Dynamic Threshold Voltage in p-GaN Gate HEMT

The p-GaN gate HEMT with a Schottky gate contact is studied in this work. The threshold voltage (V-th) of the device is found to have a dynamic nature. When the device experiences a high drain voltage V-DSQ, the gate-to-drain capacitance C-GD is charged to Q(GD)(VDSQ). The negative part of Q(GD)(V-DSQ) is mainly located in the p-GaN layer. When drain voltage drops to a lower value V-DSM, the non-equilibrium charges Delta Q(GD) = Q(GD)(V-GDQ) - Q(GD)(V-GDM) cannot be effectively released, since the discharging current is blocked by the reversely biased metal/p-GaN Schottky junction and p-GaN/2DEG PN junction. An extra V-GS is required to counteract the non-equilibrium Delta Q(GD), resulting in a change of V-th. The change in V-th is well predictable by its linear relationship to Delta Q(GD). During switching operation, V-th is adaptive along the load line. Without considering the dynamic V-th effect, there exist large discrepancies in switching transients between the modelled and the experimental results. Incorporation of the dynamic V-th in device model result in greatly enhanced accuracy in simulated transient behavior.