Variable range hopping mechanism and modeling of isolation leakage current in GaN-based high-electron-mobility transistors

Off-state leakage current of AlGaN/GaN high-electron-mobility transistors (HEMTs) with implant and mesa isolation was studied. Comparison of isolated pad structures with HEMTs shows that isolation leakage flowing through contact pads is the major leakage source in the studied GaN-based HEMTs whose gate finger is connected to an Ohmic contact pad. Then, circular metal-oxide-semiconductor field-effect-transistor devices were used to identify this isolation leakage path as surface isolation leakage at the SiN/nitride interface rather than bulk leakage in the buffer layer or implanted region. The temperature-dependent measurement shows that the two-dimensional variable range hopping mechanism dominates both the implant and mesa isolation leakage current. Mesa isolation results in a larger hopping probability and isolation leakage current than implant isolation. The isolation leakage current through gate and drain contact pads results in a non-zero switch of gate current in rectangular devices with either a Schottky gate or a metal-oxide-semiconductor gate, which is not observed in circular devices. Gate voltage for the switch of gate leakage current is linearly correlated with drain bias voltage, and the slope of the linear model represents the influence of drain bias on gate leakage current. This empirical model is independent of the fabrication process, provided that the same layout is used. With an increase in the source-drain distance from 4 mu m to 10 mu m, the ratio of gate-drain isolation resistance to gate-source isolation resistance increases from 1.38 to 2.33, leading to a decrease in the slope of empirical lines from 0.42 to 0.30.