Mechanism of surface conduction in the vicinity of Schottky gates on AlGaN∕GaN heterostructures

Lateral surface leakage current (Is) on an AlGaN∕GaN heterostructure was systematically investigated by using a two-parallel gate structure with a gap distance (LGG) of 200nm–5μm. The surface current Is systematically increased as LGG decreased. A simple resistive layer conduction that should show 1∕LGG dependence failed to account for the drastic increase in Is when LGG was reduced to less than 1μm. However, no dependence on LGG was seen in vertical current that flows in the Schottky interface. The Is showed a clear temperature dependence proportional to exp(−T−1∕3), indicating two-dimensional variable-range hopping through high-density surface electronic states in AlGaN. A pronounced reduction in surface current of almost four orders of magnitude was observed in a sample with SiNx passivation.