Gate/Drain Coupled Barrier Lowering Effect and Negative Threshold voltage Shift in Schottky-Type p-GaN Gate HEMT

To assess GaN power transistors' capability to maintain a decent E-mode operation, V-th at high V-DS is measured for Schottky-type p-GaN gate HEMT, and an excessive negative V-th shift is observed. With V-DS = 1 V, V-th is around 1.3 V at I-D = 1 mA, but V-th drops by similar to 0.6 V when measured at a high V-DS of 100 V. In comparison, ohmic-type p-GaN gate HEMT only shows a negligible V-th shift up to V-DS = 100 V. A gate/drain coupled barrier lowering (GDCBL) effect is proposed to explain the appreciable V-th shift in Schottky-type p-GaN gate HEMT. Upon high V-DS, the potential of the floating p-GaN layer is raised by the drain through the capacitive coupling between p-GaN and drain (C-DP). The positive potential of the p-GaN layer then lowers the energy barrier along the gated channel, resulting in a reduced V-th. This effect is confirmed by the dependence of measured negative V-th shift upon the property of gate/p-GaN contact and also by the numerical simulations that reveal the change in band diagrams upon high V-DS.