Charge Storage Mechanism of Drain Induced Dynamic Threshold Voltage Shift in p-GaN Gate HEMTs

The drain induced dynamic threshold voltage (V-th) shift of a p-GaN gate HEMT with a Schottky gate contact is investigated, and the underlyingmechanisms are explained with a charge storage model. When the device experiences a high drain bias V-DSQ, the gate-to-drain capacitance (C-GD) is charged to Q(GD)(V-DSQ). As the drain voltage drops to V-DSM whereVth ismeasured, CGD is expected to be discharged to Q(GD)(V-DSM). However, themetal/p-GaN Schottky junction could block the discharging current, resulting in storage of negative charges in the p-GaN layer. For the device to turn on, additional gate voltage is required to counteract the stored negative charges, resulting in a positive shift of V-th. The dynamicVth shift is an intrinsic and predictable characteristic of the p-GaN gate HEMT which is linearly correlated with Delta Q(GD) = Q(GD)(V-DSQ)-Q(GD)(V-DSM). The V-th shift is dependent on V-DSQ as well as V-DSM, indicating that the V-th shift is varying along the load line during a switching operation.