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In this letter, ambient moisture has been identified as a previously unrecognized cause of current collapse in AlGaN/GaN high-electron-mobility transistors. Unpassivated devices exposed to dry air or protected with a hydrophobic passivation, such as vapor-deposited fluorocarbon, showed negligible current collapse under 250-ns pulsed measurements. A mechanism based on the ionization and deionization of the water molecules at the device surface has been proposed to explain this behavior. The use of a hydrophobic passivation to prevent dc-to-RF dispersion works even when it is not directly in contact with the semiconductor surface, which allows the engineering of multistack passivation layers to eliminate current collapse while minimizing parasitic capacitance.