This paper presents an extensive analysis of the off-state conduction mechanisms in AlGaN/GaN Meta-Insulator-Semiconductor (MIS) transistors. Based on combined bi-dimensional numerical simulation and experimental measurements, we demonstrate the following relevant results: (i) under off-state bias conditions, the drain current can show a significant increase when the drain bias is swept up to 600 V; (ii) several mechanisms can be responsible for off-state current conduction, including band-to-band tunneling and impact ionization; (iii) two-dimensional numerical simulations indicate that band-to-band tunneling plays a major role, while impact ionization does not significantly contribute to the overall leakage. Temperature-dependent I-V measurements were also carried out to identify the origin of the vertical drain-bulk leakage. (C) 2015 Elsevier Ltd. All rights reserved.