We have investigated the effects of surface passivation on off-state leakage current and current collapse effects of high-voltage GaN-on-Si heterojunction field effect transistors (HFETs) by using low pressure chemical vapor deposition (LPCVD) of silicon nitride (SiN x ). In this work, the metaloxide-semiconductor (MOS) structure-based HFETs are realized on AlGaN/GaN epitaxy grown silicon substrates by metal-organic chemical vapor deposition (MOCVD). For a comparative study, we have fabricated two types of HFETs, standard and modified MOS-HFETs. In the modified MOS-HFETs process, the surface passivation layer of SiN x is deposited by LPCVD after the mesa isolation step, while the gate is deposited and self-aligned in the trench etched in LPCVD-SiN x layer using inductively coupled plasma reactive ion etching (ICP-RIE). The high temperature deposition of LPCVD-SiN x prevents the degradation caused by the ohmic annealing and other process-induced surface damage. Compared to the standard MOS structure, the modified MOS-HFET devices exhibit 10 times lower off-state leakage currents within high voltage range (0-800 V) and significantly alleviated current collapse effects simultaneously.

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We systematically characterized the inhomogeneous doping properties along the c-axis of Mg-doped p-GaN microrods. Axial variation of doping concentration and electrical resistance on the p-GaN rod were measured by time-of-flight secondary-ion-mass-spectrometry and four-point probe measurements, respectively. Defects-related optical information was obtained from photoluminescence spectra together with Raman experiments revealing the change of crystal quality and strain along the rod. On the basis of a correlation of these analyses, we confirmed that Mg concentration decreased along the axial direction of the rod, leading to increasing electrical resistance. This axial Mg concentration change was revealed by green luminescence because the intensity of green luminescence sensitively varied with the doping density in both high-doping and low-doping rods. Interestingly, all the resistances at the highly doped rods were higher than the lowly doped rods due to overall mobility degradation at the high-doping rods caused by a scattering effect of increased Mg impurities and strain. All analyses provided complementary information on the p-type doping process and contribute to understanding the p-doping properties of GaN rod based photonic devices. Furthermore, our axially resolved optical spectroscopic (photoluminescence and Raman) methods can provide a facile, fast, and nondestructive way to estimate the axial doping and conductivity inhomogeneity of a Mg-doped p-GaN rod without having complex, time-consuming, and destructive structural and electrical measurements.

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