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We have investigated the influence of the structural and compositional properties of silicon nitride layers on the passivation of AlGaN/GaN HEMTs grown on sapphire substrates by assessing their continuous wave (CW) and pulsed current-voltage (I-V) characteristics. We have looked at the effect of various PECVD parameters like gas composition (SiH4/NH3 ratio and ammonia-free SiNx), process pressure, plasma power and deposition temperature. The best SiNx layer is Si-rich and has a refractive index of 2.01, an extinction coefficient of 0.03 at 3.44 eV and an optical band gap of 2.87eV. The best device performance was only achieved when the optimized SiNx layer was used in conjunction with a low-power Ar-plasma after the metallization of the ohmic contacts and cleaning using an ammonia dip just before the Schottky gate metallization. The latter treatment accounted for a substantial reduction of the gate and drain leakage currents densities from 6.7 mA/mm to 400 μA/mm.

A technique to enhance the hole concentration in activated Mg-doped p-type GaN epitaxial layers is described. The method consists of depositing a porous plasma-enhanced chemical vapor deposited SiO x layer on top of p-GaN after which the sample is heated to 950°C in nitrogen ambient for 1 min followed by the removal of the SiO x layer in a buffered HF solution. A significant improvement of the conductivity of the p-GaN layer has been obtained.

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