Small- and Large-Signal Analyses of Different Low-Pressure-Chemical-Vapor-Deposition SiNx Passivations for Microwave GaN HEMTs

Three types of SiNx passivation for microwave AlGaN/GaN HEMTs were deposited with low-pressure chemical vapor deposition under different deposition conditions, resulting in different silicon contents. The performance of the HEMTs is comprehensively investigated and compared. Both small- and large-signal analyses, such as generation-recombination (G-R) trap analysis, low-frequency noise characterization, and load-pull measurement, are indispensable to evaluate the effectiveness of a surface passivation. A Si-rich SiNx passivation shows excess G-R centers, whereas a Si-poor SiNx passivation exhibits significant current slump (30%). A bilayer SiNx passivation successfully shows not only a small current slump (~9.7%) but also a suppressed G-R trapping/detrapping process. Moreover, the bilayer passivation demonstrates almost 2 orders of magnitude lower gate current noise spectra compared with the single-layer Si-rich SiNx passivation. The capacitance-voltage measurements reveal that the Si-rich SiNx layer removes the deep-level traps at the AlGaN/SiNx interface. Considering both small- and large-signal operations, it is concluded that the bilayer SiNx passivation is a suitable and versatile candidate for microwave GaN devices.