Understanding the threshold voltage instability during off-state stress in p-GaN HEMTs

In this letter, we investigate by means of experimental results and TCAD simulations the threshold voltage instability due to OFF-state drain stress in p-GaN gate AlGaN/GaN-on-Si HEMTs. When the drain of the p-GaN HEMT is biased in the OFF-state the threshold voltage ( ${V}_{\text {th}}$ ) shows a linear increase up to ~40%. This increase saturates at drain bias voltages above 50 V. The positive ${V}_{\text {th}}$ shift is attributed to the ionization of acceptor traps in the AlGaN region below the p-GaN gate with the source of these trapping sites suggested to be the p-GaN gate out-diffused Mg dopant atoms. The ionization of the Mg acceptors due to high electric field during OFF-state bias and the removal of the generated holes from the AlGaN region through the gate contact creates the charge conditions for a positive ${V}_{\text {th}}$ shift. The sharp decrease in the gate drain capacitance ( ${C}_{\text {gd}}$ ) for ${V}_{D} V, the simulated gate edge electric field reaching its peak for a drain voltage bias ${V}_{D} \sim {50}$ V and the positive threshold voltage shift observed for negative gate stress further validate the proposed model.