In this paper, we report on a methodology for theoretical prediction and optimization of charge sensitivity for ungated AlGaN/GaN high electron mobility transistorbased ion sensors operated in the reference electrode free configuration. We have performed numerical simulations of device sensitivity, specifically the change in channel electron concentration with the change in surface potential, for different Al mole fractions and AlGaN thicknesses. These results can be used for device optimization, signal analysis, and sensor calibration purposes. To validate the model, six ungated AlGaN/GaN transistor-based devices of different Al mole fractions and AlGaN thicknesses were fabricated. These devices were exposed to KOH solutions with different pH values, and the voltage change at the gate area was indirectly measured as a function of ionic concentration. The gain in conductivity across the measured range of pH values was experimentally extracted for each device and closely matched the sensitivity predicted by simulation.

In this work, GaN/InGaN/GaN nanocolumns (NCs) have been grown by molecular beam epitaxy. Selective area growth (SAG) and self-organized growth (SOG) were performed simultaneously in patterned and unpatterned regions of the same substrate, respectively. The resulting structures show different tip morphologies and structural properties due to the different polarity along the growth direction, namely Ga-polar with r-plane faceted tips for the SAG NCs and N-polar with c-plane top facet for the SOG ones. When growing Ga-polar GaN/InGaN NCs, no indium is incorporated at a substrate temperature of [Formula: see text]°C. Rather, indium incorporation takes place under the same growth conditions on the N-polar NCs. The In-incorporation is investigated by means of nano x-ray fluorescence and diffraction, high-angle annular dark-field scanning transmission electron microscopy and high-resolution transmission electron microscopy.

We have grown 2DEG AlGaN/GaN heterostructures by molecular beam epitaxy (MBE) with electron mobilities up to 21500 cm2V−1s−1 at 2 K. In-situ RHEED was applied to optimize different aspects of Ga-rich growth. This paper gives a compact overview of the experimental key aspects that significantly affect the low temperature electron mobility in AlGaN/GaN heterostructures. Growth at the transition towards Ga droplet formation produced the best results. A quantitative analysis of the magnetoresistance confirmes scattering at dislocations as the dominant scattering process at low temperature.

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