Gate-geometry dependence of dynamic Vt in p-GaN gate HEMTs

We study the pulsed dynamics of the threshold voltage of p-GaN gate HEMTs of different gate geometries. Devices with varying p-GaN lengths and gate Schottky contact lengths have been characterized. In response to a forward gate-voltage pulse, the threshold voltage of the transistor initially shifts negative. The recovery transient that follows exhibits a strong geometry dependence. We find that the length of the uncontacted portion of the p-GaN layer (the gate "offset" region) plays a key role, with devices with longer offset recovering the fastest. We attribute these phenomena to the confluence of two factors. On the one hand, there is competition between gate current continuity and a capacitive divider in controlling the gate electrostatics under dynamic conditions. Second, we have observed that DC gate current flow across the p-GaN/AlGaN/GaN PIN junction largely takes place through the offset region. Our research highlights the critical importance of gate geometry on the switching dynamics of p-GaN HEMTs.