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The detection of hydrogen in the Earth's deep underground poses a major challenge due to the lack of oxygen and continuous changes in environmental conditions. An innovative class of hydrogen gas sensors based on AlGaN/GaN High Electron Mobility Transistors (HEMTs) with Platinum (Pt) gates as a functionalization layer, has been developed and optimized for geo-sensing. The study investigated the sensor characteristics using two carrier gases to simulate underground conditions, namely air (with 20 % O 2) and N 2 (with 0 % O 2), across a range of temperatures from 50 °C to 300 °C and with hydrogen concentrations varying from 25 ppm to 400 ppm. The detection limit was found to be approximately 1 ppm of hydrogen in the atmospheric air. The gas sensor transduction is based on the modification of the conductivity of a 2-Dimensional Electron Gas (2DEG). In this study, the principle was investigated using two gases, O 2 and H 2 , with different electronegativities relative to platinum. The adsorption competition between H 2 and O 2 on platinum was evaluated, and this allowed the calculation of the ratio of thermodynamic adsorption constants between these two gases. • Investigated the Pt-AlGaN/GaN HEMT-based sensor for detecting underground H 2 gas using N 2 and air as carrier gases. • Key parameters for hydrogen sensing included the effects of temperature and hydrogen concentration. • Demonstrated that hydrogen has 2000 times greater affinity than O 2 with the Pt-AlGaN/GaN sensor functional layer. • Presented and comprehensively explained the adsorption kinetics of hydrogen with the Pt functional layer. [ABSTRACT FROM AUTHOR]

• InGaN nano-pyramid absorber is shown to release the usual challenging constraint on p-GaN doping. • SiO2 mask is shown to help trap light into the nanopyramids leading to a larger efficiency. • Nanopyramid-based solar cell lead to an efficiency at least twice that of planar solar cell. InGaN nano-structures, grown using nano selective area growth, have been shown to exhibit high crystalline quality, even for high In content InGaN alloy, and reduced polarization charge effect. They are thus very attractive for the realization of high efficiency solar cells. Compared to planar InGaN absorbers, nanopyramid-based absorbers are shown to relax the usual challenging constraint on the doping of the p-GaN layer, which would be needed to overcome the polarization-induced electric field. NP-based solar cells maintain the same performance with ten times lower p-GaN doping. Furthermore, the SiO 2 mask used for selective area growth of the nanopyramids is shown to help trap light into the nanopyramids, leading to increased optical absorption and thus efficiency. Last, InGaN nanopyramid absorber-based solar cells can allow for a higher InGaN residual donor concentration than that of the planar InGaN solar cells. Overall, an optimized In 0.3 Ga 0.7 N nanopyramid-based solar cell can lead to an efficiency twice than that of a planar InGaN-based solar cells with standard p- and n-GaN doping level. As a proof of concept, an In 0.09 Ga 0.91 N nanopyramid-based solar cell has been fabricated and is shown to have larger short circuit photocurrent and open circuit voltage than a state of the art In 0.08 Ga 0.92 N-based planar solar cell. [ABSTRACT FROM AUTHOR]