In this work, we observe the distinct VTH characteristics in the Au-free gate-first processing p-GaN/AIGaN/GaN HEMTs with two commonly used passivation layers, i.e., SiN and SiO2. The device with SiN shows a depletion-mode (D-mode) characteristic (VTH ∼ -5V) whereas the device with SiO2 passivation exhibits an enhancement-mode (E-mode) characteristic (VTH ∼ +0.7V). Furthermore, Transmission Line Measurement (TLM) devices are fabricated to investigate the effects of the passivation on two dimensional electron gas (2DEG) in p-GaN/AIGaN/GaN stack. The results indicate that a low Rsh is obtained while passivating device surface with SiN layer, suggesting that 2DEG is present, which is most probably due to an unfunctional p-GaN layer. The SIMS results indicate a high H-intensity in the p-GaN/AIGaN/GaN stack with a SiN passivation layer. Thus, P-GaN deactivation due to the formation of complex Mg-H after SiN passivation is proposed to explain the D-mode characteristic in the device with a SiN passivation layer.