Effects of post-annealing on the passivation interface characteristics of AlGaN/GaN high electron mobility transistors

We examined the effects of post-annealing in forming-gas ambient on the spin-on-dielectric (SOD)-buffered passivation as well as the conventional plasma-enhanced chemical vapor deposition (PECVD) Si₃N₄ passivation structure in association with the quantitative analysis of defects at the passivation interfaces of AlGaN/GaN high electron mobility transistors (HEMTs). Before the annealing, the interface state densities (D_it) of the PECVD Si₃N₄ are one-order higher (10¹²-10¹³ cm^-2 eV^-1) than those of the SOD SiO_x (10¹¹-10¹² cm^-2 eV^-1) as derived from C-V characterization. Clear reduction in D_it from the PECVD Si₃N₄ is extracted to a level of 10¹¹-10¹² cm^-2 eV^-1 with a stronger absorption from Si-N peak in Fourier transform infrared spectroscopy spectra after the post-annealing. On the other hand, negligible difference in D_it value is obtained from the SOD SiO_x. In this paper we propose that much lower measurement levels (~156 mA/mm) before the annealing and substantial recovery (~13% increase) after the annealing in maximum drain current density of the AlGaN/GaN HEMTs with Si₃N₄ passivations are due to the original higher density before the annealing and greater reduction in D_it of the PECVD Si₃N₄ after the annealing. Significant reduction after the annealing in gate-drain leakage current (from ~10^-3 to ~10^-5 A, 100-μm gate width) of the HEMTs with the Si₃N₄ passivation is also supposed to be attributed to the reduction of D_it.