Parallel 2DHG conduction in AlN/GaN/AlN HEMTs: Leakage mechanisms and electric field distribution

We provide direct evidence of two-dimensional hole gas (2DHG) conduction occurring in parallel with the two-dimensional electron gas (2DEG) channel in AlN/GaN/AlN high-electron-mobility transistors, using combined electrical characterization and electric-field-induced second harmonic generation. Anomalous OFF-state leakage and diode-like field distributions, which are also confirmed in isolation structures where the top heterostructures along with the 2DEG channel have been etched, reveal a buried leakage path mediated by the 2DHG. This leakage arises from Fowler–Nordheim tunneling with electron–hole pairs generation under the gate, which activates a vertical 2DEG–2DHG diode-like pathway, and from Poole–Frenkel transport that connects the 2DHG to the ohmic contacts. An equivalent circuit model is proposed to describe these mechanisms. The upper bound of the buried 2DHG sheet resistance is estimated to be 10 MΩ/sq, while the vertical resistance between the 2DHG and n++GaN is 35 MΩ mm. These findings provide a direct visualization and a comprehensive model of 2DHG-mediated leakage, offering design strategies to suppress parasitic conduction and improve device performance.