Field Plate Designs in All-GaN Cascode Heterojunction Field-Effect Transistors

Different source field plate (FP) connections are compared for the all-GaN integrated cascode device to address the capacitance matching and turn-off controllability issues reported in the conventional GaN plus Si cascode. The experimental results suggest that the cascode device with an FP connected to the source terminal can significantly suppress the off-state internode voltage, leading to minimized capacitive energy loss and reduced overvoltage stress at the internode. This is attributed to the reduced ratio of the drain-source capacitance of the depletion mode cascode part to the total capacitance at the cascode internode. An additional FP on the E-mode cascode part is proposed to further suppress the off-state internode voltage and benefit the device. Cascode devices with the source FP connecting to the enhancement mode gate have an improved switching controllability via gate resistance during turn-off and hence enhanced dv/dt immunity in the drain loop.