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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.
Date of Publication: Oct. 2012