Optical and thermal excitation are known to enhance OFF-state leakage and modify the electrostatics of AlGaN/GaN high-electron-mobility transistors (HEMTs), yet the underlying transport pathway remains insufficiently understood. In this work, a geometry-defined ring-break architecture is introduced to directly probe lateral surface conduction and its role in barrier modulation. Simultaneous measurements of gate leakage current I G and channel potential Va reveal a gate-bias-dependent regime transition characterized by a correlated collapse of surface current and electrostatic modulation beyond a critical electric field. The identical transition behavior observed in both transport and electrostatic responses establishes that this transition is governed by a field-defined stability condition of the lateral surface conduction pathway. Measurements on conventional device geometries exhibit the same transition behavior under optical and thermal conditions, confirming the generality of the mechanism. While the magnitude of Va modulation differs significantly between the two geometries, this variation is attributed to differences in the spatial extent of surface charge redistribution and electrostatic boundary conditions rather than changes in the underlying transport process. These results demonstrate that optically and thermally induced barrier modulation in AlGaN/GaN HEMTs is governed by field-sustained lateral surface conduction mediated by surface trap dynamics.

Ultra-thin, p-type conducting Nickel oxide (NiOx) thin films, obtained by room-temperature sputter deposition under an optimum ratio of Argon and Oxygen, is shown to yield a large, nearly temperature-independent, positive shift in the threshold voltage of GaN/AlGaN depletion-mode high electron mobility transistors (HEMTs). The resulting metal-oxide-semiconductor-HEMTs (MOSHEMTs) exhibit a low subthreshold swing of 62 mV/dec, with a low specific ON resistance of 0.37 mOhm.cm2 , a large drain-current ON-OFF ratio of ~ 108, a saturation output current of 333.11 mA/mm at a gate voltage of -0.5 V and a maximum transconductance of 135 mS/mm, whereas the gate-leakage current-density (~  10 microA /cm2) is among the lowest reported so far. The large band-gap (4 eV), yielding large valence (-1.8 eV) and conduction band (1.7 eV) offsets at the NiOx-AlGaN interface, is responsible for the suppression of gate leakage, while the small thickness of the film allows enhanced gate-control of the MOSHEMT channel. These performance metrics are highly encouraging for realization of enhancement-mode GaN/AlGaN MOSHEMTs, with a recessed gate structure.