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AlxGa1−xN/GaN high-electron-mobility transistor (HEMT) structures are key components in electronic devices operating at gigahertz or higher frequencies. In order to optimize such HEMT structures, understanding their electronic response at high frequencies and room temperature is required. Here, we present a study of the room temperature free charge carrier properties of the two-dimensional electron gas (2DEG) in HEMT structures with varying Al content in the AlxGa1−xN barrier layers between x=0.07 and x=0.42. We discuss and compare 2DEG sheet density, mobility, effective mass, sheet resistance, and scattering times, which are determined by theoretical calculations, contactless Hall effect, capacitance-voltage, Eddy current, and cavity-enhanced terahertz optical Hall effect (THz-OHE) measurements using a low-field permanent magnet (0.6 T). From our THz-OHE results, we observe that the measured mobility reduction from x=0.13 to x=0.42 is driven by the decrease in 2DEG scattering time, and not the change in effective mass. For x<0.42, the 2DEG effective mass is found to be larger than for electrons in bulk GaN, which in turn, contributes to a decrease in the principally achievable mobility. From our theoretical calculations, we find that values close to 0.3m0 can be explained by the combined effects of conduction band nonparabolicity, polarons, and hybridization of the electron wavefunction through penetration into the barrier layer. 

Background:Postoperative delirium (POD) is a common and serious clinical condition that occurs after anesthesia/surgery. While its clinical impact is well recognized, the underlying electrophysiologic mechanisms remain largely unknown, posing challenges for effective treatment. This study aims to investigate hippocampal neural dynamics before and after anesthesia/surgery in aged mice, which have a tendency to develop POD. Methods:This study included adult and aged mice with a POD model. POD-like behavior was assessed in N = 10 mice at baseline (the day before surgery), as well as at 9 h and 24 h after anesthesia/surgery. A behavioral battery, including the open field test, Y maze, buried food test, and novel object recognition, was used for assessment.In vivochronic brain recordings were performed on awake, restrained mice using a high-density silicon probe during the same time intervals. To further investigate hippocampal neural dynamics,in vivotwo-photon calcium imaging was also conducted. Additionally, aged mice were pretreated with indole-3-propionic acid (IPA), and its effects on POD-like behavior and neural activity were evaluated using electrophysiology and calcium imaging. Results:The first observation was that aged mice exhibited significant POD-like behavior, as measured by Z scores, compared to adult mice after anesthesia/surgery. Analysis revealed significant age-related differences in hippocampal neuronal activities. At 9 h after surgery, aged mice exhibited a marked increase in pyramidal cell activity and a reduction in interneuron activity compared to adult mice. These changes in neuronal dynamics were associated with the onset of POD-like symptoms in aged mice. By 24 h after surgery, both pyramidal cell and interneuron activity in aged mice had returned to presurgery levels, which coincided with an improvement in POD-like behavior. Additionally, IPA pretreatment modulated neuronal activity in aged mice, attenuating pyramidal cell hyperactivity and partially ameliorating interneuron dysfunction, changes associated with mitigated POD-like behavior. Conclusions:Alterations in hippocampal neural activity may significantly contribute to brain dysfunction and POD-like behavior. IPA pretreatment may modulate neural circuit imbalances in aged mice, potentially mitigating POD incidence.