HEMT NEXAFS Sensor Transistor XPS Zwitterion 2-dimensional electron gas ADSORPTION AMINO-ACIDS AlGaN/GaN heterostructures Aluminum gallium nitride Amino acids Carboxylation Chemical and biological sensing Conductivity measurements ELECTRONIC-STRUCTURE ENERGIES Electric conductivity measurement Electron gas GAN GLYCYL-GLYCINE Gallium nitride High electron mobility transistors III-V semiconductors INNERSHELL ABSORPTION-SPECTROSCOPY Intermolecular interactions Molecules Phase interfaces Preferential orientation SHELL EXCITATION