Electrical characterization of a gate-recessed AlGaN/GaN high-electron-mobility transistor with a p-GaN passivation layer

A novel passivation technique was developed that reduces the electron-surface-hopping-induced leakage current of AlGaN/GaN high-electron-mobility transistors (HEMTs) and enhances their electrical properties under high drain bias operation. The key aspect of this passivation technique entailed growing a p-type GaN layer on a traditional depletion-mode AlGaN/GaN HEMT; this p-GaN passivation layer was also used as the spacer layer of a field-plate metal. The originally exposed gate-to-source and gate-to-drain areas were passivated by the p-GaN cap layer. Thus, a surface depletion region was formed between the p-GaN passivation layer and an n-type AlGaN/GaN two-dimensional electron gas channel. This extra surface depletion region depleted the channel carriers far from the surface to reduce the probability of the carriers being trapped by surface defects. Therefore, the carrier-hopping-induced leakage current in the gate-to-drain area, which was strongly temperature-dependent, was suppressed. Low-frequency noise (LFN) measurement revealed that the traditional chemical-vapor-deposited SiO₂ layer still exhibited a surface trap center. Moreover, the slope of a plot of noise spectra versus gate overdrive voltage was approximately -3. At a cryogenic temperature, the LFN of the p-GaN - passivated device improved by 3 orders of magnitude, whereas the high surface trap density of the standard passivated device resulted in a high LFN, even at a cryogenic temperature.