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The purpose of this study was to evaluate the reliability and validity of the raw accelerometry output from research-grade and consumer wearable devices compared to accelerations produced by a mechanical shaker table. Raw accelerometry data from a total of 40 devices (i.e., n = 10 ActiGraph wGT3X-BT, n = 10 Apple Watch Series 7, n = 10 Garmin Vivoactive 4S, and n = 10 Fitbit Sense) were compared to reference accelerations produced by an orbital shaker table at speeds ranging from 0.6 Hz (4.4 milligravity-mg) to 3.2 Hz (124.7mg). Two-way random effects absolute intraclass correlation coefficients (ICC) tested inter-device reliability. Pearson product moment, Lin's concordance correlation coefficient (CCC), absolute error, mean bias, and equivalence testing were calculated to assess the validity between the raw estimates from the devices and the reference metric. Estimates from Apple, ActiGraph, Garmin, and Fitbit were reliable, with ICCs = 0.99, 0.97, 0.88, and 0.88, respectively. Estimates from ActiGraph, Apple, and Fitbit devices exhibited excellent concordance with the reference CCCs = 0.88, 0.83, and 0.85, respectively, while estimates from Garmin exhibited moderate concordance CCC = 0.59 based on the mean aggregation method. ActiGraph, Apple, and Fitbit produced similar absolute errors = 16.9mg, 21.6mg, and 22.0mg, respectively, while Garmin produced higher absolute error = 32.5mg compared to the reference. ActiGraph produced the lowest mean bias 0.0mg (95%CI = -40.0, 41.0). Equivalence testing revealed raw accelerometry data from all devices were not statistically significantly within the equivalence bounds of the shaker speed. Findings from this study provide evidence that raw accelerometry data from Apple, Garmin, and Fitbit devices can be used to reliably estimate movement; however, no estimates were statistically significantly equivalent to the reference. Future studies could explore device-agnostic and harmonization methods for estimating physical activity using the raw accelerometry signals from the consumer wearables studied herein. [ABSTRACT FROM AUTHOR]

We report MOCVD-grown Al0.87Ga0.13N/Al0.64Ga0.36N metal-oxide-semiconductor-heterojunction-field-effect-transistors on single crystal bulk AlN substrate. As compared to control devices on AlN template, thermal impedance for devices on single crystal AlN decreased to 1/3 from 31 to 10 K mm W−1, comparable to SiC and copper heat-sinks. This represents a significant thermo-electric co-design advantage over other semiconductors. As a result, the peak drain saturation current increased from 410 to 610 mAmm−1. A 3-terminal breakdown field of 3.7 MV cm−1 was measured, which to date represents state-of-the-art performance for devices with similar Al x Ga1− x N-channel composition. This translates to a measured Baliga figure of merit of 460 MWcm−2. [ABSTRACT FROM AUTHOR]

We report threshold voltage (V TH) control in ultrawide bandgap Al0.4Ga0.6N-channel metal oxide semiconductor heterostructure field-effect transistors using a high-temperature (300 °C) anneal of the high- k ZrO2 gate-insulator. Annealing switched the polarity of the fixed charges at the ZrO2/AlGaN interface from +5.5 × 1013 cmâˆ'2 to âˆ'4.2 × 1013 cmâˆ'2, pinning V TH at ∼ (âˆ'12 V), reducing gate leakage by ∼103, and improving subthreshold swing 2Ă— (116 mV decadeâˆ'1). It also enabled the gate to repeatedly withstand voltages from âˆ'40 to +18 V, allowing the channel to be overdriven doubling the peak currents to ∼0.5 A mmâˆ'1. [ABSTRACT FROM AUTHOR]

We demonstrate fully fabricated AlGaN/GaN high electron mobility transistors (HEMTs) transferred from sapphire to copper tape on flexible polyethylene terephthalate using 193 nm excimer laser liftoff (LLO). The heterojunction is structurally intact after LLO, leading to preserved electron mobility ÎĽ n ∼1630 cm2 Vâˆ'1 sâˆ'1 and carrier concentration n s ∼1013 cmâˆ'2. The maximum drain saturation current decreased by ∼18% after transfer, which is a lower reduction than other reported transfer methods. The drain current of this flexible HEMT increased monotonically under tensile stress applied using a convex-shaped plate, while the threshold voltage shifted more negative in quantitative agreement with the expected piezoelectric charge for an intact heterojunction. [ABSTRACT FROM AUTHOR]

In this work, a large-area MoS2/graphene barristor device, with an electrically tunable Schottky barrier height, has been studied for detection of various gaseous analytes. The Schottky barrier height could be modulated by over 0.65 eV, allowing the drain current to be tuned by many orders of magnitude. Using diluted NO2 and NH3 gases as analytes, the performance of the barristor device was compared with individual MoS2 and graphene based planar FETs, where the barristor device outperformed its counterparts in terms of response magnitude and limit of detection. Due to the atomically thin nature of MoS2 and graphene, electric field applied to one material could not be fully screened from the other one, which allowed both materials to act as a composite structure when analyte molecules interacted with the barristor device. This apparently reversed the dopant behavior of NO2 and NH3, while increasing the device sensitivity through subthreshold operation. Both conductance and capacitance based measurements are presented to highlight the charge transfer and barrier height modulation, to support the unique sensing mechanism observed in the 2D barristor device. A lower limit of detection in low ppb is established for NO2 and low ppm for NH3, which could be further tuned by altering gate-drain bias conditions. [ABSTRACT FROM AUTHOR]

We report on 193 nm excimer laser-based liftoff (LLO) of Al0.26Ga0.74N/GaN high electron mobility transistors (HEMTs) with thick (t > 10 μm) AlN heat spreading buffer layers grown over sapphire substrates. The use of the thick AlN heat spreading layer resulted in thermal resistance ( R t h ) of 16 K mm/W for as-fabricated devices on sapphire, which is lower than the value of ∼25–50 K mm/W for standard HEMT structures on sapphire without the heat-spreaders. Soldering the LLO devices onto a copper heat sink led to a further reduction of R t h to 8 K mm/W, a value comparable to published measurements on bulk SiC substrates. The reduction in R t h by LLO and bonding to copper led to significantly reduced self-heating and drain current droop. A drain current density as high as 0.9 A/mm was observed despite a marginal reduction of the carrier mobility (∼1800 to ∼1500 cm2/V s). This is the highest drain current density and mobility reported to-date for LLO AlGaN/GaN HEMTs. [ABSTRACT FROM AUTHOR]

We report a 1.8 times improved light extraction efficiency for truncated cone AlGaN DUV (275 nm emission) micropixel LEDs when the pixel size was reduced from 90 to 5 μm. This is a direct consequence of reducing the re-absorption of sideways-travelling photons, as the lateral absorption length was measured to be 15 μm using cathodoluminescence. We also investigated the effect of an Al2O3/Al sidewall coating on the device performance which led to an on-wafer pulsed-mode brightness of 10.3 kW cm−2 from a single 5 μm truncated cone micropixel. [ABSTRACT FROM AUTHOR]

We report a recessed-gate enhancement-mode Al2O3-ZrO2/Al0.6Ga0.4N/Al0.4Ga0.6N metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET) with drain current as high as 0.48 A mm−1 at a gate-source voltage of +12 V. This was enabled by a pseudomorphic HFET structure with graded back barrier for strain management and to screen the growth interface from the channel. The device exhibited a threshold-voltage (VTH) of 2.75 ± 0.57 V with absolute maximum VTH = 3.6 V, a +12.2 V shift from that for a depletion-mode MOSHFET fabricated on the same wafer. A 3-terminal breakdown voltage of 700 V was measured in the off-state, showing the viability of E-mode UWBG AlGaN for power electronics. [ABSTRACT FROM AUTHOR]

We present a study of the light output power and the thermal impedance of 281 nm emission AlGaN based micropixel LEDs. A modular interconnected micropixel array design is presented which enables dense packing with area and power scalability. We study 5–15 μm diameter stand-alone devices and parallel-connected micropixel arrays with 5 μm interpixel gaps. A standalone 5 μm pixel emits 291 W cm−2 at 10.2 kA cm−2 DC-drive. A power as high as 23 mW (361 W cm−2) was measured at a pulsed-pump current of 800 mA (∼15 kA cm−2) for an interconnected array. These are the smallest and brightest DUV LEDs to date. [ABSTRACT FROM AUTHOR]

We present the temperature-dependent electrical characteristics of high-current depletion (D-mode) and barrier-recessed enhancement-mode (E-mode) ultrawide bandgap (UWBG) AlxGa1−xN channel insulated gate heterojunction field-effect transistors fabricated on the same wafer. The key motivation is the higher Baliga figure of merit for devices with the UWBG AlGaN channel and their strong potential for use in high-power, high-temperature harsh environmental applications. Over a temperature range of 125 °C, the VTH shifted in the opposite direction for D- and E-mode devices with a rate of +13.5 mV/K and −23 mV/K, respectively, giving an overall shift of +1.7 V and −2.9 V. This was attributed to changes in the fixed and trapped charge densities in the dielectric and at the dielectric–AlGaN barrier interface. A single deep sub-bandgap trap level was sufficient to explain the threshold shifts in both devices. The effective channel mobility in the E-mode devices was argued to be limited by charge scattering, arising from the same charges introduced during barrier recessing that shifted VTH. [ABSTRACT FROM AUTHOR]

We report an Ultrawide Bandgap Al0.4Ga0.6N channel metal-oxide-semiconductor heterostructure field effect transistor with drain currents exceeding 1.33 A mm−1 (pulse) and 1.17 A mm−1 (DC), around a 2-fold increase over past reports. This increase was achieved by incorporating a hybrid barrier layer consisting of an AlN spacer, n-doped Al0.6Ga0.4N barrier and a thin reverse graded AlxGa1–xN (x from 0.60 to 0.30) cap layer. To enhance current spreading, a "perforated" channel layout comprising of narrow channel sections separated by current blocking islands was used. A composite ALD deposited ZrO2/Al2O3 film was used as gate dielectric. A breakdown field above 2 MV cm−1 was measured. [ABSTRACT FROM AUTHOR]

An initial study of losses in n‐AlxGa1−xN planar waveguides at λemission ≈ 280 nm using monolithically integrated AlxGa1−xN multiple quantum wells (MQWs)‐based light‐emitting diodes and detectors is presented. The epilayer structure for the integrated devices is grown on an AlN (3.5 μm thick) template over sapphire substrates. Emitter–detector optical coupling and the directional independence of radiation within the epistructure are experimentally established. A model for estimating the attenuation coefficient under these conditions is developed. The attenuation coefficient for a planar n‐Al0.65Ga0.35N waveguide is measured to be 5–6 cm−1, and it primarily arises from the free‐carrier absorption rather than surface roughness‐dependent Rayleigh scattering. [ABSTRACT FROM AUTHOR]

Due to their superior breakdown fields compared with GaN and SiC and high thermal conductivity, AlxGa1−xN (x > 0.4) channel high‐electron‐mobility transistors (HEMTs) will find applications in extreme environments such as power electronics. Herein, the high‐temperature operation of ultrawide‐bandgap (UWBG) Al0.65Ga0.35N/Al0.4Ga0.6N metal oxide semiconductor heterostructure field effect transistors (MOSHFETs) with atomic layer‐deposited (ALD) high‐k gate dielectrics TiO2, Al2O3, and ZrO2 is reported. As compared with similar geometry HFETs, these devices exhibit a simultaneous reduction in gate‐leakage current by ≈104 and a positive shift of the threshold voltage as much as 4 V. This positive threshold shift indicates the introduction of negative charges at the oxide/barrier interface and within the thin oxide, attributed to the pre‐ALD plasma treatment. The gate leakage increases weakly with temperature up to 250 °C, whereas the peak drain currents decrease from ≈0.5 to 0.3 A mm−1. An analysis of the C–V and I–V characteristics reveals that this drain current decrease is due to a reduction in channel electron mobility. The potential mechanisms responsible for this are discussed. Up to the measured temperature of 250 °C, the devices withstand repeated temperature cycles without catastrophic degradation or breakdown, underscoring the promise of these materials. [ABSTRACT FROM AUTHOR]

Ultrawide bandgap (UWBG) AlGaN-channel metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) with a ZrO2 gate dielectric achieve peak current in excess of 0.4 A/mm and current ON/OFF ratios >106 with subthreshold swings as low as 110 mV/decade. These devices have strong potential for use in power and radio frequency electronics or as true solar-blind photodetectors. In this work, we present the photoresponse analysis in UWBG AlGaN MOSHFETs. Persistent photoconductivity with the decay time above 10 minutes can be quenched by illuminating with strong UV light at 365 nm and 254 nm, suggesting deep traps to be responsible for this behavior. Upon correlating the optical response under various illumination conditions with cathodoluminescence of these devices, we identified two key trap levels at ∼2.48 ± 0.14 eV and 3.76 ± 0.06 eV, controlling the slow response time. By depth-profiling using cathodoluminescence, these traps are identified to be at the AlN/AlGaN interface at the back of the device, due to partial relaxation from the lattice mismatch between AlN and Al0.4Ga0.6N. [ABSTRACT FROM AUTHOR]

We report Al0.85 Ga0.15 N/Al0.65 Ga0.35N high-electron mobility transistors on low-defect AlN buffer layers over basal plane sapphire substrates. A new epilayer design consisting of a doped barrier layer yielded sheet resistivity values as low as 1800~\Omega /\Box . Devices with the source–drain spacing of 5.5~\mu \text{m} and a gate length of 1.8 \mu \text{m}$ exhibited peak drain–currents as high as 250 mA/mm at a gate bias of +4 V. We also show that the 3- \mu \text{m} thick low-defect AlN buffer layers over sapphire to provide sufficient thermal conduction which enables stable device operation up to at least 40 V at 250 mA/mm with no current droop. [ABSTRACT FROM AUTHOR]

Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multilayer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5–2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions. [ABSTRACT FROM AUTHOR]

We report the emission of strong coherent broadband terahertz radiation from 6H-silicon-carbide (SiC) excited with optical pulses. The measured terahertz spectral signal-to-noise ratio is better than one thousand. We determine that the terahertz radiation is generated via second order optical nonlinearity (optical rectification). We present a measurement of the ratio of nonlinear susceptibility tensor elements χzzz(2)/χzxx(2) and the complex index of refraction of silicon carbide at terahertz frequencies. [ABSTRACT FROM AUTHOR]

We present experimental results on the optical absorption spectra of epitaxial graphene from the visible to the terahertz frequency range. In the terahertz range, the absorption is dominated by intraband processes with a frequency dependence similar to the Drude model. In the near-IR range, the absorption is due to interband processes and the measured optical conductivity is close to the theoretical value of e2/4h. We extract values for the carrier densities, the number of carbon atom layers, and the intraband scattering times from the measurements. [ABSTRACT FROM AUTHOR]

Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70–120 fs range is followed by a slower relaxation process in the 0.4–1.7 ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene. [ABSTRACT FROM AUTHOR]

Publisher's Note: "Trap characterization in ultra-wide bandgap Al0.65Ga0.4N/Al0.4Ga0.6N MOSHFET's with ZrO2 gate dielectric using optical response and cathodoluminescence" [Appl. Phys. This article was originally published online on 19 November 2019 with author MVS Chandrashekhar incorrectly spelled. All online versions of the article were corrected on 20 November 2019. [Extracted from the article]