irradiation with the dose of 700 Gy, indicating there were more defects generated after irradiation.