“Leaky Dielectric” Model for the Suppression of Dynamic R_ON in Carbon Doped AlGaN/GaN HEMTs

GaN-on-Si power switching transistors that use carbon doped epitaxy are highly vulnerable to dynamic RON dispersion, leading to reduced switching efficiency. In this paper we identify the causes of this dispersion, using substrate bias ramps to isolate the leakage paths and trapping locations in the epitaxy, and simulation to identify their impact on the device characteristics. It is shown that leakage can occur both vertically and laterally and we suggest that this is associated not only with bulk transport, but also extended defects as well as hole gases at heterojunctions. For exactly the same epitaxial design it is shown using a “leaky dielectric” model that depending on the leakage paths, dynamic RON dispersion can vary between insignificant and infinite. An optimum leakage configuration is identified to minimize dispersion requiring a resistivity which increases with depth in the buffer stack. It is demonstrated that leakage through the undoped GaN channel is required over the entire gate to drain gap, and not just under the contacts, in order to fully suppress dispersion.