A combined wet and dry cleaning process for GaN(0001) has been investigated with XPS, capacitance voltage measurement. Capacitance–voltage characterization showed that combined ex-situ wet sulfide passivation and in-situ cyclic trimethylaluminum (TMA)/hydrogen plasma exposure led to reductions in the densities of both interface traps and border traps. In situ XPS studies show that after the wet sulfur treatment on GaN(0001), sulfur desorbs in vacuum at 25C prior to gate oxide deposition. Ex-situ depth profiling ARXPS post-ALD deposition shows that the a-Al₂O₃ gate oxide bonds directly to the GaN substrate leaving both the gallium surface atoms and the oxide interfacial atoms with bulk-like charge. DFT calculations predict that the oxide/GaN(0001) interface will have bulk-like charges and a low density of band gap states. This passivation is consistent with the oxide restoring the surface gallium atoms to tetrahedral bonding by eliminating the gallium empty dangling bonds on bulk terminated GaN(0001). A key application of gate oxide on GaN is GaN tunnel FETs. Tunneling FETs (TFETs) are one promising option for reducing power consumption per logic function due to their potential for subthreshold slopes steeper than the thermionic limit of 60 mV/decade. While TFETs based on narrow-gap semiconductors have been demonstrated, the narrow band gaps of these materials also lead to limited on-off current ratios, compromising their power efficiency. GaN-based TFETs offer a potential solution to this problem, since the large band gap effectively reduces off-state current, and the large polarization fields present in III-N heterostructures (which arise from the lack of inversion symmetry in the wurtzite crystal structure of the III-N system)—and GaN/InGaN/GaN heterojunctions in particular—can be used to dramatically increase tunneling currents (and thus TFET on-state currents). However, implementation of a GaN-based TFET required a low defect gate oxide/GaN interface. This has been achieved using the two-step wet-dry surface passivation.