We have studied current versus voltage characteristics of n-GaN / u-AlGaN / n-GaN double heterostructure devices under hydrostatic pressure up to 500 MPa. Devices were grown on c-plane sapphire substrates by organometallic vapor phase epitaxy using epitaxial layer overgrowth. The effect of AlGaN layer thickness and composition on the pressure sensitivity was investigated. For a fixed applied bias, we found that the current decreases approximately linearly in magnitude with increasing hydrostatic pressure over the range of voltages and pressures applied. The decrease in current magnitude can be attributed to piezoelectric effects and is consistent with model calculations. The polarization charge densities at the GaN / AlGaN interfaces change with hydrostatic pressure, which in turn modifies the internal potential barrier. Changes in the AlGaN layer thickness and composition also modify the interfacial polarization, with thicker AlGaN layers and higher AlN content increasing the effect of pressure on the observed current versus voltage characteristics. The strain gauge factors obtained for these devices range from ϳ200 to 800.

Many websites encourage their users to write reviews for a wide variety of products and services. In particular, movie reviews may influence the decisions of potential viewers. However, users face the arduous tasks of summarizing the information in multiple reviews and determining the useful and relevant reviews among a very large number of reviews. Therefore, we developed machine learning (ML) models to classify whether an online movie review has positive or negative sentiment. We utilized the Stanford Large Movie Review Dataset to build models using decision trees, random forests, and support vector machines (SVMs). Further, we compiled a new dataset comprising reviews from IMDb posted in 2019 and 2020 to assess whether sentiment changed owing to the coronavirus disease 2019 (COVID-19) pandemic. Our results show that the random forests and SVM models provide the best classification accuracies of 85.27% and 86.18%, respectively. Further, we find that movie reviews became more negative in 2020. However, statistical tests show that this change in sentiment cannot be discerned from our model predictions.

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GaN-based heterostructure devices are of interest for pressure and stress sensing applications due to their potential for use at high temperatures and in caustic environments. We have grown n-GaN/u-AlGaN/n-GaN heterostructure devices on sapphire substrates by organometallic vapor phase epitaxy (OMVPE) using the epitaxial layer overgrowth (ELO) method. The devices were fabricated with varying AlGaN layer thickness and composition. Current-voltage (I-V) characteristics were obtained to characterize the performance of these devices under hydrostatic pressures up to 500 MPa. For a fixed bias, the current was observed to decrease in magnitude with increasing hydrostatic pressure for all devices tested. The current modulation is attributed to piezoelectric effects. Specifically, the polarization charge densities at both GaN/AlGaN interfaces are sensitive to changes in the hydrostatic pressure, and these charges affect the shape of the potential barrier and the current. Changes in the AlGaN layer thickness and composition modify the interfacial polarization, with thicker AlGaN layers and higher Al content increasing the effect of pressure on the observed I-V characteristics. The decreases in current magnitude with increasing pressure are linear over the pressure range tested. In order to quantify the performance of these devices, we calculate a pressure gauge factor based on a normalized change in current divided by the change in pressure. Values obtained range from 0.1–1.0 GPa−1, consistent with our previously published results for a single device. In addition, the turn-on voltages under both forward and reverse bias conditions are observed to increase with increasing AlGaN layer thickness and composition, a result that agrees with our device model. These turn-on voltages are governed by different mechanisms in the forward and reverse bias directions. Under forward bias, the mechanism is a transition from a thermionic to a tunneling process. However, under reverse bias, the turn-on occurs when the total electric field changes sign in the AlGaN layer.

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